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On rectifiers
Physica XVII, no 8
Augustus 1951
ON RECTIFIERS b y W. CH. V A N G E E L Natuurkundig Laboratorium der N.V. Philips' Gloeilampenfabrieken Eindhoven, Nederland Synopsis
Experiments are described in which rectifiers were formed from combinations of metals, semiconductors and intermediate layers. The following combinations have been considered : (a) the contact between an excess semiconductor and a deficit semiconductor; (b) the combination aluminium I aluminium oxide] semiconductor; and (c) the combination metal i resin layer I semiconductor. In all these cases rectification occurs. The suggestion is put forward that, in all three cases, the contact between two layers with charge carriers of opposite sign is the cause of rectification.
1. Introduction. As a rule a rectifier is assumed to consist of a m e t a l and a s e m i c o n d u c t o r with a barrier laver between. A distinction is made between physical and chemical barrier layers. The physical barrier layer arises from the transfer of charge carriers from the metal to the s e m i c o n d u c t o r upon c o n t a c t being made between the two, in consequence of which a b o u n d a r y layer of a high resistance is formed in the semiconductor. When c o n t a c t is made between a metal and a deficit semiconductor, for instance, electrons pass from the metal to the semiconductor and neutralize electron holes in the latter, so t h a t a poorly c o n d u c t i n g b o u n d a r y layer is formed 1). The chemical barrier layer has to be formed b y external means, a change in the surface laver of the semiconductor being b r o u g h t about chemically in such a w a y as to give the b o u n d a r y layer a high resistance. For instance, a substance like Cu20 which has become highly conductive through absorption of additional o x y g e n can be surface-treated to remove this additional o x y g e n and leave a poorly c o n d u c t i n g surface laver to which a metal is then applied. The oxide coatings that can be applied to metals like A1 and Zr b y anodic oxidation could also be regarded as chemical barrier layers. --
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In such a case the metal has been given a poorly conducting boundary layer, and then a semiconductor is applied to the oxide layer, for instance under pressure 2). Of course, combinations of,t)hysical and chemical barrier layers are also possible. The aforementioned barrier layers are often called genetic barrier layers, because they are formed either from the semiconductor or from the metal. There is a third kind of barrier layer to be distinguished, namely the non-genetic barrier layer, consisting of a layer of an insulating substance applied between the metal and the semiconductor and thus not being formed from either of the electrodes. As an example may be mentioned a layer of resin applied between the metal and the semiconductor 8). S c h o t t k y regards the part played by these barrier layers as being not essential for-the process of rectification. He assumes that the pores in the layer of resin serve as points of contact between metal and semiconductor. Thus there are to be distinguished: 1. Physical barrier layers, which arise when contact is made between the metal and semiconductor. Thus the latter is uniform in composition and changes take place in the boundary layer only upon contact with metal. Owing to the transfer of charge carriers a layer of high resistance is formed in the boundary laver of the semiconductor. An example is germanium to which a point contact of a metal has been applied. 2. Chemical barrier layers, divided into: (a) Genetic barrier layers formed from the metal or from the semiconductor by chemical means. An example is aluminium coated with a layer of A1203 and with a semiconductor pressed oll to it. Then there is also the cuprous oxide rectifier, which can be made 1)\7 treating a layer of cuprous oxide with additional oxygen so as to form on the surface a poorly conducting Skin. (b) Non-genetic barrier layers, which are not formed from either of the electrodes; e.g. the combination platinum ] polystyrene I zinc oxide. The principal facts about rectifiers will now be briefly dealt with. In the combination metal ! barrier layer l semiconductor the direction of good transmission depends upon the nature of the semiconductor. If it is an excess semiconductor (~t-type) then the direction of good transmission for electrons is that from semiconductor
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to metal. If it is a deficit semiconductor (p-type) the direction of good transmission is from metal to semiconductor. It is considered that the most important theories of rectification are the following 4). Davydov' theoryS). Davydov assumes rectification to be due to two conductors with charge carriers of a different sign coming into contact with each other. Thus rectification should take place when contact is made between a deficit and an excess semiconductor. Electrons will pass from the excess semiconductor to the deficit semiconductor and a potential barrier will be formed. The latter will be enlarged or reduced under influence of the field applied. D a v y d o v does not set any limit to his theory; for instance, rectification may take place in insulators with ionic conduction, provided there are two kinds of charge carriers, e.g. positive and negative ions. Schottky's theory1). Upon contact being made between a metal and a semiconductor of uniform composition, charge carriers will pass from one to the other, because as a rule the thermodynamic potentials in the two materials will be different. Owing to the exchange of electrons a space charge is formed and an electrical potential difference arises which equalizes the thermodynamic potentials. In the case of a deficit semiconductor electrons pass from the metal to the semiconductor, and in case of an excess semiconductor electrons pass from the latter to the metal. In both cases there arises in the semiconductor an area which is poor in charge carriers of the character determining the natural conductivity of the semiconductor. Thus there arises a layer of high resistance, the barrier laver. The field in this barrier layer is determined by the P o i s s o n equation. The thickness of the barrier layer depends upon the direction of the field applied and determines the resistance of the barrier laver and thus also the intensity of the current. .Mott's theorv4). M o t t uses a chemical barrier layer only 10 .4 cm thick, thereby assuming the metal and the semiconductor to be practically in contact with each other. In the cuprous oxide rectifier the copper and the cuprous oxide (with extra oxygen) are separated by a thin laver of stoichiometric composition. Electrons are able to pass from the metal through the barrier laver into the holes in the Cu20 (deficit semiconductor). On either side of the bar-
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rier layer a space charge is formed, the metal being positive and the semiconductor negative, so that a potential barrier is formed, which is influenced by the voltage applied. M o t t assumes that owing to the thermal agitation the electrons cross over the potential barrier. All three theories mentioned give the good sign of rectification, both for excess and for deficit semiconductors. As a result of the experiments described in this article the author is inclined to support D a v y d o v's theory. In this paper some experimental results will be given as obtained with composite rectifiers. The following will be dealt with in turn: (a) Rectifiers formed by contact between an excess and a deficit semiconductor, e.g. (ZnO + Zn) pressed on to (CuJ + J). This notation will be used throughout, although tee following might just as well be written : (ZnO - - O) or (CuJ - - J).) The deficit semiconductors used all obeyed 0 h m's law when measured between two metals. Transition contacts did not occur. In no case was there any rectification between a metal and a deficit semiconductor. The excess semiconductors used did not fully obey 0 h m's law. In every case there was a surface layer, a barrier layer, presumably due to the action of oxygen, as a consequence of which the excess metal ions contained in the semiconductor were converted into oxide. Thus (ZnO + Zn) was converted into ZnO at the surface by oxidation. Tearing off the surface layer certainly made an improvement but it did not entirely prevent the formation of a surface barrier layer. Sometimes it happened that the grains in the semiconductor were covered with a barrier layer, so that barrier skins were formed internally, and of course these could not be removed. (b) Rectifiers made from an anodic-oxidized plate of aluminium, the oxide layer being about 0.1 l, thick. Various semiconductors were pressed on to the oxide layer, mostly deficit semiconductors being used. Only in one case could rectification be obtained with an excess semiconductor. (c) Rectifiers obtained by applying a layer of resin to a metal (Pt) and then pressing a semiconductor on to the resin. The thickness of the layer of resin was about 3 t*. In this case particular success was achieved in making good rectifiers with excess semiconductors. The rectifying effect appeared to be slow. For this reason the idea of the
ON RECTIFIERS
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pores in the resin layer forming point contacts was rejected, since point rectifiers need not be slow. Rectification never occurs when contact is made between two deficit semiconductors. It may occur when two excess semiconductors are brought into contact and the surface layer of one of them has been given a different form of conductance, for instance through absorption of oxygen, but this is often difficult to ascertain. A description is given of the simple measuring circuit. All characteristics were measured at 50 c/s. Finally the opinion is expressed that the kinds of rectifiers described are probably all based upon the principle of contact between a deficit and an excess semiconductor.
