Energy Distribution Of Defects Research Articles

Photocurrent interference spectroscopy of solids and characterization of semiconductor solar-cell materials

Photocurrent measurements are widely used to study the intrinsic optoelectronic properties of semiconductors, such as photocarrier generation efficiency and carrier mobility, as well as evaluate the performance of optoelectronic devices, such as solar cells and photodetectors. Interferometric spectroscopy precisely measures optical properties and gathers optical spectra information on semiconductors. Consequently, photocurrent-based interferometric measurements, with high signal-to-noise ratios, high resolution, and broad frequency bandwidths, can probe the energy distributions of low-density defects and impurities and investigate charge transport in materials and devices. Here, we demonstrate that photocurrent interference spectroscopy reveals the intrinsic properties of solar-cell materials: bulk crystals of GaAs and halide perovskite, and thin films of halide perovskite and quantum dot. We show that homodyne interference spectroscopy of photocurrent can monitor low-density localized states in semiconductors and that it can be used in combination with other spectroscopy techniques, such as photoluminescence measurements, to provide a deep understanding of photocurrent generation processes. Furthermore, we show that heterodyne interference spectroscopy of photocurrent can be used to investigate the frequency dependence of material parameters, such as the dielectric constant, absorption coefficient, and reflectance. As an application, we used interference spectroscopy of photocurrent to show the impact of multiexcitons on the photoabsorption and photocarrier generation processes in quantum dot solar cells. Finally, we used it to reveal the distinctive spectral characteristics at the band edge of a halide perovskite, which is considered to be an exceptional solar-cell material with high energy conversion efficiency.

Thick gate oxide extrinsic breakdown – The potential role of neutral hydrogen atom

Power electronics is currently a hot topic due to its important role in fighting climate change. Gate oxide breakdown is the Achilles heel of power devices, and it is well known that extrinsic breakdown is the chief concern. However, the root cause of extrinsic breakdown is poorly understood. Recently, a "lucky defect" model was introduced to explain extrinsic breakdown beyond the traditional "local thinning" model. In this work, the "lucky defect" model is further developed to allow it to examine the responsible defect's energy distribution. It is found that only defects with energy 1.5 eV ± 0.3 eV above the substrate conduction band can produce the breakdown distributions commonly reported. Few studied defects can satisfy this requirement. An exception is the neutral hydrogen atom, and its known properties are consistent with experimental results in the literature. If confirmed, this has important implication on how to remedy extrinsic breakdown.

Characterization of the defect in CIGSe solar cell by admittance spectroscopy

In this work, we used admittance spectroscopy to accurately characterize the defect energy distribution of defects in the absorption layer of a CuIn1−xGaxSe2 (CIGSe) solar cell. We found that the dark current of the CIGSe cell decreased by an order of magnitude, and its ideality factor dropped from 2.16 to 1.85 after annealing. The capacitance of the CIGSe solar cell before annealing was higher than that after annealing under reverse bias. Moreover, a higher free carrier concentration was found in the absorption layer of the cell before annealing. The results of our tests show that the built-in electric voltages of the CIGSe cell before and after annealing were 0.52 and 0.64 V, respectively. After annealing, the activation energy of defects in the absorption layer decreased, and their concentration remained almost constant, as measured by admittance spectroscopy. The decrease of the defect activation energy indicated a corresponding decrease in the Shockley–Read–Hall recombination probability of defects in the present cell; hence, the increase of the open circuit voltage and parallel resistance of the CIGSe solar cells after annealing resulted in improved performance.

