Transport properties in GaN-based devices

• Main Authors: Chi-Kang Li, 李季剛
• Other Authors: Yuh-Renn Wu
• Format: Others
• Language: en_US
• Published: 2013
• Online Access: http://ndltd.ncl.edu.tw/handle/66819484716038351659

博士 === 國立臺灣大學 === 光電工程學研究所 === 102 === In this dissertation, we will introduce the motivation in studying transport properties of GaN-based devices at first in Chapter 1. In Chapter 2, the algorithm for a fully self-consistent model that solves Poisson and drift-diffusion equations by the finite element method to investigate device electrical properties was derived in detail. In addition, the Monte Carlo ray-tracing technique and heat conduction equation were introduced as well. The goal of this dissertation is combining the electrical, optical, and thermal aspects to model semiconductor devices precisely. In Chapter 3-5, various types of light emitting diodes (LEDs) were studied to give a thorough analysis for designers. In Chapter 6, the Ga2O3/GaN nanowire transistor was examined to discuss the scaling rules and related short-channel effect. In Chapter 3, the current spreading effect and light extraction efficiency (LEE) of vertical LEDs were analyzed. We tested different electrode configurations in the vertical LED to optimize the current spreading effect, which in turn suppresses the average carrier density in the quantum well and reduces the efficiency droop under high injection conditions. The wall-plug-efficiency in overall cases to identify the design rules for higher power conversion efficiency will be discussed as well. In Chapter 4, lateral LEDs were investigated with top and bottom emission conditions. The simulation results and circuit model were both used, which indicate that a uniform transparent conducting layer (TCL) cannot achieve a very uniform current spreading effect. Thus, modulating the TCL is tested to further improve the current spreading effect. Different current injecting conditions were discussed to observe the variation of the current flow path and the emerged current crowding effect. In addition, we will discuss the advantage of bottom emission LEDs by addressing the current spreading effect and LEE compared to top emission LEDs. The droop effect was also examined to verify our discussion. A thorough analysis provides deep insights for achieving high efficiency lateral LEDs in this chapter. In Chapter 5, the findings of investigating core-shell multiple quantum well nanowire LEDs were presented. The core-shell nanowire LED showed a weaker droop effect than that of conventional planar LEDs because of a larger active area and stronger recombination in nonpolar quantum wells (QWs). The current spreading effect was examined to determine the carrier distribution at the sidewall of coreshell nanowire LEDs. The results revealed that a larger aspect ratio by increasing the nanowire height could increase the nonpolar-active area volume and reduce the droop effect at the same current density. Making the current spreading length exceed a greater nanowire height to utilize the nonpolar QW effectively is critical. Therefore, an appropriate TCL might be necessary. In addition, we presented a discussion on the influences of the spacing between each nanowire on corresponding nanowire diameters. Moreover, the non-uniform indium distribution along the sidewall and different current injections were analyzed for the current crowding effect in the end of Chapter 5. In Chapter 6, a three-dimensional finite element solver was applied to investigate the performance of Ga2O3/GaN nanowire transistors. We provided the simulation results to compare with experimental nanowire results of 50 nm gate length, and they show good agreement. The performance of a shorter gate length (<50 nm) was studied and scaling issues of the short-channel effect are analyzed. With a better surrounding gate design and a recessed gate approach, the optimal conditions for a 20 nm gate length were explored in this dissertation.