Photoionization Spectra of Traps Responsible for Current Collapse in GaN MESFETs
- PDF / 398,519 Bytes
- 6 Pages / 420.48 x 639 pts Page_size
- 74 Downloads / 154 Views
ABSTRACT Current collapse in GaN MESFETS is believed to result from the trapping of carriers in the high resistivity GaN layer, and can be reversed by the application of light. Light photoionizes (or photoneutralizes) the carriers, releasing them from the traps and restoring all or part of the original I-V characteristics of the device. In these investigations we have taken advantage of this effect to characterize the traps responsible for current collapse in an n-channel GaN MESFET. At fixed source-drain voltage, the incremental light-induced drain current, above that measured in the dark, and normalized per incident photon, is measured as a function of wavelength. The resulting photoionization spectrum reflects two absorption thresholds corresponding to two distinct electron traps. Because of the nature of the measurement, these traps can be identified as those responsible for current collapse in the device. INTRODUCTION Significant progress has been made over the last several years in understanding the physical characteristics of the group-rn nitride material system, as well as in developing optoelectronic and electronic devices based on these materials. The material parameters of this system promise the possibility of convenient sources and detectors in the UV and blue portions of the spectrum, as well as electronic devices capable of operating at high power, high temperature and in adverse environments. Indeed, blue-green LED's , as well as blue lasers operating for several thousand hours are now commercially available, in spite of the fact that the materials used to fabricate these devices contain relatively high concentrations of defects. Similarly for electronic devices, FETs have been successfully produced in GaN as well as in AIGaN/GaN HEMT structures. While the properties of these FETs continue to improve, traprelated phenomena still hinder the reproducible fabrication of high quality electronic devices. Two of the most commonplace trapping phenomena are persistent photoconductivity (PPC) and "current collapse". PPC is the optical excitation of photoconductivity in a material that exists for times long after the optical excitation source is removed. This process is often associated with the presence of a metastable deep defect, such as the DX center in AIGaAs, or can be induced by the transport of photoexcited carriers across macroscopic potential barriers [1]. It has been noted with respect to the AIGaAs/GaAs system that while PPC poses no direct problem for FETs, its presence does indicate the possibility of other transient device instabilities associated with charge trapping, such as shifts in the threshold voltage [2]. "Current collapse" refers to the trapping of charge at deep level centers in the structure, resulting in a dramatic reduction in the current flowing through the device, and hence in its output power. Current collapse is initiated by subjecting the device to a high electric field, such as that experienced by ramping the drain voltage up to a large value. This is thought to inject hot ca
Data Loading...
