Small-signal theory of transit-time diodes

Transit-time diodes, in which the effects of carrier diffusion and generation or recombination are not important, have recently been investigated both theoretically and experimentally. The impedance of these devices, mainly due to one type of carriers, has been computed by several authors with different simplifying assumptions. In this paper a more general expression of the impedance of the transit-time diode is given, accounting for the spatial non-uniform d.c. distributions of the electric field and of the carrier velocity, and by allowing any dependance of the carrier mobility on the d.c. electric field. The impedance is given in closed form in the case where the mobility is constant. The impedance of a metal-semiconductor-metal structure is then computed, and the frequency and bias range over which a negative resistance arises is found to be consistent with known experimental data.