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Transistor structure relation to secondary breakdown and its effects
Microelectronics and Reliability Vol, 14, pp. 451 to 455. Pergamon Press, 1975, Printed in Great Britain
TRANSISTOR STRUCTURE RELATION TO SECONDARY BREAKDOWN AND ITS EFFECTS H. AHARONI Electr. Engng. Dept., University of the Negev, Beersheva, Israel and A. BAR-LEv Fac. of Electr. Engng., Technion-I.I.T., Haifa, Israel Abstract--The paper reviews transistor characteristics while operating in the secondary breakdown (S.B.) region. Series of experimental transistors of varying epitaxial collector structure were built and subjected to S.B. The effects both visible and electrical, were investigated, summarized and related to the structure and to the electrical fields and carrier distribution in various operating conditions.
1.
INTRODUCTION
One of the well known failure modes of power transistors is the secondary breakdown (S.B.) effect, after which the transistor is either completely destroyed or has suffered some irrepairable damage. It is the purpose of this paper to analize the type of damage found in a series of experimental transistors, of varying epitaxial collector structure, and relate it to this structure and to the changes in the electrical parameters of the devices. Since S.B. is initiated at some weak spot or crystal defect in the transistor, its effect will have a large random component. Nevertheless, by making a large number of measurements, it is possible to relate and classify the type of physical damage, the structure, the causes and the after effect. By so doing a better understanding of the whole S.B. effect is accomplished. The bipolar transistor characteristics in the common emitter avalanche and S.B. ranges are shown in Fig. 1. The S.B. effect is the transfer from region I to region II, i.e. from high voltage-high current range to low voltage-higher current range. S.B. is initiated at current marked Is8 whose value depends on the base current. Once S.B. has happened, a reduction of I c by external means may cause the transistor to recover some of its properties. In that case we are talking about a non-destructive and partially reversible effect after which the three transistor zones, emitter, base and collector, are still distinct. In other cases, a permanent short will have appeared between two or all three zones.
This will be called a destructive effect. Some damage is sustained in almost every case, manifested by a change in the characteristics and sometimes in the transistor appearance only. The various effects will be described here. 2. THE TRANSISTOR STRUCTURE
The transistors used were double diffused N P N silicon types, made in an epitaxial N-type layer grown on an N + substrate. The transistor cross section is shown in Fig. 2a and its dimensions are summarized in Table 1. Figure 2b is a photograph of the transistor, viewed from above.
Vso~
A ~
e
I|
aVceo
B~ao vce
Fig. 1. Transistor characteristics in the avalanche and S.B. Ranges.
Table 1. Experimental transistor--lateral dimensions Diameter/~m Collector-base junction diameter Dsc Emitter-base junction diameter --Dne
610 440
Junctions
Outer dia. of base contact hole ring--D~o(BC ) Inner dia. of base contact hole ring D2o(EB) Emitter contact hole dia. --Do(E )
580 476 216
SiO 2
--D1A~(B ) D2A~(B) --DAt(B )
610 426 364
Outer diameter of base A contact Inner diameter of base A contact Emitter At contact diameter
451
t
mask
A t contacts
452
H. AHARON!and A. BAR-LEV Table 2, Experimental transistors, resistivities and thickness (R& after predeposition: Rs~ after drive-in)
Sheet resistances f~/[Z]
Junction depth [#m]
Sub. parameters (Sb)
Layer par. (phos.)
Base
Rs,
Rs~
Emitter
Base
Emitter
Res. [flcm]
Width [~m]
Res. [~cm]
Width [#m]
77 68
130 140
6 7
3 3
2.3 + 0-2 2.3 ± 0-2
0.01 0.01
250 270
0-96 0.96
7 17.5
Table 2 gives the resistivities and junction depths. The resistivity and thickness of the epitaxial layer was varied in the ranges shown and series of each type were made. The transistors were manufactured by standard planar technology. The variations in collector thickness W~ and resistivity Pc enable us to investigate variable field conditions in the collector from which one can draw conclusion as to the S.B. built up mechanism.
