Breakdown of thermionic emission theory for quantum wells

Carriers escape from quantum wells into barriers via carrier-polar optical phonon absorption is theoretically studied in multisubband quantum well structures. We find that carriers in each subband have their own minimum escape time when the energy difference between the band edges of the subband and the barrier matches the energy of a longitudinal optical phonon. Compared to the calculations from classical thermionic emission theory, we find that the thermionic emission theory is no longer valid when the width or the depth of quantum wells is small.