2. The measuring circuit. The measuring circuit employed is illustrated in fig. 1. By means of a transformer T an alternating volt-
g~
Fig. 1. T h e s e t - u p used for m e a s u r i n g . T = t r a n s f o r m e r to A.C. V o l t a g e 50 c/s, G = r e c t i f i e r t o b e m e a s u r e d , R = r e s i s t a n c e , E.S. = e l e c t r o n i c s w i t c h t y p e P h i l i p s G M 4580, Osc. ----- Oscilloscope t y p e P h i l i p s GM 3156.
age of 50 c/s was applied to a rectifier G and a resistor R connected in series. The current through G induces a voltage across R which is a measure of the current. By means of an electronic switch E.S. the voltage across G was applied to the horizontal deflecting plates of an oscilloscope, whilst the voltage across R was applied, likewise through an electronic switch, to the vertical deflection plates. Thus the oscilloscope traces both the voltage across the rectifier and the current flowing through it. The electronic switches were used for tracing the coordinates for zero current and zero voltage 8). The oscilloscope used was of the type GM 3156 with a cathode-ray tube DN 9-5. The post-acceleration voltage generator GM 4198 was used. As electronic switch, the apparatus GM 4580 was employed.
3. Experiments. First a few general remarks will be made. The semiconductors used consisted of pressed or sintered material. The
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rectifiers were obtained by pressing the component parts together. In every case carbon was used for the neutral contacts, on which no barrier layer occurs. All the semiconductors were first measured between plates of carbon to ascertain whether there were any barrier layers, and whether, therefore, there were any deviations from O h m's law. Since the contacts were made by pressing one element on to another, the size of the contact surface cannot be given, so that there is no point in speaking of current densities. The surface of the semiconductors varied from 0. l to 0.5 cm 2 in size. The voltage applied varied for the different cases between l and 3 V, and the intensity of the currents from 1 to 10 mA. All i(V) characteristics were measured at 50 c/s. In all the diagrams given in this paper the voltage is set out horizontally and the current through the rectifier vertically. The characteristics lie in the first and third quadrants of each diagram. Under these quadrants it is indicated which of the electrodes concerned has a negative potential with respect to the other electrode when the characteristic is traced in the respective quadrant. Fig. 3, for instance, shows that when (ZnO + Zn) has a negative potential with respect to (Cu2S + S) the current flows in the first quadrant. If, on the other hand, (Cu2S + S) is negative with respect to (ZnO + Zn), then the current flows in the third quadrant (bottom left). Thus from fig. 3 it is seen that the largest current flows when (ZnO + Zn) is negative with respect to (Cu2S + S). (1) R e c t i f i e r s obtained by contact between an excess and a deficit semiconductor. The excess semiconductor (ZnO + Zn) obtained bv heating ZnO in an atmosphere of hydrogen, was pressed against different deficit semiconductors, (Cu2S + S), (CuJ + J), (ZnO + O) *), and against C a M n Q **). Fig. 2 shows the plate of (ZnO + Zn) pressed between two plates of carbon. The current is not entirely proportional to the voltage, owing to the presence of surface barrier layers formed by oxidation *) This s u b s t a n c e was o b t a i n e d m o r e or less a c c i d e n t a l l y d u r i n ~ a process in which discs of ZnO + Zn were c o v e r e d with ~l thin l a y e r of gold, b y c a t h o d i c sputtering" ill air of low pressure. A f t e r the gold had been r u b b e d off. the s e t n i c o n d u c t o r a p p e a r e d to h a v e b e c o t . o a deficit seluicollductor w h e . used ill a r e c t i f y i n g c o m b i n a t i o . . **) A b o u t C a M n O , too little is kltown to d e t e r m i . e the c h a r a c t e r of the SelllicollductioIt In a r e c t i f y i n g c o m b i n a t i o n it b e h a v e s ;.d',vays as a deficit s e m i c o n d u c t o r .
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767
of the surface, but there is no rectification. With the same plate of ZnO the rectifiers, whose characteristics are given in figs. 3, 4 and 5, were formed. The contacts at the extremities were in all cases carbon plates. These diagrams show that rectification occurs in every case and that the. current is highest when the excess semiconductor is negative with respect to the deficit semiconductor.
/
/
/
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/
Cv 2 5 ,-5 - Z ~ O *
Zn
C- Zn 0 * Zn - C Fig. 2. Cl(ZnO + Zn)lC.
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Fig. 3. (Cu2S + S) I (ZnO -{- Zn),
l
/
/ f
/ C,., J ,'J - Z,., O "Z,~ Fig. 4. (CuJ + J) [(ZnO + Zn).
Zn 0 " 0 - Z~O ÷Zn Fig. 5. (ZnO + O) [(ZnO + Zn).
Fig. 6 gives the characteristic of another (ZnO + Zn) plate between carbon electrodes, whilst fig. 7 gives the characteristic of this same carbon plate in contact with the semiconductor CaMnO 3.
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W. CH. VAN GEEL
Another excess semi-conductor used was (TiO 2 q- Ti). Fig. 8 gives the characteristic of the TiO 2 plate between two carbon electrodes. Here again there are surface barrier layers, but no rectification occurs. Fig. 9 gives the same plate in contact with the deficit semiconductor (Cu2S-F S), where rectification occurs again and the highest current is found when the excess semiconductor is negative with respect to the deficit semiconductor.
/ /
y
/ .... ."
2""
.
