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Explanation of ultra high current gain in silicon transistors
INCITES
Explanation of ultra high current gain in silicon transistors (Received
8 April;
in revised form
3 June
1963)
COMMON-EMITTER small-signal current gains B up to at least 10,000 have been reported(r) on certain grown-diffused silicon n-p-n transistors, notably samples of 2N335, 2N336 and 2N338. The characteristic feature of this phenomenon is the way in which gain increases as the current level is reduced, in distinction from the normal fall-off at low currents. We have observed ultra-high gain (u.h.g.) in double-diffused silicon planar n-9-n transistors fabricated under such conditions that n-type inversion channels were induced under the thermally-grown silicon dioxide on the surface of the base region, and have shown that in these devices a unipolar field effect in the channel is responsible. A description in terms of transconductance is therefore more appropriate. The grown-diffused devices show similar behaviour, and because, as discussed below, u.h.g. cannot be explained in terms of minority-carrier transistor action, it is concluded that here also it is due to field effects in an inversion channel. MAY@) concluded that inversion channels were responsible for u.h.g. by forming a barrier against surface recombination of minority carriers. Although such a barrier effect is possible, in order to explain occasional values of gain of 10,000 in structures which typically have gain of less than 10 at collector currents Ic of 10-s A it is necessary to assume that the surface recombination component of base current exceeds the sum of the other components by at least three orders of magnitude. IWERSON et d.(3) found that the surface component was dominant at low currents, but the remaining components were within an order of magnitude of the surface contribution. It has also been suggested(l94) that an efficient built-in drift-field could reduce transit time and thus account for u.h.g. This must be rejected because the theoretical limit to this shortening of transit time is about 12.(5) As May reported the u.h.g. extended into inverse values of 1~ and this provides an important clue. We consider that the action is unipolar and that normal bipolar transistor action does not play a significant part until higher levels of 1,
545
are reached. The emitter, collector, inversion channel and base form the source, drain, channel and gate of a field-effect transistor (Fig. 1).
FIG. 1 The pinch-off current through the channel between emitter and collector is thus modulated by the potential between channel and base, and high current gain results because the emitter junction impedance is very high at low forward bias, provided that the channel terminates short of the base contact. Since I, consists almost entirely of channel current, there is virtually no minority carrier flow. This explains the high emitter junction impedance. Frequently the channel extends completely to the base contact and in that case u.h.g, is not observed. Experimentally one looks for devices having a moderate channel pinch-off current (say 10-s A) between collector and emitter, but a channel-free collectorbase current (< 10-S A). Figure 2 illustrates the similarity between transfer characteristics of some planar n-p-n’s known to have n-type channels (curves A, B, and C) and two selected 2N335 grown-diffused units (curves D and E). Exact correspondence could be expected only between devices having channels of the same thickness, electron distribution and surface geometry. It is significant that the 1, vs. I’BE characteristics have a slope much less than the “ideal” value of q/KT which is found in normal transistors. (We are indebted to a reviewer for pointing this out.) These conclusions were supported by measurements on a mixed array of stripe and ring-base geometries fabricated under channel-forming conditions. Only the stripe geometries in which base channels could extend uninterrupted between emitter and collector showed u.h.g. U.H.G. was also observed when emitters and collectors were interchanged, although gain at normal currents was then greatly reduced.
NOTES 4
r
channel, which become swamped by normal bipolar action at higher currents. The essential features are a channel which is continuous between emitter and collector (thus excluding ring-base patterns), but which is suppressed on the area immediately surrounding the base contact.
S. I>. ROSENBA& ‘4. LORO Research and Development Laboratories, Northern Electric Company, Ottawa, Ontario, Canada
References
’ -0.6
I -0.4
I
-0.2
VBE
I
0
ro.2
+0.4
VOLTS
FIG. 2
In conclusion, the existence of ultra high current gain is readily explained in terms of unipolar field effects in a surface inversion
1. R. A. I)ANDL and F. T. Xlxl-, Her,. Sci. I~zstrum. 31, 575 (1960). 2. 1:. ‘I’. MAY, Oak Ridge National Laboratory report ORNL-3098UC-37 (Instruments,) August 1961. 3. J. E. IWERSEN, A. R. BR.%Y and J. J. KLEIMACK, Trans. Inst. Radio Engrs. ED-9, 474 (1962). Documentary Report 4. J. HANLON, Technical MRL-TDR-62-3 (April 1962) of the Biomedical Laboratory, 6570th Aerospace Medical Research I,abs Wright-Patterson Air Force Base, Ohio. 5. J. L. MOLL and I. M. ROSS, Pror. Inst. Radio Engrs. 44, 72 (1956).
