Overshoot mechanism in transient excitation of THz and Gunn oscillations in wide-bandgap semiconductors

Ernesto Momox,Naci Balkan,Nick Zakhleniuk

doi:10.1186/1556-276x-7-647

Abstract

A detailed study of high-field transient and direct-current (DC) transport in GaN-based Gunn diode oscillators is carried out using the commercial simulator Sentaurus Device. Applicability of drift-diffusion (DD) and hydrodynamic (HD) models to high-speed, high-frequency devices is discussed in depth, and the results of the simulations from these models are compared. It is shown, for a highly homogeneous device based on a short (2 μm) supercritically doped (1017 cm−3) GaN specimen, that the DD model is unable to correctly take into account some essential physical effects which determine the operation mode of the device. At the same time, the HD model is ideally suited to solve such problems due to its ability to incorporate non-local effects. We show that the velocity overshoot near the device contacts and space charge injection and extraction play a crucial role in defining the operation mode of highly homogeneous short diodes in both the transient regime and the voltage-controlled oscillation regime. The transient conduction current responses are fundamentally different in the DD and HD models. The DD current simply repeats the velocity-field (v-F) characteristics, and the sample remains in a completely homogeneous state. In the HD model, the transient current pulse with a full width at half maximum of approximately 0.2 ps is increased about twofold due to the carrier injection (extraction) into (from) the active region and the velocity overshoot. The electron gas is characterized by highly inhomogeneous distributions of the carrier density, the electric field and the electron temperature. The simulation of the DC steady states of the diodes also shows very different results for the two models. The HD model shows the trapped stable anodic domain in the device, while the DD model completely retains all features of the v-F characteristics in a homogeneous gas. Simulation of the voltage-controlled oscillator shows that it operates in the accumulation layer mode generating microwave signals at 0.3 to 0.7 THz. In spite of the fact that the known criterion of a Gunn domain mode n0L > (n0L)0 was satisfied, no Gunn domains were observed. The explanation of this phenomenon is given.

Highlights

At present, there is a considerable interest in development of compact emitters of high-frequency radiation in the terahertz (THz) and sub-THz spectral range
Transient response of GaN samples to ultra-short high-electric field pulses In order to investigate the differences in applying the DD and HD models to simulation of the high-field transport in high-speed devices, we will first study the transient response of a GaN sample induced by highelectric field ultra-short pulses
We assume that the GaN sample is connected in series to a photoconductive switch and the external DC bias is applied to these two components

Summary

Introduction

There is a considerable interest in development of compact emitters of high-frequency radiation in the terahertz (THz) and sub-THz spectral range. Various physical mechanisms of THz generation have been suggested and investigated in recent years. Most of these mechanisms rely on either purely optical THz radiation generation (e.g. CO2 laser-pumped THz molecular lasers) or combination of optical and electrical excitation (e.g. using laser difference frequency photomixing or laserexcited uni-travelling carrier photodetectors). Development of compact sources of THz radiation with purely electronic excitation still presents a formidable challenge This is possibly in part due to the peculiar location of the THz spectrum which is between the optical and microwave (MW) regions. The aforementioned spectral regions can be accessed using optical sources (lasers) or electronic microwave emitters, respectively

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Conclusion