approximation can he made to yield the field in the collector: E = Emax + q- Nnx Emax =
--
+
2qNoV
(N D +
p -
N A -
(1)
n)
where E is the field, e the semiconductor dielectric constant, N o, NA--donor and acceptor concentrations respectively, n, p--electrons and holes concentrations. Since the field is given by the potential gradient while n, p are related to the potential through Boltzman's relations, eqn. (1) can be written:
cO2V
63X2 --
q(N qV) ~ 1) -- N A -- 2hi sinh ~
I I
Wc2qNo V < -----2,F
(the non punch-through case). V is the voltage across the collector junction, IV,,. the distance from the junction to the substrate. Figure 3 shows the two cases schematically. When appreciable current is flowing, one still has to distinguish between two cases: The emitter junction is forward biased and large emitter current is being injected and collected. The injected current is composed of one type of carrier (electrone in our N P v N transistor case). This current, if high enough, causes base widening and resistivity modulation of the low doped v region. of the collector. In order to calculate the fields and carriers distributions the continuity and transport equations have to be considered simultaneously with Poisson's equation. For the one dimensional case they are: dn (5) Je = q#e nE + qD~ d~'
dp Jh = q#hPE -- qDh dx "~ 0,
(2)
J = J~ + Jh "~J~,
where n~ is the intrinsic carrier concentration. If no appreciable current flows (i.e. n ~ p ~ 0) and assuming low collector doping level compared to the base and substrate dopings which is the usual case in double diffused epitaxial structure, a step junction Base I C o l l e c t o r
for
g
3. ANALYSIS OF THE COLLECTOR FIELD
dE q -cOx
(4)
NoW ~
for V > (W~2qNn/2e) (depletion layer punches through the collector epitaxial layer) and Ema x --
The shape of the electrical field versus distance in the collector for a given structure depends both on the reverse potential V across the collector-base junction and on the current I crossing it. To get the field, Poisson's equation has to be solved with the charges given by the sum of the impurity atom's charge and the moving carriers charge. In order to clarify the situation we shall start with the case where no current flows and later include the current effects. Poisson's equation in the space charge area is:
(3)
1 dJ~
---+Gq dx ldJ~_ q dx
Substrate
Bose
I I
I I
Collector
n
-=0, z,, G + p- = 0, zh Substrate
I
-
E I
1
Ema x
-- _ i
NA>>No' ,
No
(a)
] Nolsubl>>N
o
N,--No',
No
No(sub)>>N D
(b)
Fig. 3. Field distribution across the collector. (a) Punch-through case, (b) Non punch-through case.
(6) (7) (8)
(9)
T ol r hi EpitoxiIoyer N+(Sb)
T
Substrole
,T (a)
Fig. 2. (a) Cross section of the experimental epitaxial transistor, (b) Top view of the transistor.
[Facint_t page 452]
Fig. 7. Photographs of A t contact damage in experimental transistors. (a) Punched-through case, (b) N o n punched-through case.
#
Fig. 8. The relations between IsB and I H in an experimental transistor. Triple exposure for I R = IBs (reversed), I~ = 0 and I~ = IsF (forward). Horiz. 10 V/div; vert. 10 mA/div.
Transistor Structure Relation to Secondary Breakdown and its Effects I
I
I
f
I
I
E
I
I
t ~ 2" = 200mA (J,., = 4)
>
1oo 1 •-~
2
.(-2_ ~
qO0 8
I
/v
P
D X
Fig. 6. Modification of the collector junction field by avalanche multiplication. No avalanche, - - - - With avalanche.
--
4
--
I0
~
4
<
L
3
6
I
II
xM~,
t
20
i
30
40
Position,
I
50
[
[
60
70
80
~rn
Fig. 4. Field distribution along an NPvN power transistor at medium and high current levels. where G is the generation rate. Neglecting the small hole current component across the junction as in eqn. (6), one can express E in terms of p. Furthermore, the diffusion term in eqn. (5) can be neglected compared to the drift term. Substituting n from eqn. (5) into Poisson's eqn. (1) with p < n one gets: dE
dx
J
-
q#~E
+ -q [N,(x) - NA(x)].