.
•
i
Fig. 6. C I (ZnO + Zn) [ C.
Fig. 7. CaMnO3 I (ZnO + Zn).
J
-
/
Fig. 8. C[ (TiO 2 + Ti) ]C.
Fig. 9. (Cu2S + S)
(TiO 2 + Ti).
(2) R e c t i f i e r s with intermediate laver of A1203. An aluminium plate was coated with oxide in the well-known
ON RECTIFIERS
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w a y b y anodic oxidation, and various semiconductors were then pressed on to the oxide layer, which was 0.1 l* thick. Thus the combination AI[A12OaJ semiconductor was considered. Figures 10, l 1, 12 and 13 give the i(V) characteristics for the cases where the deficit semiconductors (CuJ + J), (ZnO + O), (Cu2S + S) and Mn02 respectively, were pressed on to the AI20 a layer. It is seen that the direction of good electron transmission is from the metal
t (u J . J - , 4 / 2 0 ~ - / /
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a
p
a
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-
z~ o ,, o-A/e o, -~/ Fig. 10. (CuJ + J) [ A1203 [ A1.
Fig. Ii. (ZnO + O) AI20 alA1.
!
Fig. 12. (Cu2S + S) I A1203 [ A1.
/
Fig. 13. MnO 2 I A1203 I A1.
through the oxide laver to the semiconductor, the highest current flowing when A1 is negative. It may also be seen that in all four cases the direction of good transmission for electrons shows loops, the same as in the case of the Physica
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W. CH. VAN GEEL
combination AlIAl2Oal electrolyte 7). This suggests that the cause of the loop is to be sought in the oxide laver and not in the semiconductor or the electrolyte. It m a y also be an indication that the rectification takes place in the oxide layer. When, instead of a deficit semiconductor, an excess semiconductor is pressed on to the AI20 a layer it is difficult to get rectification. It was managed only in the case of the excess semiconductor (TiO2+Ti) (fig. 15). Fig. 14 gives the result of measuring the TiO 2 plate between two carbon electrodes.
/
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."
y-
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Fig. 14. C I (TiO 2 + Ti) I C.
Fig. 15. (TiO 2 + Ti) I A1203 I A1.
In fig. 15 there is again a decided loop, in the direction of least transmission, but again the A1 is negative with respect to the semiconductor, so that the direction of the field is the same as that found in the aforementioned cases where a loop was formed. This is again an indication that the cause of the loop is to be sought in the oxide layer. As regards the direction of good transmission for electrons this is again the direction from semiconductor to metal, and is thus opposite to that found in the case of the combination AlfA12Oal deficit semiconductor. It is once more pointed out that the combination AIIA12Q [ (TiO 2 + Ti) was the onlv one with which rectification was achieved with an excess semiconductor in combination with AllA12Oat. Perhaps the explanation lies in the fact that the (TiO 2 4-, Ti) used
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ON R E C T I F I E R S
had a relatively low specific resistance, whereas the other excess semiconductors had a higher specific resistance. It is possible that the n-type character of the electronic conduction of this substance was more pronounced than that of the other substances. (3) R e c t i f i e r s with a resin skin as chemical b a r r i e r 1 a y e r. A skin of resin about 3 ff thick was applied to a plate of platinum. Polystyrene was used for this skin, but other kinds of resin can be used equally well. Various semiconductors were then pressed on to the resin skin. Fig. 16 represents the semiconductor (ZnO + Zn) between two carbon electrodes, whilst fig. 17 applies for the (ZnO + Zn) plate pressed against the polystyrene skin. It is seen that the direction from (ZnO + Zn) (excess semiconductor) to Pt is the direction of good transmission for electrons.
/
i O'Zn po/-Pt
Fig. 16. C i (ZnO + Zn) I C.
Fig. 17. (ZnO + Zn) I polystyrene I Pt.
Fig. 18 gives the result of a (TiO2 + Ti) plate between two carbon electrodes, and fig. 19 the result when the same plate is used with resin skin and platinum. In figs. 17 and 19 the direction from excess semiconductor through the resin skin to the metal is the direction of good transmission. Further, both figures show a loop in the direction of least transmission. Just as in the case of the rectifiers with intermediate layers of A1203 the cause of the loop was considered to lie in the A12Q layer, here too it is thought that the cause of the loop is connected with the resin skin.
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W . CH. VAN G E E L
So far only excess semiconductors had been used. When rectification was attempted by pressing deficit semiconductors on to the resin skin it was not found possible with an alternating voltage of 50 c/s. With a DC voltage reversed in polarity, and thus with a very long cycle, it was possible to obtain rectification also with the combination metal [resin layerl deficit semiconductor, but the following phenomenon was then encountered.
I/ f
L i
/ /
/
/t F i g . 18.
c I (TiO2
+ Ti) I C.
Fig. 19. (TiO2 + Ti) i polystyrene I Pt
When beginning by making the metal negative with respect to the deficit semiconductor, a current flows through the system, and after reversing the polarity (metal positive) this retains the same value for some time but then suddenly drops to a low value. Thus there is inertia in the rectification. The direction of good transmission is again from metal to deficit semiconductor. I In one single case it was possible to obtain rectification with a voltage of 50 c~s. An example of this is given in fig. 20, where the current is unstable, high currents sometimes arising in the direction of least transmission P,~jj. j . ~ - D t (fig. 20 is a drawn COl)>" of a photographic. Fig. 20. (CuJ + J) recording). polystyrene pt.
