Effects of optical absorption on the quasi-Fermi level splitting at the emitter-base junction in npn heterojunction bipolar phototransistors

This paper analyzes the effects of optical absorption in the quasi-neutral base region of npn heterojunction bipolar phototransistors (HPT's) on the extent of electron quasi-Fermi level splitting at the emitter-base heterojunction interface. The principle of current balance is used to match the thermionic-field-emission from the emitter with the base electron diffusion current, taking into account the nonuniform optical generation in the base. The increase in minority carrier concentration produced by optical generation in the base leads to a decrease in the quasi-Fermi level splitting at the heterojunction interface which reduces the net electron injection into the base, the emitter injection efficiency and current gain to an extent that depends on the incident optical power and the base-emitter bias.