Modulation of Negative Differential Resistance in Graphene Field-Effect Transistors by Tuning the Contact Resistances

Graphene and molybdenum disulfide are two-dimensional novel materials considered promising for nanoscale electronic devices. Due to high carrier mobility and in spite of lacking a bandgap, nanoscale graphene transistors have been demonstrated to reach a cut-off frequency above 400 GHz. The absence of bandgap in graphene leads to a remarkable band-to-band tunneling property in electron devices with negative differential resistance. Ultra-thin field-effect transistors fabricated with graphene as gate conducting channels have been shown experimentally to exhibit negative differential resistance (NDR) with widespread appeal for both digital and analog electronics. NDR devices like the Esaki p–n junction have been known to have applications for high frequency oscillators, fast logic switches, memories and low-power amplifiers. In this work, a semi-analytical model equation for transfer characteristics of graphene transistors is developed to successfully model the NDR. Data from three known experimental devices exhibiting NDR with gate length from 500 nm to 3 μm are shown to match well with theoretical modeled results. Numerical calculations using the model equation show that at a fixed gate bias, NDR can be modulated by tuning the value of contact resistance. The result also shows that separate onset of NDR in purely electron current or hole current can be modeled with this equation and matches experimental data.