Base-width modulation effects on the optoelectronic characteristics of n-ITO/p-NiO/n-ZnO heterojunction bipolar phototransistors.

ITO/NiO/ZnO npn heterojunction bipolar phototransistors (HBPTs) with various base widths are fabricated using a radio-frequency sputtering system. The effects of base-width modulation on the optoelectronic characteristics of the prepared HBPTs are studied. The dark current of HBPTs decreases with increasing base width because the injected electrons from the emitter are recombined in the wide base region. The photocurrent increases with decreasing base width, which is attributed to higher emitter-base injection efficiency. The responsivity increases with the collector-emitter voltage (VCE) in the HBPTs with a 100 nm base width, whereas the responsivity sharply decreases atVCE> 4 V for the HBPTs with a thinner base width (80 nm) due to the punch-through effect. In contrast, the responsivity approaches saturation at largeVCEfor HBPTs with a thicker base width (120 nm). The responsivity and detectivity decrease with increasing incident light intensity, which is caused by an increase in the base recombination loss. The HBPTs with a base width of 100 nm exhibits the largest responsivity and detectivity; their detectivity is higher than that of HBPTs with base widths of 80 and 120 nm by approximately two and three orders, respectively.