Investigation of resistor and capacitor elements in a germanium solid circuit

Solid-State

Electronics

Pergamon

Press 1962. Vol. 5, pp. 49-53.

Printed in Great Britain

INVESTIGATION OF RESISTOR ELEMENTS IN A GERMANIUM YASUO

TARUI

Electrotechnical (Received 27 July 1961;

AND CAPACITOR SOLID CIRCUIT

and SEIICHI DENDA Laboratory,

Tokyo,

Japan

in revisedform 6 October 1961)

Abstract-New possibilities are investigated for producing resistor and capacitor elements by using germanium. p-n-junctions, made by alloying with materials containing tin, are used as resistors under reverse-biased conditions. The V-I characteristic of the junction is almost linear except for the very low-voltage region, and the values of the resistance can be controlled by the junction area and the tin concentration in the alloying material. Capacitor element is developed by evaporating zinc sulphide and aluminium on germanium, successively. A free-running multivibrator is constructed by using these techniques, and is described briefly as an example. R&umB-De nouvelles possibilites sont considertes pour produire des elements de capacite et Des jonctions p-n faites par alliage de materiaux contenant resistance en employant le germanium. de l’etain sont employees comme resistances avec des conditions de polarisation inverses. La caracteristique V-Z de la jonction est presque entierement lintaire, a l’exception de la region de tensions tres basses, et les valeurs de la resistance peuvent &tre controlees par la surface de jonction et la concentration d’etain dans le materiau allie. Un element de capacite est developpe en evaporant du sulfure de zinc et de l’aluminium successivement sur le germanium. Un multivibrateur a fonctionnement libre est construit en utilisant ces techniques et on le decrit brievement comme exemple. Zusammenfassung-Fiir die Herstellung von Widerstlnden und Kapazitaten unter Verwendung von Germanium ergeben sich neue Moglichkeiten. p+z-ubergange, die man durch Ledierung mit Zinn-enthaltenden Stoffen erhllt, werden als Widerstlnde benutzt, wobei die Vorspannung umgekehrt wird. Die F-I-Kennlinie des Ubergangs ist fast linear, ausser im Gebiet sehr niedriger Spannung, und der Wert des Widerstands lasst sich durch die Fllche des tfbergangs und den Zinngehalt des legierten Materials kontrollieren. Kapazitlten werden durch sukzessives Aufdampfen von Zinksulfid und Aluminium auf Germanium hergestellt. Es folgt eine kurze Beschreibung eines mit diesen Verfahren hergestellten frei laufenden Multivibrators.

1. INTRODUCTION AN

tained by a bulk resistance or a surface-diffusedlayer resistance, but high-resistance elements are difficult to achieve. The reverse-biased p-n barriers can possibly be used as resistor elements. However their resistance is usually very high. It has been found that the resistance can be controllably reduced. p-n-junctions, made by alloying Sn-Pb-Sb to p-type germanium, have been developed which give resistance values down to 10 kQ or less. Capacitor elements made by evaporating zinc

trend in modern electronics is toward decreased size and increased reliability of electronic systems. In recent years, a new solid circuits, integrated category of circuits, circuits, etc., have been solving this problem by replacing every circuit component with semiconductors and by saving connexions by integrating every component in a semiconductor block. In these techniques, resistor elements are obD

IMPORTANT

49

50

YASUO

TARUI

an d SEIICHI

sulphide film on germanium have been developed. Values of capacitance up to 12,000 pF/cms have been obtained. 2. ALLOYED

RESISTORS

Several methods are already known(l) for making resistor elements with semiconductors, e.g. the bulk resistor, or the surface-diffused-layer resistor, made by the masking-diffusion technique or photo-engraving. The latter was tried for germanium. It was

DENDA

though it is well known as an ohmic material to n-type germanium. In fact, it acts as a neutral element in germanium or silicon, with its four valence electrons. If it is completely neutral,(*j an alloy of tin with other donor elements should be a good junction material for p-type germanium, as in the case of lead. Alloying materials involving tin give a rectification characteristic with a large leakage current in the backward direction, as shown in Fig. 1. In these experiments, an alloying material

FIG. 1. Characteristics of voltage vs. current and voltage vs. photo-response of a tin-doped resistor element.

found, however, that this kind of resistor is impractical except for a considerably low value of resistance, because of the large leakage conductance between the parent material and the diffused layer. The reverse-biased p-n-junction has a high resistance. If this resistance can be reduced controllably, the results are quite promising. Generally, alloying agents for making p-n junctions to one type of germanium serve as an ohmic contact to the opposite type. However, experiments have shown that tin does not make a good rectifying junction for P-type germanium

containing lead, tin and antimony was used for p-type germanium. In this case lead serves purely as a carrier metal. Various compositions of the alloy were investigated and a typical mixture used was 62:31:7::Pb:Sn:Sb. Fig. 2 shows the values of resistance obtained versus tin content in the alloy. Here the values of resistance are defined as the gradient of the V-1 characteristic at 4 V of reverse bias. The value of resistance varies proportionally to the junction area. The relationship is shown in Fig. 3. Chemical etching changes the resistance

RESISTOR

lOOk/

AND

0 Weight

CAPACITOR

ELEMENTS

I

I

50

100

% of Sn in metal

’

FIG. 2. Resistance vs. tin content.

value only by reducing the area of the junctions. This shows that the current passes through the whole area of the junction, but not the surface layer through which the flow of the usual leakage component takes place. The value of resistance is controlled by the cooling process. A furnace having a very small

