Phototransistors in digital optical communication systems

Analytic solutions for the sensitivity of long-wavelength (1-1.6 \mum) optical receivers using heterojunction phototransistors (HPT's) require that the Shockley base-emitter resistance reshould not be modulated by the optical signal. This implies that the ratio α of the dc bias and signal components of the collector current must be much greater than unity. In this paper, the sensitivity of such receivers is expressed explicitly in terms of α to indicate the range of values over which the derivation is valid. "State-of-the-art" HPT's are expected to give better results than the present-day pin-FET hybrid only at bit rates >1 Gbit/s. Reducing the input capacitance and increasing the current gain hFEof the HPT would give improved performance; but if the requirement\alpha > 1is to be satisfied, the load resistance must eventually be restricted to low values, giving sensitivity still comparable to the pin-FET hybrid up to several hundred megabaud. For\alpha < 1, an integrating receiver design may be operable, although a bounded disparity transmission code may be needed to avoid misequalization for frequencies below the 3 dB frequency of the HPT stage. The HPT for such a receiver would require a high current gain hFEand a low input capacitance C2satisfyingC_{2} < 2 \times 10^{-3} \sqrt{h_{FE}}pF, and very low base-collector leakage and base-emitter defect currents.