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4. Surnmary o/the results. Rectifiers formed by contact between an excess and a deficit semiconductor were first considered, and in all cases there was rectification. Against this it may be said that the excess semiconductors used have a barrier layer on their surface, so that the contact between the two kinds of semiconductors was not direct. Therefore the true combination was really: carbon i barrier layer I excess semiconductor I barrier layer [ deficit semiconductor I carbon. This combination gave rectification in every case. When the deficit semiconductor was eliminated from the combination there was no rectification, but then the current was not entirely proportional to the voltage. If the excess semiconductor was eliminated from the combination there was again no rectification but 0 h m ' s law was strictly complied with. It cannot, therefore, be said with certainty whether, in the entire absence of the surface barrier layer on the excess semiconductor, rectification would still occur. The second type of rectifier considered was that with intermediate layers of A120v Typical in this case was the occurrence of a loop in the dynamic characteristic when a field was applied across the intermediate layer, so that the A1 became negative. This applied both to the combination with a deficit semiconductor and to that with an excess semiconductor or an electrolyte. In a previous publication 6) an explanation has already been given for the occurrence of the loop by supposing that in the oxide layer, at the side facing the A1, there is a layer, having the properties of an excess semiconductor, underlying an insulating layer, the barrier layer. The thickness of that insulating layer decreases or increases, as a consequence of electrolysis, according to the direction of the field applied. When a field is applied such as to make the A1 negative, the thickness of the excess semiconducting layer in the oxide layer increases, while the thickness of the insulating skin decreases. Thus, with decreasing voltage in the direction of transmission the barrier layer is thinner than in the case of increasing voltage, so that with decreasing voltage the currents are higher than with increasing voltage. This accounts for the loop and may given an indication of why the loop occurs in the case of the rectifiers with resin skin. We now have to explain the occurrence of rectification. When we start from the aforementioned fact that the A1203 layer is built up
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W. CtI. VAN G E E L
of a layer, having the properties of an excess semiconductor, underlying an insulating layer, and we press a deficit semiconductor on to the oxide layer, then we have the combination excess semiconductor J insulating barrier layer 1 I deficit semiconductor. Thus the type with A1203 intermediate layers is reduced to the type of contact between deficit and excess semiconductors already considered. There is the possibility - - which has already been alluded to in a previous paper s) __ that, in addition to the layer with properties of an excess semiconductor and the insulating layer, the A12Q layer also comprises, on the side facing the electrolyte, a layer having the properties of a deficit semiconductor. This layer m a y arise from the absorption of oxygen b y the A1203 layer on the electrolyte side, or else it m a y be that the A12Q layer does not contain enough A1, either of which possibilities m a y lead to deficit semiconductance. When a deficit semiconductor is pressed on to the A1203 layer having itself a deficit semiconductor at the surface, the direction of rectification remains the same. In essence nothing is changed. It has already been seen how difficult it is to get rectification b y pressing an excess semiconductor on to the A1203 layer. This can be understood to a certain extent. If the A1203 layer consists of a layer with excess semiconductance and an insulating layer, then, when pressing on an excess semiconductor, we have the combination excess semiconductor I barrier layer I excess semiconductor. In the case where the A1203 layer has a surface layer with deficit semiconductance, when an excess semiconductor is pressed on to it we get the combination, excess semiconductor I insulator I deficit semiconductor I excess semi conductor. It is clear that in both these cases it depends greatly upon the excess semiconductor applied whether or not the system will rectify. In the first case of the combination, excess semiconductor ] barrier layer ] excess semiconductor, perfect symmetry is possible, so that the recitification will bepend upon the difference in properties of the two excess semiconductors. The second case is complicated by two rectifying systems being connected opposite to each other (excess
ON RECTIFIERS
775 t
semiconductor I deficit semiconductor I excess semiconductor), and it is then a question which of the two systems conducts current best. The third type considered was t h a t of a rectifier having a resin skin as barrier layer. It is striking that in figs. 17 and 19 a loop occurs in the direction of least transmission. Here, too, explanation might be sought in a varying thickness of the barrier layer, as supposed to be the case with the A120a layers. Of course, the thickness of the resin skin itself does not change. As is the case with the A120a skin, a part of the resin skin would therefore have to change in thickness to account for the occurrence of the loop, and we should have to suppose that there is ionic conductance in the skin. Now ionic conductance in resin skins is a phenomenon that frequently occurs 8). Account may be taken of the possibility of positive and negative ions being present in the resin skin. The possibility referred to by D a v y d o v 5) than presents itself, namely that rectification takes place in insulators with ionic conductance. D a v y d o v's theory amounts actually to the formation of space charges, giving rise to a counter - - e.m.f., as often occurs in insulators. Another analogy between the A120a layers and the resin skins lies in the fact that rectifiers made with the aid of these layers show inertia and at high frequencies no longer rectify. At about 200 c/s a resin skin rectifier no longer rectifies. This, as in the case of A120a, points to a continuously changing structure in the resin skin. For rectification a certain structure is needed in the skin, brought about by a certain field direction. When the polarity is reversed that structure is destroyed. Restoring takes time, and if that time is too short there is no rectification. Here attention should be drawn to the conception that rectification takes place only because there are pores in the resin skin allowing point contacts to be made between the metal and the semiconductor. This idea is objected because with point-contact rectitiers there is no inertia at such a low frequency. It has not yet been possible to form a correct idea of the structure in the resin skin that would give rise to rectification. By probe testing it is proposed to ascertain something about the field conditions in the resin skin. Another phenomenon to which attention has to be paid is the difference in behaviour of the combination when pressing on an excess or a deficit semiconductor. There seems but little doubt that
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ON
RECTIFIERS
a transfer of charge carriers from the semiconductor to the resin skin will have to be assumed. It is intended to carry out extensive tests with resin skin rectifiers. Thanks are due to Dr P. W. H a a ij m a n for having prepared the various semiconductors.
.Note. After the dressing of this article, the author was informed of an article, written in Russian by J o f f e, A. V., Journ. techn. Phys. U.S.S.R. 18 (1948) 1498 in which the rectification of the contact between excess and deficit semi-conductor has also been described. Eindhoven, September 1950. Received 17-4-51.
REFERENCES 1) S e h o t tky, W . , Z . P h y s . 1 1 3 (1939) 3 6 7 - 4 1 4 ; 1 1 8 (1942) 3 5 9 - 5 9 2 . 2) v a n G e e 1, W . C h., Z. P h y s . 6Y (1931) 765-78,5. 3) J u s 6 , W . , N a t u r e 1:12 (1933) 242; v a n G e e l , W . Ch., N a t u r e I::~:?.( 1 9 3 3 ) 7 1 1 ; Quintin, M., C. R. A c a d . Sci. P a r i s 1 9 8 (1934) 3 4 7 - 3 4 9 ; H a r t m a n n , W., P h y s . Z. I~7 (1936) 8 6 2 - 8 6 5 . 4) H e n i s c h, H . K., Metal-rectifiers, Clarendon Press Oxford 1949; M o t t, N. F. and G u r n e y, R. W . , Electronic processes in ionic crystals, Clarendon Press, Oxford (1946), p. 81 ; T e s z n e r, S., B u l l e t i n de la Soc. ft. des 61ectriciens .~ (1949) 401. 5) D a v y d o v, B., T e c h n . P h y s . U . S . S . R . 5 (1938) 87-9,5. 6) v a n G e e l, W . C h. a n d B o u m a, 13. C., P h i l i p s Res. R e p . 5 (1950) 4 6 1 - 4 7 5 . 7) D e k k e r , A.J. and van Geel, W. C11., Philips Res. R e p . ~ (1950) 3 0 3 - 3 1 9 . 8) B 6 n i n g, P a u 1, Electrische Isolierstoffe, Sammlung Vieweg, Braunschweig
(1938).
