Demonstration of an n-ZnO/p-Si/n-Si heterojunction bipolar phototransistor for X-ray detection

We fabricated an n-ZnO/p-Si/n-Si heterojunction bipolar phototransistor by the low cost RF magnetron sputtering method and characterized its photoresponse to both the 780 nm visible light and the X-ray in this paper. The device structure and the film transmittance were characterized by the SEM, XRD and transmittance spectra. The phototransistor shows high sensitivity, linearity and repeatable response to both the 780 nm visible light and the X-ray illumination indicating the promising potential of the n-ZnO/p-Si/n-Si detector. The sensitivity of the phototransistor to the X-ray is about 280 μC·Gy⁻¹·cm⁻² under the bias voltage of 10 V with the X-ray tube voltage of 30 kV. The phototransistor also shows a fast response with a decay time of about 7.75 μs, which corresponds to the lifetime of the sputtered p-Si film. The temperature dependent dark current of the device indicated the activation energy of 0.32 eV, which is probably caused by the deep-level defect in the high resistivity silicon substrate. The photo current of the phototransistor to the 780 nm visible light linearly increases with increasing the bias voltage in the saturation region indicating an Early voltage of about 5.6 V, from which the effective impurity density of the p-Si film was deduced to be ∼1 × 10¹⁶ cm⁻³ revealing the low activation rate of the sputtering method, and the unactivated dopant could act as recombination centers and responsible for the short carrier lifetime of the p-Si film and the fast response of the phototransistor. This paper demonstrated that the n-ZnO/p-Si/n-Si heterojunction bipolar transistor has potential for the development of an X-ray detector.