Compact Modeling of Distributed Effects in 2-D Vertical Tunnel FETs and Their Impact on DC and RF Performances

Abstract

In this paper, distributed effects along the channel are investigated for 2-D vertical tunnel FETs by developing a model based on a succession of unit cells along the channel, each of which includes lateral FET conduction and vertical tunnel conduction components. The distributed model shows that there are tradeoffs between these two conduction mechanisms in both dc and RF characteristics. At dc, the overall device current is often limited by one of the two mechanisms, which is lateral conduction for many of the examples discussed in this paper. Channel length has opposite effects on current from the two mechanisms, so that ON-state current can typically be optimized by proper choice of channel length. Tunneling current density is highly nonuniform along the channel for long channel length. For RF applications, lateral conduction limitations increase the total input capacitance, particularly $\text{C}_{\text {gd}}$ , and can lead to capacitance peaking at specific bias voltages near device turn-on. Unlike lateral TFET design, scaling down the channel length significantly improves the cutoff frequency. The distributed model is implemented inVerilog-A and is directly useful for circuit simulations. Parasitic capacitances and contact resistances are also taken into account when evaluating RF characteristics for practical design purposes.