栅注入晶体管结构的p-GaN/InAlN/GaN常关型功率器件研究

GaN-based normally-off planar power devices are the key technology of electronic power switch and chip integration applications. However, there exist great challenges such as low-threshold voltage, low-breakdown voltage, and large-leakage for the present common AlGaN/GaN high-speed-HEMT based normally-off electronic devices. In this project, we will investigate gate-injection-transistor structural novel GaN-based normally-off devices from new material, novel-structure to new technology. By replacing AlGaN barrier layer with InAlN with the Al fraction of 0.83, better lattice match can be achieved and the electronic barrier layer is increased. As a result, the gate-leakage and special on-resistance are reduced. By replacing p-GaN gate-Ohmic contact with p-GaN gate-Schottky contact, higher threshold voltage and lower gate-leakage can be achieved, which is due to that the p-GaN Schottky barrier can improve the gate-voltage and block majority holes. To decrease gate-leakage and prevent the front thermal breakdown, we will use the KOH wet etching and electrochemical corrosion to remove masa-etching defect layers and thereby increase breakdown voltage and improve threshold voltage, which provides scientific guidance on developing GaN-based normally-off power switch devices.

常关型GaN基平面功率器件是电力电子开关及芯片集成应用的关键技术，当前各种普遍以AlGaN/GaN高速电子器件HEMT为基础的常关型器件存在阈值电压低、击穿电压小及栅漏电流大等共性关键难题。本项目拟从新结构、新材料及新工艺入手，发展栅注入式GaN基常关型晶体管器件。将Al组份0.83的InAlN新材料替代传统AlGaN用作器件势垒层，实现异质结层晶格匹配，增大电子阻挡层势垒高度，以减小器件导通电阻和栅极漏电流；使用p-GaN肖特基代替传统p-GaN欧姆接触作为器件栅电极，通过p-GaN肖特基结势垒反向分压和对p-GaN层空穴多子的阻挡作用，以提高器件阈值电压和降低栅极漏电流；研究KOH湿法腐蚀和光电化学腐蚀法去除刻蚀台面缺陷，减少或阻断器件漏电路径，防止刻蚀台面边缘提前热击穿，实现器件击穿电压的增大和阈值电压的提高，为GaN基常关型功率器件研究提供科学指导。

作为第三代半导体材料，GaN具有临界击穿电场高、禁带宽度大、电子饱和漂移速度大等优良特性。同时由于极化效应，GaN可以与AlGaN等材料形成具有高面密度和高迁移率的二维电子气沟道，从而使得AlGaN/GaN HEMT可以同时获得较高的击穿电压和较低的导通电阻，因此被认为是最具有潜力的新型功率开关器件发展方向。但是由于GaN与AlGaN晶格不匹配，AlGaN/GaN HEMT器件存在着诸多可靠性问题，此外，AlGaN/GaN HEMT还存在着栅极电场集中效应以及缓冲层泄漏电流等问题，使得器件提前击穿，未能充分发挥GaN材料的高耐压特性。. 基于以上问题，本论文提出两种具有p型栅极的常关型GaN基功率器件，并对其机理、结构参数及电学特性展开了研究。针对传统AlGaN/GaN HEMT器件的可靠性问题，使用In组分为17%的InAlN势垒层代替AlGaN势垒层。本项目中提出p-GaN栅常关型InAlN/GaN HEMT替代AlGaN/GaN HEMT，以提高器件可靠性，增大饱和电流，减小开起的比导通电阻，提高器件最大截止频率，以及减少器件亚阈值摆幅。由于InAlN/GaN异质结存在着超强的自发极化效应使沟道达到更高浓度的2DEG，大幅度的提高了器件的电学性能。相比于p-GaN栅常关型AlGaN/GaN HEMT，在栅电压为5V时，InAlN/GaN HEMT器件导通电阻降低23%，开态电路增强161%；同时，InAlN/GaN HEMT器件的截至频率达到11.5GHz，高于AlGaN/GaN HEMT 器件39%。然而，InAlN/GaN HEMT相对较低的击穿电压限制其在功率电子领域的应用，相关研究表明表面钝化层厚度和介电常数对器件击穿电压有重要影响，会使栅电极靠漏极边沿附近峰值电场降低以及2DEG沟道区域电场分布均匀。因此，项目通过在InAlN/GaN HEMT器件中采用高介电常数的TiO2作为钝化层材料，使得器件击穿电压提高25%，且达到1024V，基于p-GaN栅的常关型InAlN/GaN HEMT设计仿真大量结果表明其在功率电子应用方面具有较大潜力。