A new comparison between solid-state thermionics and thermoelectrics
  It is shown that equations for electrical current in solid-state thermionic and thermoelectric devices converge for devices with a width equal to the mean free path of electrons, yielding a common expression for intensive electronic efficiency in the two types of devices. This result is used to demonstrate that the materials parameters for thermionic and thermoelectric devices are equal, rather than differing by a multiplicative factor as previously thought. 
T. E. Humphrey and M. F. O'Dwyer