(10)
8
These equations were solved numerically by Schilling [3] for a typical power NPvN transistor at various current levels. Two of his results, for medium J and for high J, are shown schematically in Fig. 4 for the same
Vc~. In the high current case the high field zone moves to the farther end of the epitaxial layer. Due to Joule heating the generation term G increases exponentially with temperature. This usually starts at some crystal fault where field and current density are higher than the average. Because of the bad thermal conductivity of Si (K = 0.84 Joule/sec cm K with a temperature
coefficient of 0.84 × 293/T°K) the spot will become hotter and more electrically conductive than its surrounding area. Additional current will be channeled through it, increasing the power regeneratively, creating a hot spot and bringing the transistor into S.B. At this stage a certain instability may occur: The surrounding area temperature grows and some of the current is transferred to other weak spots in the junction. The available current for the original hot spot will diminish and it will start to cool. The conductivity-temperature curve of extrinsic Si is shown schematically in Fig. 5 and the hot spot which was first at point A, where conductivity is becoming generation controlled, will now move to point B where the conductivity is mobility controlled and again carry more current. Thus very low frequency light emitting oscillations may appear in the current carried by a hot spot. Sometimes a visible second spot can be seen near the original one and the characteristic shows a multiple voltage level breakdown. A different situation exists if the base is reversed biased. Only leakage current flows to the collector and the field are shown in Fig. 3 till the maximum field at the junction causes ionization and avalanche multiplication of both kinds of carriers. The fields under avalanche situation were calculated by Egawa [4] assuming equal ionization constants per holes and electrons which are strongly field dependent according to
°t=fl=aeb~ a = 2500 cm-1; J E u I
>~
453
(11)
b = 3-07 × 10-5 Vcm.
The continuity equations have then to be modified by inclusion of generation by ionization. A typical result of the collector field modification is shown in Fig. 6 for the punch-through case. Both types of carriers now exist, moving across the depletion layer in opposite directions, modifying the original field distribution. We shall refer to these results as we analyze the various S.B. effects observed.
O.I
>.
OOI
A 0.001
[~---1
t
[ 0.02
,
1 0-04
,
I/Temperafure,
I 006
,
I
0 08
,
*K
Fig. 5. Typical conductivity-temperature curve of extrinsic silicon.
4. THE
VISIBLE
S.B.
EFFECTS
In general the S.B. effects were different for the thin punch through case. In this section we shall review those effects which are visible through a microscope.
454
H. AHARON!and A. BAR-LEv
A. The Hot Spot
5. C H A N G E S
IN ELECTRICAL FOLLOWING
At the onset of avalanche light is emitted and can be seen from microplasmas located near the collector base junction periphery. When the current increases, a hot spot appears coincident with the voltage drop in the characteristics of Fig. 1, signifying S.B. The hot spot radiates in the red and infra-red, its intensity increasing with the current. Even with fixed current the light intensity oscillates somewhat. These very low frequency oscillations in light intensity can be explained by the analysis of hot spot formation in the last section. Since A, is seen to melt the temperature over the hot spot must be above the A r S i entectic (576°C). The spot size was around 35-40 #m diameter and somewhat varied with the current.
B. Changes in the Contacts
The intense heat of the hot spot usually melts the A~ from the emitter contact above it. With no base current the damage is concentrated around a small spot in the case of a punched through collector, but is much more extensive, covering most of the emitter, in the non punched-through case. As can be seen in the photographs of Fig. 7. Most of the A~ showed colour changes in this case. The damage increased relatively slowly. In the punched-through case the damage was about one tenth in area, developed much faster and did not increase with time. These differences can be explained by the fact that in Fig. 7b the high power zone extended much deeper !nto the collector layer (see Fig. 4), was further from the junction and the damage was sustained by heat transfer to all the rest of the transistor. In Fig. 7a case the high field region is limited to the narrow collector layer, very near the junction, which is immediately affected by it. Areas further away do not have time to get affected before the device is destroyed. The damage to the transistor is usually fatal here, while in the first non punchedthrough case the device is still operable (it should be noted that Si.melts at 1420°C while A~ is severely affected already at 600°C).
i
Base contact A Emitter contoct
"~
r///////////////////////J --
4:
PROPERTIES
S.B.