I/S,

IN A GERMANIUM

CIRCUIT

51

thermal time constant (N O-5 set) was used for this purpose. Rapid cooling with this heater gives a value of resistance about one decade lower than that of Fig. 2 in spite of equal tin content and area. Resistors smaller than 50 kQ were made by this method. As for the mechanism of the anomalous leakage current of the resistor, some study has been carried out. Photo-response has been measured by illumination with 400 c/s chopped light at various applied voltages. The results are given in Fig. 1, showing rather flat response. This means that the excess current is not the result of multiplication at the p-n barrier. The temperature dependence of the linear conductance part is given in Fig. 4, showing that this current is neither space-charge-layer generation current nor the usual surface leakage current, since these should have large temperature dependence. There is the possibility that the effects are due to the spreading resistance of small ohmic spots at the barrier. The voltage distribution of the back

1000/T,

mm-’

FIG. 3. Resistance vs. junction area.

SOLID

FIG. 4. Temperature

“K

dependence of azresistor element.

52

YASUO

TARUI

and

surface of reverse-biased, thinly lapped p-n junctions was also measured, but no clear spot was found: so, even if this is the case, the ohmic spots should be distributed uniformly. The possibilities of precipitation(s) of a kind of metal in the junction which is known to have a softening effect on the V-I characteristic are also conceivable and are now under investigation. Though the mechanism is not clearly analysed yet, this resistor is of practical importance because it is easy to make and has a superior space factor compared with other types of resistance in solid circuits. 3. ZnS

CAPACITORS

In the case of germanium, the reverse-biased p-n-junction capacitor is not practical because of the leakage current. Consequently evaporation of dielectric material was adopted. ZnS was used

SEIICHI

DENDA

since it evaporates at comparatively low temperature and has a good adhesion to germanium. The germanium substrate was heated to 300 “C for good adhesion to ZnS. After evaporation, the sample was gradually cooled. Aluminium was evaporated as the opposite electrode. By controlling the thickness of the ZnS layer, capacitance up to 12,000 pF/cms was obtained with good reproducibility. The thickness of the ZnS layer’was measured by grinding the capacitance surface at an angle of 5” and observing under a microscope. A dielectric constant of 6.5 _t 0.4 was obtained from the capacitance value and thickness and area of the electrode. Figure 5 shows the leakage-current characteristics of a typical unit. It shows that the leakage current below 10 V is less than lo-10 A. Some units showed interesting leakage characteristics though they were not good as capacitors. An example of these units is shown in Fig. 6. With a

10

FIG.

5.

Leakage-current characteristics capacitor element.

of

a typical

FIG. 6. Ixakage-current

characteristics CIICC.

of Y2 depend-

RESISTOR

AND CAPACITOR

ELEMENTS

IN A GERMANIUM

SOLID

CIRCUIT

positive polarity of Ge+ the leakage current follows the relation of Va dependence. At negative polarity, the leakage current saturates as the voltage increases. A current having a V dependence seems to be a space-charge-limited current.@) Detailed discussion, however, will be given elsewhere. 4. FABRICATION OF A SOLID CIRCUIT As an example, a free running multivibrator was constructed using these techniques. The circuit is shown in Fig. 7. The transistors are of

20

&

ColleL~ terminal Lead

FIG. 8.

View of the solid circuit.

(6) Making internal connexions. A cast unit is shown in Fig. 8 (with some dimensions given) and Fig. 9 is a photograph of the unit.

lOOh

out

V

5. CONCLUSIONS

t:

FIG.

7. Diagram of the solid circuit.

the alloy-diffused type. The is adopted for construction.

following

procedure

(1) Diffusing of antimony to p-type germanium to form the n-type surface layer about 10 p deep. (2) Alloying of the base electrode of the transistor. (3) Alloy-diffusing of the emitter electrode close to the base electrode at 650 “C, 30 min. At the same time the ohmic contact (indium, serving as collector electrode) is alloyed and mesa-shaped. (4) Evaporating of ZnS film on the backsurface of the transistor face, and successive evaporation of aluminium. (5) Alloying of the resistance dots. TWO of them are 100 kQ heated at 7.50 “C, two others are 10 kQ heated at about 600 “C and cooled rapidly.

The reverse-biased alloyed +-junctions including tin as an alloy material showed large leakage conductance. The mechanism of conductance of this resistor was investigated; the temperature dependence, photoconductivity and potential distribution were measured. The precise however, is not yet known. mechanism, Tentatively, ohmic spots and their spreading resistances will most likely explain the observed phenomena. Evaporated ZnS and aluminium layers constitute the capacitance elements in the solid circuit. Further investigation on this leakage current and the mechanism of conductance of the resistor elements are now in progress

authors wish to acknowledge the guidance of Drs. H. WADA and M. KIKUCHI and the co-operation of Miss R. INOUE and Mr. N. NARUKAMI

Acknowledgements-The

in this program.

REFERENCES 1. J.

W.

LATHROP, R.

E.

LEE and C.

33, 20, 69 (1960). TRUMBORE, J. Electrochem.

H.

PHIPPS,

Electronics, 2. F. A.

Sot.

103,

593

(1956). 3. A. GOETZBERGER,J, Appl. Phys. 31, 1821 (1960). 4. N. F. MOTT, Electronic Porcesses in Ionic Crystals. Oxford University Press (1940).