DC, RF, and Thermal Characterization of High Electric Field Induced Degradation Mechanisms in GaN-on-Si High Electron Mobility Transistors

Abstract

Gallium Nitride (GaN) high electron mobility transistors (HEMTs) are becoming increasingly popular in power amplifier systems as an alternative to bulkier vacuum tube technologies. GaN offers advantages over other III-V semiconductor heterostructures such as a large bandgap energy, a low dielectric constant, and a high critical breakdown field. The aforementioned qualities make GaN a prime candidate for high-power and radiation-hardened applications using a smaller form-factor. Several different types of semiconductor substrates have been considered for their thermal properties and costeffectiveness, and Silicon (Si) has been of increasing interest due to a balance between both factors.In this thesis, the DC, RF, and thermal characteristics of GaN HEMTs grown on Si-substrates will be investigated through a series of accelerated lifetime experiments. A figure of merit known as the critical voltage is explored and used as the primary means by which the GaN-on-Si devices are electrically strained. The critical voltage is defined as the specific voltage bias by which a sudden change in device performance is experienced due to a deformation of the target GaN HEMT's epitaxial structure. Literature on the topic details the inevitable formation of pits and cracks localized under the drain-side of the gate contact that promote electrical degradation of the devices via the inverse piezoelectric effect. Characteristic changes in device performance due to high field strain are recorded and physical mechanisms behind observed degraded performance are investigated.The study assesses the performance of roughly 60 GaN-on-Si HEMTs in four experimental settings. The first experiment investigates the critical voltage of the device in the off-state mode of operation and explores device recovery post-stress. The second experiment analyzes alterations in DC and RF performance under varying thermal loads and tracks the dependence of the critical voltage on temperature. The third experiment examines electron trapping within the HEMTs as well as detrapping methodologies. The final experiment links the changes in RF performance induced by high field strain to the small-signal parameters of the HEMT. Findings from the research conclude the existence of process-dependent defects that originate during the growth process and lead to inherent electron traps in unstressed devices. Electron detrapping due to high electric field stress applied to the HEMTs was observed, potentially localized within the AlGaN layer or GaN buffer of the HEMT. The electron detrapping in turn contributed to drain current recovery and increased unilateral performance of the transistor in the RF regime. Thermal experiments resulted in a positive shift in critical voltage, which enhanced gate leakage current at lower gate voltage drives. v ACKNOWLEDGMENTS I would like to first and foremost thank Dr. Dennis Derickson for giving me the opportunity to work on this research initiative. Without Dr. Derickson's guidance and mentorship through the entirety of the project, the...

Investigation of Degradation Effects Due to Gate Stress in GaN-on-Si High Electron Mobility Transistors Through Analysis of Low Frequency Noise

“…Some examples of these images are shown in Degradation also manifests in a change in the I ds vs. V ds family of curves, discussed more in §5.2, that shows an increase in I ds due to a negative shift in the threshold voltage, V th (this trend is also observed in data presented in §6.1) [18]. …”

“…One of the most easily observable signs of degradation is increased gate leakage current. Previous research has shown that gate leakage current, I g , increases when exposed to a high electric field (for reasons that will be discussed in subsequent sections) [18], [23]. The change in gate leakage current was found to be dependent on both the gate stress voltage and the amount of time the gate was exposed to the stress [23].…”

“…Previous investigations applying high field stress have speculated that an increase in I g after stress is due to stress induced defects formed through the inverse piezoelectric effect [18] McWhorter speculated that the trap time constant is related to the depth in the material based on Equation (2-7) which is based on the theory that 1/f noise is generated by thermionic emission [29], [31]. Therefore, if the material has a homogenous distribution of traps, then g(τ) = 1/τ will result; any deviation from a homogenous distribution will result in a nonconstant dN trap /dx and a different g(τ) distribution.…”

“…Other measurable types of degradation include changes in the I ds -V ds characteristics, the I ds -V gs transfer characteristics, and shifts in the threshold voltage, V th , and transconductance, g m . Degradation effects can be classified as either permanent or reversible with some traits experiencing partial or full recovery to the pre-stress state over time [18].…”

“…Detrapping is then performed by applying increasing steps of V gs or temperature while the resulting current from the detrapping electrons is measured. After detrapping, DC performance reverts back to the pre-trapped state [18]. Other variations of this experiment have been used to measure the decrease in I ds with more negative V gs to induce more trapping [23].…”