  Solid-state thermionic power generators are an alternative to thermoelectric
modules. In this manuscript, we develop an analytical model to investigate the
performance of these generators in the non-linear regime. We identify
dimensionless parameters determining their performance and provide measures to
estimate acceptable range of thermal and electrical resistances of thermionic
generators. We find the relation between the optimum load resistance and the
internal resistance and suggest guide lines for the design of thermionic power
generators. Finally, we show that in the nonlinear regime, thermionic power
generators can have efficiency values higher than the state of the art
thermoelectric modules.
