Photo-Induced Negative Differential Transconductance in Back-Gated Layered MoSe₂/p-Ge Heterojunction Field Effect Transistors

This paper reports the light-induced negative differential transconductance (NDT) in an n-type MoSe2 and highly doped p-type germanium (Ge) heterojunction transistor. A fully complementary-metal–oxide–semiconductor compatible fabrication of heterojunction transistors in a backgated geometry has been developed. The room-temperature transfer characteristic of the device showed an ON/OFF ratio of ∼100, which further increased to ∼105 as the temperature dropped to 80 K. The effect of illumination on the transfer characteristic was studied for different wavelengths. We observed a light-induced NDT effect when the device was illuminated at 1550 nm wavelength. We have associated this NDT effect to the resonant tunneling of charge carriers at the interface of the MoSe2/Ge heterojunction. The effect of incident light power and the operating temperature was also studied for a clear understanding of the observed NDT effect. It was observed that the resonant tunneling window is dependent on the lateral electric field provided by the drain to source bias (VDS). The peak to valley ratio of the hump observed in the transfer characteristic increased for drain voltages (VDS) < 100 mV, and for higher VDS values, it decreased sharply. The NDT effect results in a sign changing transconductance, which provides a possibility of using these heterojunction transistors for multivalued logics.