A calculation of the effect of screening on phonon drag thermopower in a Si MOSFET

Abstract

Phonon drag thermopower Sg calculations are presented for a quasi-2D electron gas at the (100) plane in a Si MOSFET and compared with previous theory and experiment. Without screening -Sg is improved to a factor of approximately 16 times larger than experiment (from approximately 35). Multi-sub-band screening is considered and its importance assessed in the experimental range (2-6 K and electron densities to 1016 m-2) but a single sub-band approximation is found to be sufficient and gives thermopowers just approximately 40% different from experiment. Further improvement is made by adopting the Fang and Howard variational wavefunction and better material parameters. The best agreement is found to be about 5% at temperatures in the mid-range (4 K) and for the highest electron densities. The same excellent qualitative agreement with experiment is retained including the peak in -Sg/T3 whose presence is explained by enhanced phonon absorption around the Kohn resonance (q approximately=2kF). The peak positions move up in temperature (by about 0.75 K) because the dominant q-value increases and reduces the effect of screening on Sg at higher T. In GaAs/GaAlAs heterojunctions Sg is much less sensitive to screening effects because the screening constant is much smaller in GaAs than it is in Si.