High field transport phenomena in wide bandgap semiconductors

Abstract

The Ensemble Monte Carlo (EMC) method is widely used in the field of computational electronics related to the simulation of the state of the art devices. Using this technique our specific intention is to scrutinize the high-field transport phenomena in wide bandgap semiconductors (Such as GaN, AlGaN and AlN). For this purpose, we have developed an EMC-based computer code. After a brief introduction to our methodology, we present detailed analysis of three different types of devices, operating under high-field conditions, namely, unipolar n-type structures, avalanche photodiodes (APD) and finally the Gunn diodes. As a testbed for understanding impact ionization and hot electron effects in sub-micron sized GaN, AlN and their ternary alloys, an n +−n−n + channel device is employed having a 0.1 µm-thick n region. The time evolution of the electron density along the device is seen to display oscillations in the unintentionally doped n-region, until steady state is established. The fermionic degeneracy effects are observed to be operational especially at high fields within the anode n +-region. For AlxGa1−xNbased systems, it can be noted that due to alloy scattering, carriers cannot acquire the velocities attained by the GaN and AlN counterparts. Next, multiplication and temporal response characteristics under a picosecond pulsed optical illumination of p +-n-n + GaN and n-type Schottky Al0.4Ga0.6N APDs are analyzed. For the GaN APD, our simulations can reasonably reproduce the available measured data without any fitting parameters. In the case of AlGaN, the choice of a Schottky contact APD is seen to improve drastically the field confinement resulting in satisfactory gain characteristics. Moreover, alloy scattering is seen to further slow down the temporal response while displacing the gain threshold to higher fields. Finally, the dynamics of large-amplitude Gunn domain oscillations from 120 GHz to 650 GHz are studied in detail by means of extensive EMC simulations. The basic operation is checked under both impressed single-tone sinusoidal bias and external tank circuit conditions. The width of the doping-notch is observed to enhance higher harmonic efficiency at the expense of the fundamental frequency up to a critical value, beyond which sustained Gunn oscillations are ceased. The degeneracy effects due to the Pauli Exclusion principle and the impact ionization are also considered but observed to have negligible effect within the realistic operational bounds. Finally, the effects of lattice temperature, channel doping and DC bias on the RF conversion efficiency are investigated