The current at which the transistor enters S.B. is marked lsn in Fig. 1. If the collector current is reduced after S.B. the transistor will go out of region II in Fig. 1 and back into the high voltage zone. The current at which this happens will be called I n . T h e relationship between lsB and I n was found to depend on the magnitude and direction of the base current before S.B. When in reverse direction, the larger I B is before S.B., the more must the collector current be reduced afterwards to get it out of S.B. (I n < Is8 ). When in forward direction, however, the larger 1B, the less must the collector current be reduced before the transistor comes out of S.B. (I n ~- lsB ). When I n = 0 before S.B. we always found lsB > I n. This can clearly be seen in Fig. 8. It is possible to explain the differences between the forward and reverse base current cases by relating them to the current distribution in each case. Figure 9a shows the current distribution under forward base current, where current crowding effect distributes it all around the emitter periphery and when a hot spot forms it carries only a small part of the total current. The spot has relatively small area and immediately I c is slightly reduced it will disappear and the transistor will come out of S.B. The current distribution under reversed base is shown in Fig. 9b and in this case the pinch-in effect predominates: All the collector current concentrates at the emitter center and once a hot spot forms it will carry the whole current. To get the transistor out of S.B. one then has to reduce I c to a much lower level (I n small) before the hot spot disappears. Hence I n < lsn in this case. The collector leakage currents always grow after S.B., even if it was not destructive. BVen o may remain essentially the same, but the reverse emitter characteristic will have become soft. BVce o is the most sensitive of the breakdown voltages and will usually become lower once the transistor underwent S.B. hvE grows with temperature and will therefore become higher near SB. The output resistance is reduced due
VIllA V / / / / / / / / / / / / / ~ f / / ½ / / U / / U / / I
t l
/IHill
l) I
P
n
/7+
(a)
Fig. 9. (a) Current distribution in a transistor with forward base current (crowding effect). (b) Transistor with reversed base current (pinch-in effect).
Transistor Structure Relation to Secondary Breakdown and its Effects to the increased collector leakage. It should be noted that non-destructive S.B. could be accomplished only if a pulsed supply was used, feeding a single half sine wave to the collector. A d.c. supply always results in destructive S.B. The transistor will suffer a higher damage by S.B. if the base carries a reverse current. This relates to the pinch-in effect that happens then [5], forcing the collector current to concentrate at the center of the device, as shown in Fig. 9. Local heating is then higher, thus increasing the damage. Once S.B. has occurred, it will usually happen the next time at a reduced IsB which keeps coming down when the device is allowed to breakdown repeatedly. If the same current is maintained, then S.B. will occur after a shorter application time. This results from the weak spot at which S.B. started, becoming weaker still because of the intense local heating there.
455
the collectors depend on whether the base was reverse of forward biased prior to S.B. The location of the high field point varies with the collector resistivity and with the base current. The further it is from the metalurgical base-collector junction the less will be the electrical damage. The device will then heat up as a whole and, upon removal of the power, will regain in many cases its electrical properties though not its "looks". In transistors with narrow epitaxial layers the high field region is forced to be very near to the junction and development of high heat there following S.B. will usually destroy the device, though visibly it is less affected. In general the damage can vary from complete destruction through marked changes in parameters and increased leakage currents to no noticeable electrical change, besides somewhat reduced life expectancy and a tendency to go into S.B. at somewhat lower ratings the next time it is subjected to high voltage and current.
6. CONCLUSIONS The effects of S.B., both visible and electrical, were related to the collector epitaxial layer thickness and resistivity. It was shown that in the case of a thick collector with medium or high resistivity the visible damage was extensive, while the electrical changes and damage were smaller than in the case of thin collector epitaxial layers. The fields and carrier distribution in
REFERENCES
1. H. A. Schafft, Proc. IEEE, 55, 8, 1272 (1967). 2. H. Aharoni, D.Sc. Thesis, Technion--Israel Institute of Technology (1972). 3. R. B. Schilling, RCA Rev., 32, 339 (1971). 4. H. Egawa, IEEE Trans. ED-13, 11,754 (1966). 5. H. G. Grutchfield and T. J. Moutoux, IEEE Trans. ED-13, 11,743 (1966).
