Negative-effective-mass ballistic field-effect transistor: Theory and modeling

We consider p+pp+ diodes, in which the middle p region (base) consists of a p-type quantum well current-conducting channel that is controlled by a gate potential. Hole concentrations in the channel are assumed to be such that a ballistic current flows only in the lowest quantized subband. This subband contains a negative-effective-mass (NEM) section in the dispersion relation. We carry out numerical simulation for realistic designs of this ballistic field-effect transistor (FET) and compare them to simple analytical estimates. We show that three types of self-organized terahertz current oscillations appear in these FETs. Two of these types originate from the NEM instability, while the third arises from the two-stream instability, predicted before for conventional ballistic diodes and FETs. Frequencies of the NEM oscillations are controlled effectively by a gate potential. They are substantially higher than frequencies of two-stream oscillations. The NEM oscillation frequencies exceed 2.5 THz for large enhancing gate potentials.