Augustus 1951
ON RECTIFIERS b y W. CH. V A N G E E L Natuurkundig Laboratorium der N.V. Philips' Gloeilampenfabrieken Eindhoven, Nederland Synopsis
Experiments are described in which rectifiers were formed from combinations of metals, semiconductors and intermediate layers. The following combinations have been considered : (a) the contact between an excess semiconductor and a deficit semiconductor; (b) the combination aluminium I aluminium oxide] semiconductor; and (c) the combination metal i resin layer I semiconductor. In all these cases rectification occurs. The suggestion is put forward that, in all three cases, the contact between two layers with charge carriers of opposite sign is the cause of rectification.
1. Introduction. As a rule a rectifier is assumed to consist of a m e t a l and a s e m i c o n d u c t o r with a barrier laver between. A distinction is made between physical and chemical barrier layers. The physical barrier layer arises from the transfer of charge carriers from the metal to the s e m i c o n d u c t o r upon c o n t a c t being made between the two, in consequence of which a b o u n d a r y layer of a high resistance is formed in the semiconductor. When c o n t a c t is made between a metal and a deficit semiconductor, for instance, electrons pass from the metal to the semiconductor and neutralize electron holes in the latter, so t h a t a poorly c o n d u c t i n g b o u n d a r y layer is formed 1). The chemical barrier layer has to be formed b y external means, a change in the surface laver of the semiconductor being b r o u g h t about chemically in such a w a y as to give the b o u n d a r y layer a high resistance. For instance, a substance like Cu20 which has become highly conductive through absorption of additional o x y g e n can be surface-treated to remove this additional o x y g e n and leave a poorly c o n d u c t i n g surface laver to which a metal is then applied. The oxide coatings that can be applied to metals like A1 and Zr b y anodic oxidation could also be regarded as chemical barrier layers. --
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W. e l l . VAN G E E L
In such a case the metal has been given a poorly conducting boundary layer, and then a semiconductor is applied to the oxide layer, for instance under pressure 2). Of course, combinations of,t)hysical and chemical barrier layers are also possible. The aforementioned barrier layers are often called genetic barrier layers, because they are formed either from the semiconductor or from the metal. There is a third kind of barrier layer to be distinguished, namely the non-genetic barrier layer, consisting of a layer of an insulating substance applied between the metal and the semiconductor and thus not being formed from either of the electrodes. As an example may be mentioned a layer of resin applied between the metal and the semiconductor 8). S c h o t t k y regards the part played by these barrier layers as being not essential for-the process of rectification. He assumes that the pores in the layer of resin serve as points of contact between metal and semiconductor. Thus there are to be distinguished: 1. Physical barrier layers, which arise when contact is made between the metal and semiconductor. Thus the latter is uniform in composition and changes take place in the boundary layer only upon contact with metal. Owing to the transfer of charge carriers a layer of high resistance is formed in the boundary laver of the semiconductor. An example is germanium to which a point contact of a metal has been applied. 2. Chemical barrier layers, divided into: (a) Genetic barrier layers formed from the metal or from the semiconductor by chemical means. An example is aluminium coated with a layer of A1203 and with a semiconductor pressed oll to it. Then there is also the cuprous oxide rectifier, which can be made 1)\7 treating a layer of cuprous oxide with additional oxygen so as to form on the surface a poorly conducting Skin. (b) Non-genetic barrier layers, which are not formed from either of the electrodes; e.g. the combination platinum ] polystyrene I zinc oxide. The principal facts about rectifiers will now be briefly dealt with. In the combination metal ! barrier layer l semiconductor the direction of good transmission depends upon the nature of the semiconductor. If it is an excess semiconductor (~t-type) then the direction of good transmission for electrons is that from semiconductor
ON R E C T I F I E R S
763
to metal. If it is a deficit semiconductor (p-type) the direction of good transmission is from metal to semiconductor. It is considered that the most important theories of rectification are the following 4). Davydov' theoryS). Davydov assumes rectification to be due to two conductors with charge carriers of a different sign coming into contact with each other. Thus rectification should take place when contact is made between a deficit and an excess semiconductor. Electrons will pass from the excess semiconductor to the deficit semiconductor and a potential barrier will be formed. The latter will be enlarged or reduced under influence of the field applied. D a v y d o v does not set any limit to his theory; for instance, rectification may take place in insulators with ionic conduction, provided there are two kinds of charge carriers, e.g. positive and negative ions. Schottky's theory1). Upon contact being made between a metal and a semiconductor of uniform composition, charge carriers will pass from one to the other, because as a rule the thermodynamic potentials in the two materials will be different. Owing to the exchange of electrons a space charge is formed and an electrical potential difference arises which equalizes the thermodynamic potentials. In the case of a deficit semiconductor electrons pass from the metal to the semiconductor, and in case of an excess semiconductor electrons pass from the latter to the metal. In both cases there arises in the semiconductor an area which is poor in charge carriers of the character determining the natural conductivity of the semiconductor. Thus there arises a layer of high resistance, the barrier laver. The field in this barrier layer is determined by the P o i s s o n equation. The thickness of the barrier layer depends upon the direction of the field applied and determines the resistance of the barrier laver and thus also the intensity of the current. .Mott's theorv4). M o t t uses a chemical barrier layer only 10 .4 cm thick, thereby assuming the metal and the semiconductor to be practically in contact with each other. In the cuprous oxide rectifier the copper and the cuprous oxide (with extra oxygen) are separated by a thin laver of stoichiometric composition. Electrons are able to pass from the metal through the barrier laver into the holes in the Cu20 (deficit semiconductor). On either side of the bar-
764
W. CH. VAN G E E L
rier layer a space charge is formed, the metal being positive and the semiconductor negative, so that a potential barrier is formed, which is influenced by the voltage applied. M o t t assumes that owing to the thermal agitation the electrons cross over the potential barrier. All three theories mentioned give the good sign of rectification, both for excess and for deficit semiconductors. As a result of the experiments described in this article the author is inclined to support D a v y d o v's theory. In this paper some experimental results will be given as obtained with composite rectifiers. The following will be dealt with in turn: (a) Rectifiers formed by contact between an excess and a deficit semiconductor, e.g. (ZnO + Zn) pressed on to (CuJ + J). This notation will be used throughout, although tee following might just as well be written : (ZnO - - O) or (CuJ - - J).) The deficit semiconductors used all obeyed 0 h m's law when measured between two metals. Transition contacts did not occur. In no case was there any rectification between a metal and a deficit semiconductor. The excess semiconductors used did not fully obey 0 h m's law. In every case there was a surface layer, a barrier layer, presumably due to the action of oxygen, as a consequence of which the excess metal ions contained in the semiconductor were converted into oxide. Thus (ZnO + Zn) was converted into ZnO at the surface by oxidation. Tearing off the surface layer certainly made an improvement but it did not entirely prevent the formation of a surface barrier layer. Sometimes it happened that the grains in the semiconductor were covered with a barrier layer, so that barrier skins were formed internally, and of course these could not be removed. (b) Rectifiers made from an anodic-oxidized plate of aluminium, the oxide layer being about 0.1 l, thick. Various semiconductors were pressed on to the oxide layer, mostly deficit semiconductors being used. Only in one case could rectification be obtained with an excess semiconductor. (c) Rectifiers obtained by applying a layer of resin to a metal (Pt) and then pressing a semiconductor on to the resin. The thickness of the layer of resin was about 3 t*. In this case particular success was achieved in making good rectifiers with excess semiconductors. The rectifying effect appeared to be slow. For this reason the idea of the
ON RECTIFIERS
765
pores in the resin layer forming point contacts was rejected, since point rectifiers need not be slow. Rectification never occurs when contact is made between two deficit semiconductors. It may occur when two excess semiconductors are brought into contact and the surface layer of one of them has been given a different form of conductance, for instance through absorption of oxygen, but this is often difficult to ascertain. A description is given of the simple measuring circuit. All characteristics were measured at 50 c/s. Finally the opinion is expressed that the kinds of rectifiers described are probably all based upon the principle of contact between a deficit and an excess semiconductor.
