Current amplification in nonhomogeneous-base structure and static induction transistor structure

Current-amplification mechanisms for a nonhomogeneous-base bipolar phototransistor (BPT) and a static induction phototransistor (SIPT) are discussed in this paper, and comparisons are made to the homogeneous- and or the gradually-graded-base BPT. It is shown that the current-transport mechanism of the BPT with a nonhomogeneously doped ⋅ ⋅ ⋅p+pp+p ⋅ ⋅ ⋅(⋅ ⋅ ⋅ n+nn+n ⋅ ⋅ ⋅) base is similar to that of the SIPT if the p (n) base potential is capacitively controllable both by the p+ (n+) base and collector voltages. For the n-channel SIPT in the open-gate mode of operation, the potential-barrier height for holes stored in the p+-gate region is higher than the potential-barrier height for electrons in the source region by the value of VbiGS−VbiG*S, where the VbiGS is the built-in potential between the p+-gate region and the n+-source region of the SIPT and VbiG*S is the potential-barrier height between the intrinsic gate G* in the channel and the n+-source region. In fact, a very high dc optical gain G of more than 108 (at 3×10−15 W incident optical power level, λ=6550 Å) has been obtained. An average gain-bandwidth product of more than 108 Hz has been obtained at subpicowatt incident power levels. The steady-state and transient optical response of the SIPT with an open gate is studied experimentally and theoretically. It is concluded that if the base region of a homogeneous-base BPT is replaced by a nonhomogeneous-base structure and or SIPT structure, the sensitivity and response time will be much improved.