Low capacitance, high speed phototransistors with a large absorption region

Increasing the sensitivity of optical receivers is of paramount importance to reduce the energy cost of optical communications. For this, the signal to noise ratio (SNR) of the front-end detector and amplifier must be improved. A very efficient way of achieving this is to introduce gain right at the detection level. Avalanche photo detectors (APDs) are a common approach for this, but bipolar phototransistors (BPTs) also offer a potential solution to the problem, without the need for high voltage or the detriment brought in by the inherent excess noise in avalanche mechanisms. Nevertheless BPTs have never reached very high speeds [2] or low enough capacitance to truly compete with APDs [3]. Here we propose a new way of making BPTs, which greatly enhances the speed of the phototransistor, while enabling it to keep a large absorption volume and low capacitance.