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The influence of gate metal with thermal annealed process adopt to control the distance between gate and channel on pseudomorphic Al0.24Ga0.76As/ In0.22Ga0.78As double heterojunction high electron mobility transistors (DH-HEMTs) were studied. Compared to device with gate-recess process, the distance of gate-to-channel could be controlled through the thermal annealed process and therefore exhibit a lower series resistance. Measured transconductance of 150 mS/mm and an open- drain voltage gain of 136 for the DH-HEMT with an as deposited gate are enhanced to 175 mS/mm and 160 for the DH-HEMT with a 330-{degree sign}C annealed gate. Good device linearity is also obtained with a low second-harmonic to fundamental ratio of 3.55 %. In addition, good microwave performances such as unit-current gain- and maximum power gain- frequency were also obtained from devices with gate-annealed process.

N-p-n InGaP/GaAs Dual-Emitter heterojunction phototransistor (DEPT) with/without InGaP-passivation layer have been fabricated to investigate the influence of surface leakage on the device optical performance. The comparison between DEPT with a voltage-biased emitter and HPT with a voltage-biased base is also included. There are four (three) operating regions appearing in the optical characteristics of the DEPT (HPT): negative-saturation, negative-tuning, positive-tuning, and positive-saturation (cut-off, tuning, and saturation) regions. The InGaP-passivated DEPT with the extrinsic base surface passivated by InGaP, exhibits the maximum optical gains of 46.57, 46.86 and 47.39 while the non-passivated one shows those of 32.02, 33.55 and 33.57 under the optical powers of 8.62, 13.2 and 17.5 μW, respectively. However, the optical gains are only in the range of 0.93~2.0 (0.83~1.64) for the InGaP- passivated HPT (non-passivated HPT) for all the illuminating conditions.

We report on the InGaP/GaAs dual-emitter heterojunction phototransistors (DEPTs) with an emitter biased using a voltage for comparison to heterojunction phototransistors (HPTs) with a floating base operated in the p-i-n and transistor modes and to the HPTs with a base biased using a voltage. The optical gain of the DEPT is presented to be tunable with both of the voltage applied to the emitter and the incident optical power. On the contrary, a conventional HPT's configuration does not simultaneously exhibit both. The power-tunable optical gain is available when the base of the HPT is floated. Otherwise, the voltage-tunable optical is expected for the HPT with a voltage biased base. Experimental results show that (1) the HPT with a voltage biased base exhibits a gain-tuning efficiency of 4.4 V- 1 and (2) the DEPT with an emitter biased using a voltage exhibits a gain-tuning efficiency of 43.4 V-1.

This paper reports on a four-terminal dual-emitter heterojunction phototransistor (4T-DEPT) with a base biased by current source in comparison with a three-terminal dual-emitter heterojunction photoreansistor (3T-DEPT) without the additional current bias. While only voltage can be used to tune the optical performance of the 3T-DEPT, two kinds of operation modes, voltage- (VE21) and current- (IBdc) control modes, are considered for the 4T-DEPT. In addition to the power- and voltage-tunable optical gains, current-tunable one is also available in the 4T-DEPT operation. When the 4T-DEPT operates under the incident optical power (Pin) of 0.423 μW, it shows the maximum current-dependent gain-tuning efficiency of 62.66 μA-1 at VE21 = 0.06 V and IBdc = 0.001 μA. However, it is only 45.03 V-1 for the maximum voltage-dependent gain-tuning efficiency at VE21 = 0.06 V and IBdc = 0.25 μA.

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Temperature-dependent dark and optical characteristics of the InGaP/GaAs heterojunction phototransistors (HPTs) with and without sulfur treatment are studied. As compared to the HPT without (NH 4 ) 2 S treatment (HPT A), treatment at 50°C for 20 min leads to a reduced p-i-n dark current (/dark) and a reduced collector dark current (IC dark ) for the HPT (HPT D) in the emitter-floated and base-floated configurations, respectively. Moreover, the effective reduction of the surface defects also induces an enhanced p-i-n photocurrent (I ph ). The enhanced I ph combined with the promoted dc current gain results in an enhanced optical gain (G) and signal-to-noise ratio (SNR). For HPT A (D) under P in = 107.6 nW at 298 K, the G is 1.42 (20.3) while the SNR is 42 (94) dB. Experimental results indicate that the treated HPTs, compared to the untreated one, are more sensitive to low-power illumination.