Optical comparator: A new application for avalanche phototransistors

We describe an InP/InGaAs heterojunction avalanche phototransistor which can be switched from a low-current "off" state to a high-current "on" state with an optical pulse. The transition is characterized by a region of negative dynamic resistance. In the switching mode where the effective current gain is determined by the circuit series resistance, gains exceeding 10⁵have been achieved. The switching rise time is an order of magnitude less (≃20 ns) than the rise time in the normal phototransistor mode. In addition, we show that this type of device can be used as an optical comparator which discriminates between optical pulses relative to a fixed threshold power level. The threshold level can be varied over three orders of magnitude by changing the bias voltage.