Low-Dark-Current Heterojunction Phototransistors with Long-Term Stable Passivation Induced by Neutralized (NH4)2S Treatment

To investigate the effects of surface leakage on the temperature-dependent dark and optical performance of a heterojunction phototransistor (HPT), three sets of HPTs were prepared. They were unpassivated (HPT A), passivated with diluted (NH4)2S (HPT B), and passivated with neutralized (NH4)2S (HPT C). Passivation treatment successfully suppresses surface-defect-induced effects. The passivated HPTs exhibit a reduced dark current and an enhanced signal-to-noise ratio (SNR). HPT A exhibits a room-temperature collector dark current (ICdark) of 0.59 nA at a VCE of 1 V, while those of HPTs B and C are 0.03 and 0.89 pA, respectively. The room-temperature SNR values for HPTs A, B, and C at a Pin of 8.3 (107.6) nW are 5.9 (42), 59 (91), and 91 (122) dB, respectively. After a three-week air exposure, the room-temperature SNR at a Pin of 107.6 nW decreases to 25 (67) dB for HPT A (B), while it is 116 dB for HPT C. The decrease for HPT C is only 6 dB. A long-term stable passivation with good thermal stability has been achieved by neutralized (NH4)2S treatment.