Phonon-Assisted Tunneling in Large-Size Direct-Bandgap Semiconductor Devices

Trap-assisted tunneling (TAT) is one of the probable reasons for a degraded subthreshold swing in TFETs. Phonon-assisted tunneling (PAT), which is a first step towards an implementation of TAT, has not been studied in large semiconductor devices. Therefore, we present a multi-band PAT current formalism within the framework of the quantum transmitting boundary method. We derived the electron-phonon coupling terms using the envelope function approximation, used to calculate direct band-to band tunneling (BTBT) currents. We applied a low-wavevector approximation for local Fröhlich-based phonon-assisted inter-band tunneling in direct-bandgap semiconductors.
We study the PAT current density in up to 100nm-long and 20nm-wide p-n diodes with the 2-band model of our formalism. We find that the electron-phonon coupling is non-efficient across the tunneling junction, as a result of the phase-shift between same-band envelop functions. We further obtain that the PAT currents depend on the device’s length since there are observable contributions of the PAT process in the leads. Finally, we have compared the doping-dependence of BTBT and PAT currents.