InP/InGaAs heterojunction bipolar phototransistors with improved sensitivity

The fabrication and device characteristics of back-illuminated InP/InGaAs n-p-n heterojunction phototransistors will be described. These devices consist of a "wide-bandgap" InP emitter with smaller bandgap InGaAs base and collector layers. Uniform spectral response is observed in the wavelength range from 0.95 µm to 1.6 µm. The DC optical gain increases from approximately 40 at an input power of 1 nW to over 1000 for an input power level of 5 µW. The small-signal gain is characteristically 2 to 3 times higher than the EC gain. The cut-off frequency f T is an increasing function of the incident light level; for 1 µW of incident power f T ≃ 300 MHz. The possible applications of these phototransistors in fiber optic systems will be discussed.