2. The measuring circuit. The measuring circuit employed is illustrated in fig. 1. By means of a transformer T an alternating volt-
g~
Fig. 1. T h e s e t - u p used for m e a s u r i n g . T = t r a n s f o r m e r to A.C. V o l t a g e 50 c/s, G = r e c t i f i e r t o b e m e a s u r e d , R = r e s i s t a n c e , E.S. = e l e c t r o n i c s w i t c h t y p e P h i l i p s G M 4580, Osc. ----- Oscilloscope t y p e P h i l i p s GM 3156.
age of 50 c/s was applied to a rectifier G and a resistor R connected in series. The current through G induces a voltage across R which is a measure of the current. By means of an electronic switch E.S. the voltage across G was applied to the horizontal deflecting plates of an oscilloscope, whilst the voltage across R was applied, likewise through an electronic switch, to the vertical deflection plates. Thus the oscilloscope traces both the voltage across the rectifier and the current flowing through it. The electronic switches were used for tracing the coordinates for zero current and zero voltage 8). The oscilloscope used was of the type GM 3156 with a cathode-ray tube DN 9-5. The post-acceleration voltage generator GM 4198 was used. As electronic switch, the apparatus GM 4580 was employed.
3. Experiments. First a few general remarks will be made. The semiconductors used consisted of pressed or sintered material. The
766
W. CH. VAN GEEL
rectifiers were obtained by pressing the component parts together. In every case carbon was used for the neutral contacts, on which no barrier layer occurs. All the semiconductors were first measured between plates of carbon to ascertain whether there were any barrier layers, and whether, therefore, there were any deviations from O h m's law. Since the contacts were made by pressing one element on to another, the size of the contact surface cannot be given, so that there is no point in speaking of current densities. The surface of the semiconductors varied from 0. l to 0.5 cm 2 in size. The voltage applied varied for the different cases between l and 3 V, and the intensity of the currents from 1 to 10 mA. All i(V) characteristics were measured at 50 c/s. In all the diagrams given in this paper the voltage is set out horizontally and the current through the rectifier vertically. The characteristics lie in the first and third quadrants of each diagram. Under these quadrants it is indicated which of the electrodes concerned has a negative potential with respect to the other electrode when the characteristic is traced in the respective quadrant. Fig. 3, for instance, shows that when (ZnO + Zn) has a negative potential with respect to (Cu2S + S) the current flows in the first quadrant. If, on the other hand, (Cu2S + S) is negative with respect to (ZnO + Zn), then the current flows in the third quadrant (bottom left). Thus from fig. 3 it is seen that the largest current flows when (ZnO + Zn) is negative with respect to (Cu2S + S). (1) R e c t i f i e r s obtained by contact between an excess and a deficit semiconductor. The excess semiconductor (ZnO + Zn) obtained bv heating ZnO in an atmosphere of hydrogen, was pressed against different deficit semiconductors, (Cu2S + S), (CuJ + J), (ZnO + O) *), and against C a M n Q **). Fig. 2 shows the plate of (ZnO + Zn) pressed between two plates of carbon. The current is not entirely proportional to the voltage, owing to the presence of surface barrier layers formed by oxidation *) This s u b s t a n c e was o b t a i n e d m o r e or less a c c i d e n t a l l y d u r i n ~ a process in which discs of ZnO + Zn were c o v e r e d with ~l thin l a y e r of gold, b y c a t h o d i c sputtering" ill air of low pressure. A f t e r the gold had been r u b b e d off. the s e t n i c o n d u c t o r a p p e a r e d to h a v e b e c o t . o a deficit seluicollductor w h e . used ill a r e c t i f y i n g c o m b i n a t i o . . **) A b o u t C a M n O , too little is kltown to d e t e r m i . e the c h a r a c t e r of the SelllicollductioIt In a r e c t i f y i n g c o m b i n a t i o n it b e h a v e s ;.d',vays as a deficit s e m i c o n d u c t o r .
O N RECTIFIERS
767
of the surface, but there is no rectification. With the same plate of ZnO the rectifiers, whose characteristics are given in figs. 3, 4 and 5, were formed. The contacts at the extremities were in all cases carbon plates. These diagrams show that rectification occurs in every case and that the. current is highest when the excess semiconductor is negative with respect to the deficit semiconductor.
/
/
/
°'t'
/
/
i
i
l
'
°, /
/ j .~ [/'
i ! /
/
Cv 2 5 ,-5 - Z ~ O *
Zn
C- Zn 0 * Zn - C Fig. 2. Cl(ZnO + Zn)lC.
/
Fig. 3. (Cu2S + S) I (ZnO -{- Zn),
l
/
/ f
/ C,., J ,'J - Z,., O "Z,~ Fig. 4. (CuJ + J) [(ZnO + Zn).
Zn 0 " 0 - Z~O ÷Zn Fig. 5. (ZnO + O) [(ZnO + Zn).
Fig. 6 gives the characteristic of another (ZnO + Zn) plate between carbon electrodes, whilst fig. 7 gives the characteristic of this same carbon plate in contact with the semiconductor CaMnO 3.
768
W. CH. VAN GEEL
Another excess semi-conductor used was (TiO 2 q- Ti). Fig. 8 gives the characteristic of the TiO 2 plate between two carbon electrodes. Here again there are surface barrier layers, but no rectification occurs. Fig. 9 gives the same plate in contact with the deficit semiconductor (Cu2S-F S), where rectification occurs again and the highest current is found when the excess semiconductor is negative with respect to the deficit semiconductor.
