Surface states in AlGaN/GaN high electron mobility transistors: Quantitative profiles and dynamics of the surface Fermi level

TL;DR

This paper introduces an optical spectroscopy method to quantitatively profile and monitor the dynamics of surface state charge density and Fermi level in AlGaN/GaN HEMTs under different bias conditions, aiding surface charge studies.

Contribution

The paper presents a novel optical spectroscopy technique combined with a model to measure surface state charge profiles and Fermi level dynamics in fully fabricated HEMTs under various biases.

Findings

Quantitative surface charge profiles obtained from photocurrent derivatives.

Real-time monitoring of Fermi level dynamics during device operation.

Method applicable under any bias condition for in-situ surface state analysis.

Abstract

We present a method to obtain quantitative profiles of surface state charge density and monitor its dynamics under various stress conditions in high electron mobility transistor (HEMT) devices. The method employs an optical spectroscopy of the channel current at various bias conditions. We test the method on a classical AlGaN/GaN HEMT structure. To analyze the results, we propose a model, according to which the energy distribution of the surface charge density may be obtained from the derivative of the channel photocurrent. The proposed method is applied to fully fabricated transistors and can be measured under any device bias combination. This way, it is possible to explore the effect of device operating conditions on the surface state charge. This feature should be especially useful in studies of the various surface charge migration effects in nitride HEMTs. An important byproduct of the method is a quantitative assessment of the energy position of the surface Fermi level and its dynamics under various bias conditions.