Limits to the Signal Delay in Ballistic, Nanoscale Transistors: Semiclassical and Quantum Results

When considering high-frequency signal propagation in bipolar devices, it is customary to invoke a signal velocity in excess of the electron propagation velocity in order to account for the effect of image charges. In this work, we examine the applicability of this idea to a field-effect transistor structure, and further extend the ideas to incorporate quantum resonances. The focus of this work is on ballistic devices. We find that, by incorporating quantum effects, the signal delay may be less than that expected from the simple semiclassical approach; however, the condition on the wavevectors in order to achieve such a result is very strict, and the improvement is only marginal in most cases. More importantly, it is shown that the signal delay is likely to be worse than that given by the semiclassical result.