Amorphous silicon germanium carbide photo sensitive bipolar junction transistor with a base-contact and a continuous tunable high current gain

• Current controllable a-Si:H phototransistor
• Controllable high current gain
• Current and phototunable transistor

In this paper, the design, fabrication and characterization of an amorphous silicon germanium carbide (a-SiGeC:H) photo sensitive bipolar junction transistor (PS-BJT) with three terminals are presented. Whereas the current gain of similar transistor devices presented in the past (Wu et al., 1984; Hwang et al., 1993; Nascetti and Caputo, 2002; Chang et al., 1985a,b; Wu et al, 1985; Hong et al., 1990) can only be controlled with photo induced charge generation, the n–i–δp–i–n structure developed features a contacted base to provide the opportunity to adjust the current gain optically and electrically, too. Electron microscope-, current-/voltage- and spectral measurements were performed to study the PS-BJT behavior and calculate the electrical and optical current gain. The spectral response maximum of the base–collector diode has a value of 170 mA/W applying a base–collector voltage of − 1 V and is located at 620 nm. The base–emitter diode reaches a sensitivity of 25.7 mA/W at 530 nm with a base-emitter voltage of − 3 V. The good a-Si:H transport properties are validated in a μτ-product of 4.6 × 10− 6 cm2 V s, which is sufficient to reach a continuous base- and photo-tunable current gain of up to − 126 at a base current of IB = + 10 nA and a collector–emitter voltage of VCE = − 3 V. The transistor obtains a maximum collector current of − 65.5 μA (VCE = − 3 V) and + 56.2 μA (VCE = + 3 V) at 10,000 lx 5300 K white-light illumination. At 3300 lx, the electrical current gain reaches a value of + 100 (VCE = + 2 V) at IB = 10 nA. With a negative base current of IB = − 10 nA the electrical gain can be adjusted between 87 (VCE = + 2 V) and − 106 (VCE = -3 V), respectively. When no base charge is applied, the transistor is “off” for VCE > − 3 V. Reducing the base current increases the electrical current gain. Operating with a voltage VCE of just ± 2 V, the device presented in this paper obtains no optical gain with an incident light power of 0.31 μW and 578 nm wavelength. The PS-BJT exhibits promising characteristics with a maximum current density greater than 65 μA μm− 1 and a tunable high current gain β. Possible applications exist in fields of control and drive circuits, in display and image devices, in photo interrupters, in photo sensitive matrices or in optoelectronic switches.