Velocity overshoot decay mechanisms in compound semiconductor field-effect transistors with a submicron characteristic length

Jang Jyegal

doi:10.1063/1.4922332

Abstract

Velocity overshoot is a critically important nonstationary effect utilized for the enhanced performance of submicron field-effect devices fabricated with high-electron-mobility compound semiconductors. However, the physical mechanisms of velocity overshoot decay dynamics in the devices are not known in detail. Therefore, a numerical analysis is conducted typically for a submicron GaAs metal-semiconductor field-effect transistor in order to elucidate the physical mechanisms. It is found that there exist three different mechanisms, depending on device bias conditions. Specifically, at large drain biases corresponding to the saturation drain current (dc) region, the velocity overshoot suddenly begins to drop very sensitively due to the onset of a rapid decrease of the momentum relaxation time, not the mobility, arising from the effect of velocity-randomizing intervalley scattering. It then continues to drop rapidly and decays completely by severe mobility reduction due to intervalley scattering. On the other hand, at small drain biases corresponding to the linear dc region, the velocity overshoot suddenly begins to drop very sensitively due to the onset of a rapid increase of thermal energy diffusion by electrons in the channel of the gate. It then continues to drop rapidly for a certain channel distance due to the increasing thermal energy diffusion effect, and later completely decays by a sharply decreasing electric field. Moreover, at drain biases close to a dc saturation voltage, the mechanism is a mixture of the above two bias conditions. It is suggested that a large secondary-valley energy separation is essential to increase the performance of submicron devices.

Highlights

Overshoot of drift velocity, so-called velocity overshoot, exhibited by carriers under nonstationary transport conditions constitutes one of the most important transport phenomena that influence the performance of semiconductor devices when their characteristic dimensions are on the scale of submicrometers
The velocity overshoot phenomenon is crucial for highfrequency submicron field-effect transistors (FETs), such as small GaAs metal-semiconductor fieldeffect transistors (MESFETs), that are fabricated from high-electron-mobility compound semiconductor materials or alloys
Velocity overshoot occurs in semiconductors owing to the nonequivalence of the energy relaxation time and the momentum relaxation time for carriers, and a detailed description of its physical mechanism in the case of carrier transport in bulk materials can be found in Refs. 2 and 6

Summary

INTRODUCTION

Materials and many different types of submicron devices. Related research for semiconductor materials includes wide-bandgap semiconductors with a large secondary-valley separation energy, such as GaN, InN, ZnO, etc., that have attracted great interest recently for possible applications to high-power high-frequency devices, as well as for applications to optoelectronic devices.[3,4,5]. A two-dimensional numerical simulation was performed typically for a submicron GaAs MESFET, which has a short gate length of 0.3 μm, using a hydrodynamic transport model, to analyze the underlying physical mechanisms of velocity overshoot decay behaviors of electrons in the compound semiconductor submicron FETs that are commercially important. It will be shown in the present paper that the physical mechanisms are characterized differently depending on device bias conditions, i.e., device operating points in the saturation region, near saturation drain voltages, or in the linear region of the drain current (dc) characteristics

PHYSICAL MODEL EQUATIONS AND SPECIFICATIONS

OVERSHOOT DECAY MECHANISM IN THE LINEAR DC REGION

CONCLUSION