Explanation of ultra high current gain in silicon transistors (Received
8 April;
in revised form
3 June
1963)
COMMON-EMITTER small-signal current gains B up to at least 10,000 have been reported(r) on certain grown-diffused silicon n-p-n transistors, notably samples of 2N335, 2N336 and 2N338. The characteristic feature of this phenomenon is the way in which gain increases as the current level is reduced, in distinction from the normal fall-off at low currents. We have observed ultra-high gain (u.h.g.) in double-diffused silicon planar n-9-n transistors fabricated under such conditions that n-type inversion channels were induced under the thermally-grown silicon dioxide on the surface of the base region, and have shown that in these devices a unipolar field effect in the channel is responsible. A description in terms of transconductance is therefore more appropriate. The grown-diffused devices show similar behaviour, and because, as discussed below, u.h.g. cannot be explained in terms of minority-carrier transistor action, it is concluded that here also it is due to field effects in an inversion channel. MAY@) concluded that inversion channels were responsible for u.h.g. by forming a barrier against surface recombination of minority carriers. Although such a barrier effect is possible, in order to explain occasional values of gain of 10,000 in structures which typically have gain of less than 10 at collector currents Ic of 10-s A it is necessary to assume that the surface recombination component of base current exceeds the sum of the other components by at least three orders of magnitude. IWERSON et d.(3) found that the surface component was dominant at low currents, but the remaining components were within an order of magnitude of the surface contribution. It has also been suggested(l94) that an efficient built-in drift-field could reduce transit time and thus account for u.h.g. This must be rejected because the theoretical limit to this shortening of transit time is about 12.(5) As May reported the u.h.g. extended into inverse values of 1~ and this provides an important clue. We consider that the action is unipolar and that normal bipolar transistor action does not play a significant part until higher levels of 1,
545
are reached. The emitter, collector, inversion channel and base form the source, drain, channel and gate of a field-effect transistor (Fig. 1).
FIG. 1 The pinch-off current through the channel between emitter and collector is thus modulated by the potential between channel and base, and high current gain results because the emitter junction impedance is very high at low forward bias, provided that the channel terminates short of the base contact. Since I, consists almost entirely of channel current, there is virtually no minority carrier flow. This explains the high emitter junction impedance. Frequently the channel extends completely to the base contact and in that case u.h.g, is not observed. Experimentally one looks for devices having a moderate channel pinch-off current (say 10-s A) between collector and emitter, but a channel-free collectorbase current (< 10-S A). Figure 2 illustrates the similarity between transfer characteristics of some planar n-p-n’s known to have n-type channels (curves A, B, and C) and two selected 2N335 grown-diffused units (curves D and E). Exact correspondence could be expected only between devices having channels of the same thickness, electron distribution and surface geometry. It is significant that the 1, vs. I’BE characteristics have a slope much less than the “ideal” value of q/KT which is found in normal transistors. (We are indebted to a reviewer for pointing this out.) These conclusions were supported by measurements on a mixed array of stripe and ring-base geometries fabricated under channel-forming conditions. Only the stripe geometries in which base channels could extend uninterrupted between emitter and collector showed u.h.g. U.H.G. was also observed when emitters and collectors were interchanged, although gain at normal currents was then greatly reduced.
NOTES 4
r
channel, which become swamped by normal bipolar action at higher currents. The essential features are a channel which is continuous between emitter and collector (thus excluding ring-base patterns), but which is suppressed on the area immediately surrounding the base contact.
S. I>. ROSENBA& ‘4. LORO Research and Development Laboratories, Northern Electric Company, Ottawa, Ontario, Canada
References
’ -0.6
I -0.4
I
-0.2
VBE
I
0
ro.2
+0.4
VOLTS
FIG. 2
In conclusion, the existence of ultra high current gain is readily explained in terms of unipolar field effects in a surface inversion
1. R. A. I)ANDL and F. T. Xlxl-, Her,. Sci. I~zstrum. 31, 575 (1960). 2. 1:. ‘I’. MAY, Oak Ridge National Laboratory report ORNL-3098UC-37 (Instruments,) August 1961. 3. J. E. IWERSEN, A. R. BR.%Y and J. J. KLEIMACK, Trans. Inst. Radio Engrs. ED-9, 474 (1962). Documentary Report 4. J. HANLON, Technical MRL-TDR-62-3 (April 1962) of the Biomedical Laboratory, 6570th Aerospace Medical Research I,abs Wright-Patterson Air Force Base, Ohio. 5. J. L. MOLL and I. M. ROSS, Pror. Inst. Radio Engrs. 44, 72 (1956).