Terahertz Generation in Submicron Nitride-based Semiconductor Devices

Abstract

In this thesis, the electron dynamics and transport properties of III-nitride semiconductors materials and devices are studied, with an emphasis on their application to the generation high-frequency electromagnetic radiation. Numerical simulation models, including Monte Carlo, drift-diffusion, and thermal diffusion are utilized to model transport in the hot-electron and moderate-field regimes.

The Monte Carlo method is first applied to the study of the distribution function and the basic characteristics of hot electrons in III-nitrides under moderate electric fields. It is found that in relatively low fields (of the order of kV/cm) polar-optical phonon emission dominates the electron kinetics giving rise to a spindle-shaped distribution function and an extended portion of a quasisaturation of the current-voltage (I-V) characteristics.

The Monte Carlo program developed for the study of the III-nitrides is then extended to include the quantum mechanical spin evolution of electrons in bulk GaAs at room temperature. The spin relaxation time and characteristic decay lengths of spin polarized electrons are determined.

Next, the conditions for microwave power generation in a submicrometer GaN diode are investigated. By applying a high-field electron transport model based on the local quasistatic approximation, it is shown that oscillations in GaN diodes can be supported in the terahertz-frequency range near the LSA regime. The shape of the diode voltage and electronic current waveforms are examined in terms of the circuit parameters and operating frequencies over the bandwidth of active generation. Based on a Fourier series analysis of the diode voltage and current, the generated power and dc-to-RF conversion efficiency at the fundamental and the lowest higher harmonic frequencies are estimated. The calculation results clearly indicate that submicrometer GaN diodes (channel doping of $1 imes 10^{17}$ cm$^{-3}$) can achieve large output powers ($>$ 1 W) in the absence of Gunn domain formation, over a wide range of frequencies, near 0.5 terahertz.

Finally, conditions for pulsed dc regimes of terahertz power generation are theoretically investigated in a vertical nanoscale $n^+nn^+$ GaN-based diode coupled to an external resonant circuit. A combined electrothermal model is adopted allowing for a detailed analysis of the dynamical local distributions of the electric field, drift velocity, and lattice temperature via self-consistent simulation of the high-field electron transport in the active channel and the thermal transport in the device structure. The main performance parameters including, generation power, efficiency, and operation frequency are determined for stable generation with short pulses of a few ns and a few tens of ns of duty cycle. The presented results can be used for optimization and design of two-terminal GaN-based high-power THz generators for pulsed regime operation.