Phototransistors, APD-FET, and PINFET optical receivers for 1-1.6-µm wavelength

The PINFET hybrid is the most sensitive receiver currently available for long-wavelength direct detection digital optical systems operating below 1 Gbit/s. State-of-the-art values for input capacitance and current gain in heterojunction phototransistors (HPT´s) may yield better sensitivity at bit-rates above 1 Gbit/s. At lower bit-rates, a reduced capacitance and/or higher gain would be needed, together with a low base-collector leakage current, a low base-emitter defect current, and probably the use of a bounded disparity transmission code. Avalanche photodiodes (APD´s) in principle offer scope for much improvement, but will require a low level of shot noise on the bulk leakage current and either a low excess noise factor for the photocurrent avalanche or a low level of gain-independent pre-amplifier noise. Comprehensive estimates of receiver sensitivity for APD´s in systems operating at 100 Mbit/s and 1 Gbit/s indicate the relative importance of the various noise sources for the case where the excess noise factors for the photocurrent and leakage current avalanches are equal. However, the PINFET hybrid is still likely to compete with state-of-the-art APD´s for bit-rates up to several hundred Mbit/s.