Defects in GaN based transistors

Abstract

Electrically active defects in AlGaN/GaN high electron mobility transistors (HEMTs) are the source of intense study due to their linkage to the mechanisms for GaN HEMT degradation upon a variety of stress conditions. The ability to directly characterize traps and identify their sources in GaN HEMTs is challenging, however, and this is due to a combination of the large bandgap of these materials and the complex electrostatics present in these device structures. Furthermore, the targeted extreme operating conditions intended for GaN HEMTs, whether designed for RF or power applications, greatly exacerbate the ability to identify those defects that are most relevant to device degradation under actual operation. Over the past few years, however, we have developed several electronic defect characterization methods based on deep level optical spectroscopy (DLOS) and thermally-based deep level transient spectroscopy (DLTS), which have been adapted from basic studies of defects in GaN and AlGaN to become applicable to working HEMTs. These so-called constant drain current (CID) DLOS/DLTS methods are able to directly provide individual trap concentrations and energy levels for traps that may exist throughout the AlGaN/GaN HEMT bandgap, and can discern between traps under the gate or in the transistor access regions. This talk will first review the CID-DLOS/DLTS methodology and then will focus on the application of these methods to stressed and un-stressed AlGaN/GaN HEMTs. Direct correlations of the formation of several specific traps are made with a variety of HEMT degradation mechanisms induced by stresses that include RF, DC and also proton irradiation

Identification of an RF degradation mechanism in GaN based HEMTs triggered by midgap traps

“…While emphasis was placed on discussion of the properties and location of the E C -0.57 eV trap, observed using CI D -DLTS in the drain-controlled CI D -DLTS mode, which senses traps in the drain access region, it is noteworthy that drain controlled CI D -DLOS measurements performed on these same GaN HEMTs detected much deeper traps within the bandgap in the drain-access regions. However for those devices, no systematic correlation was observed between the evolution of very deep trap states and the output power degradation seen [5][6][7][8], with the entire degradation being fully explained by the behavior and properties of the Ec-0.57 eV trap.…”

“…Box-car analysis of the ΔR D thermal transients for a given time rate-window gives rise to negative peaks which are representative of electron traps. The trap concentration is extracted using the amplitude of measured ΔR D CI D -DLTS peaks in conjunction with the Hall measured n s and μ n values using relations that have been previously published [5][6][7][8]. Arrhenius analysis of the CI D -DLTS peaks for different rate windows yields the thermal trap activation energy (referred to the conduction band edge) and apparent thermal capture cross-section of the trap.…”

“…To that end, constant drain current deep level transient spectroscopy (CI D -DLTS), based on trap detection via thermally-stimulated emission, and deep level optical spectroscopy (CI D -DLOS), based on optically-stimulated emission to observe very deep traps, were developed for application to GaN HEMTs. [5][6][7][8]. Arehart et al reported the initial application of these methods to RF stressed HEMTs, revealing the presence of a trap at E C -0.57 eV in the drain access region that was directly linked to power loss in those devices.…”

Review—Ionizing Radiation Damage Effects on GaN Devices

“…91 Coming back to the point that radiation exposure increases the concentration of defects already present in the GaN, Figure 16 shows a schematic representation of energy levels in the gap of both n-and p-type GaN before and after proton irradiation. 71,78 Note how many of the defects already present in the material increase after proton irradiation. Figure 17 provides a summary of the carrier removal rates in n-and p-GaN films, and InAlN/GaN and AlGaN/GaN HEMT structures exposed to proton, electron, neutron or gamma-ray fluences at different energies.…”

“…In p-GaN implanted with 100 keV protons, degradation of luminescent and properties starts at doses ∼10 12 cm −2 , while decreases in hole concentration were evident for Figure 16. Schematic of position in the bandgap of defects in n-and p-GaN before and after proton irradiation (after Sasikumar et al 71,78 Both threshold doses are more than an order of magnitude lower than in proton implanted n-GaN and may involve complex formation between the radiation defects and Mg acceptors. The main deep centers introduced by proton damage in pGaN have activation energies 0.3 eV, 0.6 eV and 0.9 eV.…”