TRANSISTOR STRUCTURE RELATION TO SECONDARY BREAKDOWN AND ITS EFFECTS H. AHARONI Electr. Engng. Dept., University of the Negev, Beersheva, Israel and A. BAR-LEv Fac. of Electr. Engng., Technion-I.I.T., Haifa, Israel Abstract--The paper reviews transistor characteristics while operating in the secondary breakdown (S.B.) region. Series of experimental transistors of varying epitaxial collector structure were built and subjected to S.B. The effects both visible and electrical, were investigated, summarized and related to the structure and to the electrical fields and carrier distribution in various operating conditions.
1.
INTRODUCTION
One of the well known failure modes of power transistors is the secondary breakdown (S.B.) effect, after which the transistor is either completely destroyed or has suffered some irrepairable damage. It is the purpose of this paper to analize the type of damage found in a series of experimental transistors, of varying epitaxial collector structure, and relate it to this structure and to the changes in the electrical parameters of the devices. Since S.B. is initiated at some weak spot or crystal defect in the transistor, its effect will have a large random component. Nevertheless, by making a large number of measurements, it is possible to relate and classify the type of physical damage, the structure, the causes and the after effect. By so doing a better understanding of the whole S.B. effect is accomplished. The bipolar transistor characteristics in the common emitter avalanche and S.B. ranges are shown in Fig. 1. The S.B. effect is the transfer from region I to region II, i.e. from high voltage-high current range to low voltage-higher current range. S.B. is initiated at current marked Is8 whose value depends on the base current. Once S.B. has happened, a reduction of I c by external means may cause the transistor to recover some of its properties. In that case we are talking about a non-destructive and partially reversible effect after which the three transistor zones, emitter, base and collector, are still distinct. In other cases, a permanent short will have appeared between two or all three zones.
This will be called a destructive effect. Some damage is sustained in almost every case, manifested by a change in the characteristics and sometimes in the transistor appearance only. The various effects will be described here. 2. THE TRANSISTOR STRUCTURE
The transistors used were double diffused N P N silicon types, made in an epitaxial N-type layer grown on an N + substrate. The transistor cross section is shown in Fig. 2a and its dimensions are summarized in Table 1. Figure 2b is a photograph of the transistor, viewed from above.
Vso~
A ~
e
I|
aVceo
B~ao vce
Fig. 1. Transistor characteristics in the avalanche and S.B. Ranges.
Table 1. Experimental transistor--lateral dimensions Diameter/~m Collector-base junction diameter Dsc Emitter-base junction diameter --Dne
610 440
Junctions
Outer dia. of base contact hole ring--D~o(BC ) Inner dia. of base contact hole ring D2o(EB) Emitter contact hole dia. --Do(E )
580 476 216
SiO 2
--D1A~(B ) D2A~(B) --DAt(B )
610 426 364
Outer diameter of base A contact Inner diameter of base A contact Emitter At contact diameter
451
t
mask
A t contacts
452
H. AHARON!and A. BAR-LEV Table 2, Experimental transistors, resistivities and thickness (R& after predeposition: Rs~ after drive-in)
Sheet resistances f~/[Z]
Junction depth [#m]
Sub. parameters (Sb)
Layer par. (phos.)
Base
Rs,
Rs~
Emitter
Base
Emitter
Res. [flcm]
Width [~m]
Res. [~cm]
Width [#m]
77 68
130 140
6 7
3 3
2.3 + 0-2 2.3 ± 0-2
0.01 0.01
250 270
0-96 0.96
7 17.5
Table 2 gives the resistivities and junction depths. The resistivity and thickness of the epitaxial layer was varied in the ranges shown and series of each type were made. The transistors were manufactured by standard planar technology. The variations in collector thickness W~ and resistivity Pc enable us to investigate variable field conditions in the collector from which one can draw conclusion as to the S.B. built up mechanism.