Characterization of defects in CIGSe solar cells through admittance spectroscopy

We use admittance spectroscopy to characterize the energy distribution of defects in CIGSe solar cells before and after annealing to investigate the mechanism of the annealing process improving the performances of solar cells. In this work, we anneal the prepared CIGSe solar cells in compressed air at 150 ℃ for 10 min. We measure dark <i>I-V</i>, <i>C-V</i>, admittance spectra, and illumination <i>I-V</i> tests on CIGSe solar cells before and after annealing to characterize the changes in the performances of solar cells before and after annealing, respectively. The test results of dark <i>I-V</i> characteristics show that the reverse dark current of CIGSe solar cell decreases by about an order of magnitude after annealing, and the ideal factor of the cell also decreases from 2.16 (before annealing) to 1.85 (after annealing). This means that the annealing process reduces the recombination of carriers in CIGSe solar cell. Under reverse bias, the capacitance of CIGSe solar cell is higher than that after annealing, and its <i>C-V</i> characteristics linearly fitted with 1/<i>C</i><sup> 2</sup> <i>vs. V</i>. The fitting results show that the slope of the curve increases after annealing, which means that the annealing process results in a decrease in the free carrier concentration in the absorption layer of CIGSe solar cell, specifically, the carrier concentration contributed by defects after annealing decreases. In addition, the built-in potential before and after annealing of CIGSe solar cell are also obtained through fitting, which are 0.52 V and 0.64 V in value, respectively. The admittance spectrum test results of CIGSe solar cell before and after annealing show that the defect activation energy in the absorption layer significantly decreases after annealing, but the defect concentration remains almost unchanged. The decrease in defect activation energy means that the Shockley Read Hall (SRH) recombination probability of defects in copper indium gallium selenium solar cell decreases. In addition, the test results of the optical <i>I</i>-<i>V</i> characteristics of the battery indicate that the open circuit voltage and parallel resistance of the solar cell significantly increase after annealing, which is consistent with the test results of the dark <i>I-V</i> characteristics, <i>C-V</i> characteristics, and admittance spectroscopy of the solar cell. Therefore, the annealing process of CIGSe solar cells leads to theweakening of the SRH recombination of carriers in the absorption layer of the cell, thereby improving the performance of the solar cell.

Numerical Investigation of Photo-Generated Carrier Recombination Dynamics on the Device Characteristics for the Perovskite/Carbon Nitride Absorber-Layer Solar Cell.

The nitrogenated holey two-dimensional carbon nitride (C2N) has been efficaciously utilized in the fabrication of transistors, sensors, and batteries in recent years, but lacks application in the photovoltaic industry. The C2N possesses favorable optoelectronic properties. To investigate its potential feasibility for solar cells (as either an absorber layer/interface layer), we foremost detailed the numerical modeling of the double-absorber-layer-methyl ammonium lead iodide (CH3NH3PbI3) -carbon nitride (C2N) layer solar cell and subsequently provided in-depth insight into the active-layer-associated recombination losses limiting the efficiency (η) of the solar cell. Under the recombination kinetics phenomena, we explored the influence of radiative recombination, Auger recombination, Shockley Read Hall recombination, the energy distribution of defects, Band Tail recombination (Hoping Model), Gaussian distribution, and metastable defect states, including single-donor (0/+), single-acceptor (-/0), double-donor (0/+/2+), double-acceptor (2/-/0-), and the interface-layer defects on the output characteristics of the solar cell. Setting the defect (or trap) density to 1015cm-3 with a uniform energy distribution of defects for all layers, we achieved an η of 24.16%. A considerable enhancement in power-conversion efficiency ( η~27%) was perceived as we reduced the trap density to 1014cm-3 for the absorber layers. Furthermore, it was observed that, for the absorber layer with double-donor defect states, the active layer should be carefully synthesized to reduce crystal-order defects to keep the total defect density as low as 1017cm-3 to achieve efficient device characteristics.

