Reset Operation of Quantum-Well Infrared Phototransistors

An improved operation of charge-sensitive infrared (IR) phototransistor (CSIP) is demonstrated by adding a reset function. The phototransistor is fabricated in a GaAs/AlGaAs double quantum-well (QW) structure. The upper QW is lithographically defined to form an isolated island. Under IR illumination (lambda ~ 14 mum), excited electrons escape from the isolated QW island. The QW island is thereby positively charged up, which, in turn, causes the conductance through the lower QW to increase. The performance of the detector, however, was restricted by the reduction of sensitivity, which arises from a distortion in the electrostatic potential profile caused by the accumulation of positive charges on the upper QW island. This drawback is removed by introducing a front gate on a channel leading to the isolated QW island. Applying positive pulses (duration 1 mus) to the gate, neutralizes the isolated QW island and reset the CSIP to the highly sensitive state. Typically, a current responsivity on the order of ~104 A/W is realized along with a dynamical range as large as > 109.