/ /
y
/ .... ."
2""
.
.
•
i
Fig. 6. C I (ZnO + Zn) [ C.
Fig. 7. CaMnO3 I (ZnO + Zn).
J
-
/
Fig. 8. C[ (TiO 2 + Ti) ]C.
Fig. 9. (Cu2S + S)
(TiO 2 + Ti).
(2) R e c t i f i e r s with intermediate laver of A1203. An aluminium plate was coated with oxide in the well-known
ON RECTIFIERS
769
w a y b y anodic oxidation, and various semiconductors were then pressed on to the oxide layer, which was 0.1 l* thick. Thus the combination AI[A12OaJ semiconductor was considered. Figures 10, l 1, 12 and 13 give the i(V) characteristics for the cases where the deficit semiconductors (CuJ + J), (ZnO + O), (Cu2S + S) and Mn02 respectively, were pressed on to the AI20 a layer. It is seen that the direction of good electron transmission is from the metal
t (u J . J - , 4 / 2 0 ~ - / /
j0
•
+
#
a
p
a
°
-
z~ o ,, o-A/e o, -~/ Fig. 10. (CuJ + J) [ A1203 [ A1.
Fig. Ii. (ZnO + O) AI20 alA1.
!
Fig. 12. (Cu2S + S) I A1203 [ A1.
/
Fig. 13. MnO 2 I A1203 I A1.
through the oxide laver to the semiconductor, the highest current flowing when A1 is negative. It may also be seen that in all four cases the direction of good transmission for electrons shows loops, the same as in the case of the Physica
X'VII
49
770
W. CH. VAN GEEL
combination AlIAl2Oal electrolyte 7). This suggests that the cause of the loop is to be sought in the oxide laver and not in the semiconductor or the electrolyte. It m a y also be an indication that the rectification takes place in the oxide layer. When, instead of a deficit semiconductor, an excess semiconductor is pressed on to the AI20 a layer it is difficult to get rectification. It was managed only in the case of the excess semiconductor (TiO2+Ti) (fig. 15). Fig. 14 gives the result of measuring the TiO 2 plate between two carbon electrodes.
/
t"
."
y-
/
Fig. 14. C I (TiO 2 + Ti) I C.
Fig. 15. (TiO 2 + Ti) I A1203 I A1.
In fig. 15 there is again a decided loop, in the direction of least transmission, but again the A1 is negative with respect to the semiconductor, so that the direction of the field is the same as that found in the aforementioned cases where a loop was formed. This is again an indication that the cause of the loop is to be sought in the oxide layer. As regards the direction of good transmission for electrons this is again the direction from semiconductor to metal, and is thus opposite to that found in the case of the combination AlfA12Oal deficit semiconductor. It is once more pointed out that the combination AIIA12Q [ (TiO 2 + Ti) was the onlv one with which rectification was achieved with an excess semiconductor in combination with AllA12Oat. Perhaps the explanation lies in the fact that the (TiO 2 4-, Ti) used
771
ON R E C T I F I E R S
had a relatively low specific resistance, whereas the other excess semiconductors had a higher specific resistance. It is possible that the n-type character of the electronic conduction of this substance was more pronounced than that of the other substances. (3) R e c t i f i e r s with a resin skin as chemical b a r r i e r 1 a y e r. A skin of resin about 3 ff thick was applied to a plate of platinum. Polystyrene was used for this skin, but other kinds of resin can be used equally well. Various semiconductors were then pressed on to the resin skin. Fig. 16 represents the semiconductor (ZnO + Zn) between two carbon electrodes, whilst fig. 17 applies for the (ZnO + Zn) plate pressed against the polystyrene skin. It is seen that the direction from (ZnO + Zn) (excess semiconductor) to Pt is the direction of good transmission for electrons.
/
i O'Zn po/-Pt
Fig. 16. C i (ZnO + Zn) I C.
Fig. 17. (ZnO + Zn) I polystyrene I Pt.
Fig. 18 gives the result of a (TiO2 + Ti) plate between two carbon electrodes, and fig. 19 the result when the same plate is used with resin skin and platinum. In figs. 17 and 19 the direction from excess semiconductor through the resin skin to the metal is the direction of good transmission. Further, both figures show a loop in the direction of least transmission. Just as in the case of the rectifiers with intermediate layers of A1203 the cause of the loop was considered to lie in the A12Q layer, here too it is thought that the cause of the loop is connected with the resin skin.
772
W . CH. VAN G E E L
So far only excess semiconductors had been used. When rectification was attempted by pressing deficit semiconductors on to the resin skin it was not found possible with an alternating voltage of 50 c/s. With a DC voltage reversed in polarity, and thus with a very long cycle, it was possible to obtain rectification also with the combination metal [resin layerl deficit semiconductor, but the following phenomenon was then encountered.
I/ f
L i
/ /
/
/t F i g . 18.
c I (TiO2
+ Ti) I C.
Fig. 19. (TiO2 + Ti) i polystyrene I Pt
When beginning by making the metal negative with respect to the deficit semiconductor, a current flows through the system, and after reversing the polarity (metal positive) this retains the same value for some time but then suddenly drops to a low value. Thus there is inertia in the rectification. The direction of good transmission is again from metal to deficit semiconductor. I In one single case it was possible to obtain rectification with a voltage of 50 c~s. An example of this is given in fig. 20, where the current is unstable, high currents sometimes arising in the direction of least transmission P,~jj. j . ~ - D t (fig. 20 is a drawn COl)>" of a photographic. Fig. 20. (CuJ + J) recording). polystyrene pt.