Quantitative evaluation of plasma-damaged SiN/Si structures using bias-dependent admittance analysis

Plasma process-induced damage (PID) to SiN dielectric films was investigated by using an impedance (admittance)-based technique. Multi-layered equivalent circuits were introduced to assign the spatial and energy distribution of defects created in the SiN/Si system. We propose to use admittance as the principal parameter for damaged SiN/Si systems after Ar and He plasma exposures. The change in the border trap density was determined from the admittance in accumulation, whereas the interface state density and energy profile that was created was determined from the admittance in depletion. Plasma source-dependent damage-creation mechanisms are discussed. It was found that the extracted border trap density in the He plasma-damaged sample was larger than that in the Ar plasma-damaged sample under the same ion dosage. The proposed characterization scheme is useful for assessing PID to dielectric/Si systems.

Harvesting of Infrared Part of Sunlight to Enhance Polaron Transport and Solar Water Splitting

AbstractThere is increasing evidence that defects such as oxygen vacancies are a double‐edged sword for photoelectrochemical (PEC) water splitting. Although surface oxygen vacancies can largely improve the catalytic activity, their bulk counterparts may bind polarons, drag down the carrier transport, and thus degrade the PEC performance. However, it is very challenging to precisely control the spatial and energy distributions of defects. Instead, the infrared part of sunlight, normally discarded in PEC water splitting, is harvested to thermally activate the polarons. With the prototypical BiVO4 photoanode (absorbs blue–ultraviolet light), even when is undoped, a high solar‐to‐hydrogen efficiency of 5.3% is achieved in conjunction with a photothermal substrate (absorbs infrared light) and in tandem with a perovskite solar cell (absorbs red–green). Detailed characterizations reveal that the temperature increase of the system can not only accelerate the polaron hopping, but also activate the polarons bound by defects. This study, by demonstrating the use of the PEC‐inactive infrared part of sunlight to enhance transport kinetics at the device level, allows for panchromatic sunlight harvesting to improve the overall PEC performance.

Extensive assessment of the charge-trapping kinetics in InGaAs MOS gate-stacks for the demonstration of improved BTI reliability

Gate-stack reliability has been a major roadblock in the realization of InGaAs-channel based logic technology. Excessive charge trapping in the gate-oxide causes time-dependent drift in transistor threshold voltage (Vth). The extent to which Vth drifts under certain stress conditions depends on (i) the defect ground-state energy distributions in the oxide bandgap, and (ii) the specific activation energy distributions for charge capture/emission process. In this work, semi-empirical modelling of ground-state defect energy distributions is revisited to determine the Total Operating Voltage Range of InGaAs-based MOSFETs for a DC-BTI lifetime of 10 years under DC operating conditions. Non-radiative Multiphonon (NMP) theory is subsequently used to describe the microscopic physics of charge (de-)trapping kinetics in terms of activation energy barriers for charge capture (EAc) and emission (EAe) into/from gate-stack defects. These activation energy barriers are visualized using Capture/Emission Time (CET) maps, which are efficient and powerful tools to predict the BTI degradation under different AC operating conditions as well. We demonstrate that the enhanced BTI reliability of a gate-stack with a novel ASM interfacial layer (ASM-IL), as compared to the bilayer gate-stack of Al2O3/HfO2, results from the favorable reconfiguration of the defect energy distribution in the oxide bandgap, as well as their activation energies for capture and emission process.

Total-Ionizing-Dose Effects and Low-Frequency Noise in 30-nm Gate-Length Bulk and SOI FinFETs With SiO2/HfO2Gate Dielectrics

Total-ionizing-dose (TID) effects and low-frequency noise are evaluated in 30-nm gate-length bulk and silicon-on-insulator (SOI) FinFETs for devices with fin widths of 10–40 nm. Minimal threshold voltage shifts are observed at 2 Mrad(SiO2), but large increases in low-frequency noise are found, and significant changes in defect-energy distributions are inferred. Radiation-induced leakage current is enhanced for narrow- and short-channel bulk FinFETs. Short-channel SOI FinFETs show enhanced degradation compared with longer-channel devices. Narrow- and short-channel SOI devices exhibit high radiation tolerance. Significant random telegraph noise (RTN) is observed in smaller devices due to prominent individual defects.

