Fig. 1. (a) Schematic illustration of the PG-HFEPT and (b) its cross-sectional view. (c) Schematic diagram and optical image of the interdigitated electrodes, mentioning the device dimensions.

Fig. 2. (a) Transmission and reflection spectra of the epitaxial structure with p-type photogate. (b) XRD RSM for the epitaxial structure taken around (105) reflection, in which the dashed lines correspond to fully strained (R%=0%) and fully relaxed (R%=100%) states of the Al0.4GaN channel layer. Simulated energy band diagram for the devices (c) without and (d) with the p-type AlGaN photogate.

Fig. 3. Dark and illuminated I-V characteristics of the HFEPTs (a) with and (b) without the p+-Al0.4GaN top photogate. The inset of (b) is the linear plot of the I-V curve. (c) I-V characteristics of the Ti/Al/Ni/Au Ohmic electrodes for the two HFEPTs. (d) Spectral responsivity of the PG-HFEPT under 1 V and 3 V bias. The inset is the corresponding spectral photocurrent.

Fig. 4. (a) Schematic diagram of the photogenerated carriers and their movement in the absorber, barrier, and channel layers of PG-HFEPT. Schematic band diagrams of (b) the top p-type photogate heterojunction and (c) the virtual back-photogate heterojunction together with the barrier layer outside the p-photogate under dark and illumination conditions.

Fig. 5. (a) Iph-V curves of the PG-HFEPT measured at different incident power densities of a 260 nm LED. (b) Dependence of Iph on incident power density. (c) Responsivity versus voltage characteristics under different incident power densities. (d) Dependence of responsivity on incident power density.

Fig. 6. (a) Noise power density spectra of the PG-HFEPT and HFEPT measured at different biases. (b) Impulse response of the two phototransistors measured at 3 V bias. The inset is the time-dependent photoresponse of the two phototransistors at 3 V bias measured at 260 nm irradiation with a 30 s on/off cycle.