approximation can he made to yield the field in the collector: E = Emax + q- Nnx Emax =
--
+
2qNoV
(N D +
p -
N A -
(1)
n)
where E is the field, e the semiconductor dielectric constant, N o, NA--donor and acceptor concentrations respectively, n, p--electrons and holes concentrations. Since the field is given by the potential gradient while n, p are related to the potential through Boltzman's relations, eqn. (1) can be written:
cO2V
63X2 --
q(N qV) ~ 1) -- N A -- 2hi sinh ~
I I
Wc2qNo V < -----2,F
(the non punch-through case). V is the voltage across the collector junction, IV,,. the distance from the junction to the substrate. Figure 3 shows the two cases schematically. When appreciable current is flowing, one still has to distinguish between two cases: The emitter junction is forward biased and large emitter current is being injected and collected. The injected current is composed of one type of carrier (electrone in our N P v N transistor case). This current, if high enough, causes base widening and resistivity modulation of the low doped v region. of the collector. In order to calculate the fields and carriers distributions the continuity and transport equations have to be considered simultaneously with Poisson's equation. For the one dimensional case they are: dn (5) Je = q#e nE + qD~ d~'
dp Jh = q#hPE -- qDh dx "~ 0,
(2)
J = J~ + Jh "~J~,
where n~ is the intrinsic carrier concentration. If no appreciable current flows (i.e. n ~ p ~ 0) and assuming low collector doping level compared to the base and substrate dopings which is the usual case in double diffused epitaxial structure, a step junction Base I C o l l e c t o r
for
g
3. ANALYSIS OF THE COLLECTOR FIELD
dE q -cOx
(4)
NoW ~
for V > (W~2qNn/2e) (depletion layer punches through the collector epitaxial layer) and Ema x --
The shape of the electrical field versus distance in the collector for a given structure depends both on the reverse potential V across the collector-base junction and on the current I crossing it. To get the field, Poisson's equation has to be solved with the charges given by the sum of the impurity atom's charge and the moving carriers charge. In order to clarify the situation we shall start with the case where no current flows and later include the current effects. Poisson's equation in the space charge area is:
(3)
1 dJ~
---+Gq dx ldJ~_ q dx
Substrate
Bose
I I
I I
Collector
n
-=0, z,, G + p- = 0, zh Substrate
I
-
E I
1
Ema x
-- _ i
NA>>No' ,
No
(a)
] Nolsubl>>N
o
N,--No',
No
No(sub)>>N D
(b)
Fig. 3. Field distribution across the collector. (a) Punch-through case, (b) Non punch-through case.
(6) (7) (8)
(9)
T ol r hi EpitoxiIoyer N+(Sb)
T
Substrole
,T (a)
Fig. 2. (a) Cross section of the experimental epitaxial transistor, (b) Top view of the transistor.
[Facint_t page 452]
Fig. 7. Photographs of A t contact damage in experimental transistors. (a) Punched-through case, (b) N o n punched-through case.
#
Fig. 8. The relations between IsB and I H in an experimental transistor. Triple exposure for I R = IBs (reversed), I~ = 0 and I~ = IsF (forward). Horiz. 10 V/div; vert. 10 mA/div.
Transistor Structure Relation to Secondary Breakdown and its Effects I
I
I
f
I
I
E
I
I
t ~ 2" = 200mA (J,., = 4)
>
1oo 1 •-~
2
.(-2_ ~
qO0 8
I
/v
P
D X
Fig. 6. Modification of the collector junction field by avalanche multiplication. No avalanche, - - - - With avalanche.
--
4
--
I0
~
4
<
L
3
6
I
II
xM~,
t
20
i
30
40
Position,
I
50
[
[
60
70
80
~rn
Fig. 4. Field distribution along an NPvN power transistor at medium and high current levels. where G is the generation rate. Neglecting the small hole current component across the junction as in eqn. (6), one can express E in terms of p. Furthermore, the diffusion term in eqn. (5) can be neglected compared to the drift term. Substituting n from eqn. (5) into Poisson's eqn. (1) with p < n one gets: dE
dx
J
-
q#~E
+ -q [N,(x) - NA(x)].
