GATE EC 2020 Set 1 — Question 43
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Electronic Devices → BJT → Current Gain Parameters
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Question
The base of an BJT T1 has a linear doping profile as shown below. The base of another BJT T2 has a uniform doping of . All other parameters are identical for both the devices. Assuming that the hole density profile is the same as that of doping, the common-emitter current gain of T2 is
- A.approximately 2.0 times that of T1.
- B.approximately 0.3 times that of T1.
- C.approximately 2.5 times that of T1.
- D.approximately 0.7 times that of T1.
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Correct answer
(MTA)
Solution
The common-emitter current gain is related to the base transport factor and emitter efficiency. A key factor is the total base charge (Gummel number), . For T2 (uniform doping):
For T1 (linear doping profile from to ):
If we assume (assuming base recombination dominates and is constant), then:
However, in a graded base (T1), an internal electric field is created which aids minority carrier transport, increasing . The collector current is proportional to . For a linear profile, this leads to a significantly higher compared to a uniform base with the same peak doping. According to the official GATE 2020 answer key, this question is marked as MTA (Marks To All), indicating that none of the provided options are technically correct or the question is ambiguous.
For T1 (linear doping profile from to ):
If we assume (assuming base recombination dominates and is constant), then:
However, in a graded base (T1), an internal electric field is created which aids minority carrier transport, increasing . The collector current is proportional to . For a linear profile, this leads to a significantly higher compared to a uniform base with the same peak doping. According to the official GATE 2020 answer key, this question is marked as MTA (Marks To All), indicating that none of the provided options are technically correct or the question is ambiguous.
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