ON RECTIFIERS
773
4. Surnmary o/the results. Rectifiers formed by contact between an excess and a deficit semiconductor were first considered, and in all cases there was rectification. Against this it may be said that the excess semiconductors used have a barrier layer on their surface, so that the contact between the two kinds of semiconductors was not direct. Therefore the true combination was really: carbon i barrier layer I excess semiconductor I barrier layer [ deficit semiconductor I carbon. This combination gave rectification in every case. When the deficit semiconductor was eliminated from the combination there was no rectification, but then the current was not entirely proportional to the voltage. If the excess semiconductor was eliminated from the combination there was again no rectification but 0 h m ' s law was strictly complied with. It cannot, therefore, be said with certainty whether, in the entire absence of the surface barrier layer on the excess semiconductor, rectification would still occur. The second type of rectifier considered was that with intermediate layers of A120v Typical in this case was the occurrence of a loop in the dynamic characteristic when a field was applied across the intermediate layer, so that the A1 became negative. This applied both to the combination with a deficit semiconductor and to that with an excess semiconductor or an electrolyte. In a previous publication 6) an explanation has already been given for the occurrence of the loop by supposing that in the oxide layer, at the side facing the A1, there is a layer, having the properties of an excess semiconductor, underlying an insulating layer, the barrier layer. The thickness of that insulating layer decreases or increases, as a consequence of electrolysis, according to the direction of the field applied. When a field is applied such as to make the A1 negative, the thickness of the excess semiconducting layer in the oxide layer increases, while the thickness of the insulating skin decreases. Thus, with decreasing voltage in the direction of transmission the barrier layer is thinner than in the case of increasing voltage, so that with decreasing voltage the currents are higher than with increasing voltage. This accounts for the loop and may given an indication of why the loop occurs in the case of the rectifiers with resin skin. We now have to explain the occurrence of rectification. When we start from the aforementioned fact that the A1203 layer is built up
774
W. CtI. VAN G E E L
of a layer, having the properties of an excess semiconductor, underlying an insulating layer, and we press a deficit semiconductor on to the oxide layer, then we have the combination excess semiconductor J insulating barrier layer 1 I deficit semiconductor. Thus the type with A1203 intermediate layers is reduced to the type of contact between deficit and excess semiconductors already considered. There is the possibility - - which has already been alluded to in a previous paper s) __ that, in addition to the layer with properties of an excess semiconductor and the insulating layer, the A12Q layer also comprises, on the side facing the electrolyte, a layer having the properties of a deficit semiconductor. This layer m a y arise from the absorption of oxygen b y the A1203 layer on the electrolyte side, or else it m a y be that the A12Q layer does not contain enough A1, either of which possibilities m a y lead to deficit semiconductance. When a deficit semiconductor is pressed on to the A1203 layer having itself a deficit semiconductor at the surface, the direction of rectification remains the same. In essence nothing is changed. It has already been seen how difficult it is to get rectification b y pressing an excess semiconductor on to the A1203 layer. This can be understood to a certain extent. If the A1203 layer consists of a layer with excess semiconductance and an insulating layer, then, when pressing on an excess semiconductor, we have the combination excess semiconductor I barrier layer I excess semiconductor. In the case where the A1203 layer has a surface layer with deficit semiconductance, when an excess semiconductor is pressed on to it we get the combination, excess semiconductor I insulator I deficit semiconductor I excess semi conductor. It is clear that in both these cases it depends greatly upon the excess semiconductor applied whether or not the system will rectify. In the first case of the combination, excess semiconductor ] barrier layer ] excess semiconductor, perfect symmetry is possible, so that the recitification will bepend upon the difference in properties of the two excess semiconductors. The second case is complicated by two rectifying systems being connected opposite to each other (excess
ON RECTIFIERS
775 t
semiconductor I deficit semiconductor I excess semiconductor), and it is then a question which of the two systems conducts current best. The third type considered was t h a t of a rectifier having a resin skin as barrier layer. It is striking that in figs. 17 and 19 a loop occurs in the direction of least transmission. Here, too, explanation might be sought in a varying thickness of the barrier layer, as supposed to be the case with the A120a layers. Of course, the thickness of the resin skin itself does not change. As is the case with the A120a skin, a part of the resin skin would therefore have to change in thickness to account for the occurrence of the loop, and we should have to suppose that there is ionic conductance in the skin. Now ionic conductance in resin skins is a phenomenon that frequently occurs 8). Account may be taken of the possibility of positive and negative ions being present in the resin skin. The possibility referred to by D a v y d o v 5) than presents itself, namely that rectification takes place in insulators with ionic conductance. D a v y d o v's theory amounts actually to the formation of space charges, giving rise to a counter - - e.m.f., as often occurs in insulators. Another analogy between the A120a layers and the resin skins lies in the fact that rectifiers made with the aid of these layers show inertia and at high frequencies no longer rectify. At about 200 c/s a resin skin rectifier no longer rectifies. This, as in the case of A120a, points to a continuously changing structure in the resin skin. For rectification a certain structure is needed in the skin, brought about by a certain field direction. When the polarity is reversed that structure is destroyed. Restoring takes time, and if that time is too short there is no rectification. Here attention should be drawn to the conception that rectification takes place only because there are pores in the resin skin allowing point contacts to be made between the metal and the semiconductor. This idea is objected because with point-contact rectitiers there is no inertia at such a low frequency. It has not yet been possible to form a correct idea of the structure in the resin skin that would give rise to rectification. By probe testing it is proposed to ascertain something about the field conditions in the resin skin. Another phenomenon to which attention has to be paid is the difference in behaviour of the combination when pressing on an excess or a deficit semiconductor. There seems but little doubt that
776
ON
RECTIFIERS
a transfer of charge carriers from the semiconductor to the resin skin will have to be assumed. It is intended to carry out extensive tests with resin skin rectifiers. Thanks are due to Dr P. W. H a a ij m a n for having prepared the various semiconductors.
.Note. After the dressing of this article, the author was informed of an article, written in Russian by J o f f e, A. V., Journ. techn. Phys. U.S.S.R. 18 (1948) 1498 in which the rectification of the contact between excess and deficit semi-conductor has also been described. Eindhoven, September 1950. Received 17-4-51.
REFERENCES 1) S e h o t tky, W . , Z . P h y s . 1 1 3 (1939) 3 6 7 - 4 1 4 ; 1 1 8 (1942) 3 5 9 - 5 9 2 . 2) v a n G e e 1, W . C h., Z. P h y s . 6Y (1931) 765-78,5. 3) J u s 6 , W . , N a t u r e 1:12 (1933) 242; v a n G e e l , W . Ch., N a t u r e I::~:?.( 1 9 3 3 ) 7 1 1 ; Quintin, M., C. R. A c a d . Sci. P a r i s 1 9 8 (1934) 3 4 7 - 3 4 9 ; H a r t m a n n , W., P h y s . Z. I~7 (1936) 8 6 2 - 8 6 5 . 4) H e n i s c h, H . K., Metal-rectifiers, Clarendon Press Oxford 1949; M o t t, N. F. and G u r n e y, R. W . , Electronic processes in ionic crystals, Clarendon Press, Oxford (1946), p. 81 ; T e s z n e r, S., B u l l e t i n de la Soc. ft. des 61ectriciens .~ (1949) 401. 5) D a v y d o v, B., T e c h n . P h y s . U . S . S . R . 5 (1938) 87-9,5. 6) v a n G e e l, W . C h. a n d B o u m a, 13. C., P h i l i p s Res. R e p . 5 (1950) 4 6 1 - 4 7 5 . 7) D e k k e r , A.J. and van Geel, W. C11., Philips Res. R e p . ~ (1950) 3 0 3 - 3 1 9 . 8) B 6 n i n g, P a u 1, Electrische Isolierstoffe, Sammlung Vieweg, Braunschweig
(1938).