(10)
8
These equations were solved numerically by Schilling [3] for a typical power NPvN transistor at various current levels. Two of his results, for medium J and for high J, are shown schematically in Fig. 4 for the same
Vc~. In the high current case the high field zone moves to the farther end of the epitaxial layer. Due to Joule heating the generation term G increases exponentially with temperature. This usually starts at some crystal fault where field and current density are higher than the average. Because of the bad thermal conductivity of Si (K = 0.84 Joule/sec cm K with a temperature
coefficient of 0.84 × 293/T°K) the spot will become hotter and more electrically conductive than its surrounding area. Additional current will be channeled through it, increasing the power regeneratively, creating a hot spot and bringing the transistor into S.B. At this stage a certain instability may occur: The surrounding area temperature grows and some of the current is transferred to other weak spots in the junction. The available current for the original hot spot will diminish and it will start to cool. The conductivity-temperature curve of extrinsic Si is shown schematically in Fig. 5 and the hot spot which was first at point A, where conductivity is becoming generation controlled, will now move to point B where the conductivity is mobility controlled and again carry more current. Thus very low frequency light emitting oscillations may appear in the current carried by a hot spot. Sometimes a visible second spot can be seen near the original one and the characteristic shows a multiple voltage level breakdown. A different situation exists if the base is reversed biased. Only leakage current flows to the collector and the field are shown in Fig. 3 till the maximum field at the junction causes ionization and avalanche multiplication of both kinds of carriers. The fields under avalanche situation were calculated by Egawa [4] assuming equal ionization constants per holes and electrons which are strongly field dependent according to
°t=fl=aeb~ a = 2500 cm-1; J E u I
>~
453
(11)
b = 3-07 × 10-5 Vcm.
The continuity equations have then to be modified by inclusion of generation by ionization. A typical result of the collector field modification is shown in Fig. 6 for the punch-through case. Both types of carriers now exist, moving across the depletion layer in opposite directions, modifying the original field distribution. We shall refer to these results as we analyze the various S.B. effects observed.
O.I
>.
OOI
A 0.001
[~---1
t
[ 0.02
,
1 0-04
,
I/Temperafure,
I 006
,
I
0 08
,
*K
Fig. 5. Typical conductivity-temperature curve of extrinsic silicon.
4. THE
VISIBLE
S.B.
EFFECTS
In general the S.B. effects were different for the thin punch through case. In this section we shall review those effects which are visible through a microscope.
454
H. AHARON!and A. BAR-LEv
A. The Hot Spot
5. C H A N G E S
IN ELECTRICAL FOLLOWING
At the onset of avalanche light is emitted and can be seen from microplasmas located near the collector base junction periphery. When the current increases, a hot spot appears coincident with the voltage drop in the characteristics of Fig. 1, signifying S.B. The hot spot radiates in the red and infra-red, its intensity increasing with the current. Even with fixed current the light intensity oscillates somewhat. These very low frequency oscillations in light intensity can be explained by the analysis of hot spot formation in the last section. Since A, is seen to melt the temperature over the hot spot must be above the A r S i entectic (576°C). The spot size was around 35-40 #m diameter and somewhat varied with the current.
B. Changes in the Contacts
The intense heat of the hot spot usually melts the A~ from the emitter contact above it. With no base current the damage is concentrated around a small spot in the case of a punched through collector, but is much more extensive, covering most of the emitter, in the non punched-through case. As can be seen in the photographs of Fig. 7. Most of the A~ showed colour changes in this case. The damage increased relatively slowly. In the punched-through case the damage was about one tenth in area, developed much faster and did not increase with time. These differences can be explained by the fact that in Fig. 7b the high power zone extended much deeper !nto the collector layer (see Fig. 4), was further from the junction and the damage was sustained by heat transfer to all the rest of the transistor. In Fig. 7a case the high field region is limited to the narrow collector layer, very near the junction, which is immediately affected by it. Areas further away do not have time to get affected before the device is destroyed. The damage to the transistor is usually fatal here, while in the first non punchedthrough case the device is still operable (it should be noted that Si.melts at 1420°C while A~ is severely affected already at 600°C).
i
Base contact A Emitter contoct
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r///////////////////////J --
4:
PROPERTIES
S.B.
