Fig. 1. (a) Cross-sectional diagram of a phototransistor with ohmic-contact electrodes. (b) Configuration diagram of a planar phototransistor with interdigital electrodes. (c) Photograph of the planar interdigital electrode phototransistors on a chip with an area of 1  cm×1  cm. (d) AFM topography of sample surface with scanning area of 2  μm×2  μm. (e) XRD RSM of the sample recorded around (101¯5) reflection. The calculated full relaxation (R=100%) and full strain (R=0%) lines are also shown. (f) Room-temperature PL spectrum measured using a 213-nm laser as the excitation source.

Fig. 2. (a) Simulated energy band diagram of the phototransistor under zero bias voltage in the dark and 240 nm illumination of 0.1  μW/cm2 (upper panel) and the corresponding charge contributions (lower panel). (b) Directions of the spontaneous and piezoelectric polarization in our compressively strained Al0.55Ga0.45N/Al0.47Ga0.53N/AlN heterostructure. (c) Polarization charge distribution in the epitaxial structure under dark condition. (d) Comparison of the electric field distribution before and after 240-nm DUV illumination. (e) Cross-sectional view of the electron concentration distribution in the phototransistor before (left) and after (right) 240-nm DUV illumination.

Fig. 3. (a) C-V curves measured under the dark and the DUV illumination. (b) Depth profile of the apparent carrier density under the DUV illumination.

Fig. 4. Photoresponse characteristics of the phototransistor. (a) I-V curves under the dark and DUV illumination conditions. (b) Spectral response of the phototransistor under different bias voltages. (c) Time dependence of the response current to the 10 s/10 s on/off cycle incident DUV light with a time span of 80 days. (d) Transient response of the phototransistor to the 213-nm periodic pulse signals at 8 V. A magnified view of one of the impulse responses is shown in the inset. (e) Noise power spectral density measured at different bias voltages in the dark.

Fig. 5. (a) Photograph of the measuring equipment for recording I-V-Pinc curves. (b) The I-V-Pinc curves of the phototransistor at the semi-logarithmic scale. (c) The linear scales at a DUV incident light power ranging from 0.6 fW to 6.7 nW. Pinc dependence of (d) the Iph and the PDCR and (e) D* at 10 V bias. (f) The theoretical Iph under the combination of photogating effects in the barrier and the channel layers. Schematic diagram of the energy band in the phototransistor under (g) dark, (h) low power irradiation, and (i) high power irradiation conditions.