The current at which the transistor enters S.B. is marked lsn in Fig. 1. If the collector current is reduced after S.B. the transistor will go out of region II in Fig. 1 and back into the high voltage zone. The current at which this happens will be called I n . T h e relationship between lsB and I n was found to depend on the magnitude and direction of the base current before S.B. When in reverse direction, the larger I B is before S.B., the more must the collector current be reduced afterwards to get it out of S.B. (I n < Is8 ). When in forward direction, however, the larger 1B, the less must the collector current be reduced before the transistor comes out of S.B. (I n ~- lsB ). When I n = 0 before S.B. we always found lsB > I n. This can clearly be seen in Fig. 8. It is possible to explain the differences between the forward and reverse base current cases by relating them to the current distribution in each case. Figure 9a shows the current distribution under forward base current, where current crowding effect distributes it all around the emitter periphery and when a hot spot forms it carries only a small part of the total current. The spot has relatively small area and immediately I c is slightly reduced it will disappear and the transistor will come out of S.B. The current distribution under reversed base is shown in Fig. 9b and in this case the pinch-in effect predominates: All the collector current concentrates at the emitter center and once a hot spot forms it will carry the whole current. To get the transistor out of S.B. one then has to reduce I c to a much lower level (I n small) before the hot spot disappears. Hence I n < lsn in this case. The collector leakage currents always grow after S.B., even if it was not destructive. BVen o may remain essentially the same, but the reverse emitter characteristic will have become soft. BVce o is the most sensitive of the breakdown voltages and will usually become lower once the transistor underwent S.B. hvE grows with temperature and will therefore become higher near SB. The output resistance is reduced due
VIllA V / / / / / / / / / / / / / ~ f / / ½ / / U / / U / / I
t l
/IHill
l) I
P
n
/7+
(a)
Fig. 9. (a) Current distribution in a transistor with forward base current (crowding effect). (b) Transistor with reversed base current (pinch-in effect).
Transistor Structure Relation to Secondary Breakdown and its Effects to the increased collector leakage. It should be noted that non-destructive S.B. could be accomplished only if a pulsed supply was used, feeding a single half sine wave to the collector. A d.c. supply always results in destructive S.B. The transistor will suffer a higher damage by S.B. if the base carries a reverse current. This relates to the pinch-in effect that happens then [5], forcing the collector current to concentrate at the center of the device, as shown in Fig. 9. Local heating is then higher, thus increasing the damage. Once S.B. has occurred, it will usually happen the next time at a reduced IsB which keeps coming down when the device is allowed to breakdown repeatedly. If the same current is maintained, then S.B. will occur after a shorter application time. This results from the weak spot at which S.B. started, becoming weaker still because of the intense local heating there.
455
the collectors depend on whether the base was reverse of forward biased prior to S.B. The location of the high field point varies with the collector resistivity and with the base current. The further it is from the metalurgical base-collector junction the less will be the electrical damage. The device will then heat up as a whole and, upon removal of the power, will regain in many cases its electrical properties though not its "looks". In transistors with narrow epitaxial layers the high field region is forced to be very near to the junction and development of high heat there following S.B. will usually destroy the device, though visibly it is less affected. In general the damage can vary from complete destruction through marked changes in parameters and increased leakage currents to no noticeable electrical change, besides somewhat reduced life expectancy and a tendency to go into S.B. at somewhat lower ratings the next time it is subjected to high voltage and current.
6. CONCLUSIONS The effects of S.B., both visible and electrical, were related to the collector epitaxial layer thickness and resistivity. It was shown that in the case of a thick collector with medium or high resistivity the visible damage was extensive, while the electrical changes and damage were smaller than in the case of thin collector epitaxial layers. The fields and carrier distribution in
REFERENCES
1. H. A. Schafft, Proc. IEEE, 55, 8, 1272 (1967). 2. H. Aharoni, D.Sc. Thesis, Technion--Israel Institute of Technology (1972). 3. R. B. Schilling, RCA Rev., 32, 339 (1971). 4. H. Egawa, IEEE Trans. ED-13, 11,754 (1966). 5. H. G. Grutchfield and T. J. Moutoux, IEEE Trans. ED-13, 11,743 (1966).