Identification of defects and defect energy distribution in the perovskite layer of MAPbI3−xClx perovskite solar cell

Here we elucidate the defects and defect energy distribution in the perovskite layer of MAPbI3−xClx perovskite solar cell. The electrochemical impedance spectroscopy (EIS) result depicts that the interfacial recombination of electrons and holes taking place in MAPbI3−xClx/TiO2 junction. Furthermore, an increase in interfacial charge recombination resistance (Rct) was observed with decrease in temperature suggesting a reduction in the recombination rate. A non-linear dependence of series resistance (RS) with temperature was observed. From admittance spectroscopy, it can be conclude that defects are present in MAPbI3−xClx perovskite layer at energy level sites of 0.625 eV. This study may help to understand and discuses about the electronic defects and temperature instability of PSCs.

Total-Ionizing-Dose Response of MoS2Transistors With ZrO2and h-BN Gate Dielectrics

The total-ionizing-dose response of few layer MoS2 transistors with ZrO2 or h-BN gate dielectrics is investigated under various bias conditions. Defects in MoS2 and surrounding dielectric layers significantly affect radiation-induced trapping. For devices with ZrO2 dielectrics, much larger negative Vth shifts and peak transconductance degradation are observed for irradiation under negative and ground bias than under positive bias. The h-BN devices exhibit positive threshold voltage shifts under negative-bias irradiation. For both ZrO2 and h-BN passivated devices, the peak transconductance degradation results from charge trapping at the surface of the MoS2 or in nearby oxides. Changes in defect energy distributions of MoS2 FETs during X-ray irradiation are characterized via temperature-dependent low-frequency noise measurements. Density functional theory calculations are performed to provide insight into the pertinent defects.

Gate Bias and Length Dependences of Total Ionizing Dose Effects in InGaAs FinFETs on Bulk Si

We evaluate the total ionizing dose (TID) responses of InGaAs nMOS FinFETs with different gate lengths irradiated with 10-keV X-rays under different gate biases. The largest degradation after irradiation occurs at V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">G</sub> = -1 V. Radiation-induced trapped positive charge dominates the TID response of InGaAs FinFET transistors, consistent with previous results for InGaAs multifin capacitors. Shorter gate-length devices show larger radiation-induced charge trapping than longer gate-length devices, most likely due to the electrostatic effects of trapped charge in the surrounding SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> isolation and SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> /Si <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> spacer oxides. The 1/f noise measurements indicate a high trap density and a nonuniform defect-energy distribution, consistent with a strong variation of effective border-trap density with surface potential.

A Sensitivity Map-Based Approach to Profile Defects in MIM Capacitors From &lt;inline-formula&gt; &lt;tex-math notation="LaTeX"&gt;${I}$ &lt;/tex-math&gt; &lt;/inline-formula&gt;–&lt;inline-formula&gt; &lt;tex-math notation="LaTeX"&gt;${V}$ &lt;/tex-math&gt; &lt;/inline-formula&gt;, &lt;inline-formula&gt; &lt;tex-math notation="LaTeX"&gt;${C}$ &lt;/tex-math&gt; &lt;/inline-formula&gt;–&lt;inline-formula&gt; &lt;tex-math notation="LaTeX"&gt;${V}$ &lt;/tex-math&gt; &lt;/inline-formula&gt;, and &lt;inline-formula&gt; &lt;tex-math notation="LaTeX"&gt;${G}$ &lt;/tex-math&gt; &lt;/inline-formula&gt;–&lt;inline-formula&gt; &lt;tex-math notation="LaTeX"&gt;${V}$ &lt;/tex-math&gt; &lt;/inline-formula&gt; Measurements

We present a defect spectroscopy technique to profile the energy and spatial distribution of defects within a material stack from leakage current (J-V), capacitance (C-V), and conductance (G-V) measurements. The technique relies on the concept of sensitivity maps (SMs) that identify the bandgap regions, where defects affect those electrical characteristics. The information provided by SMs are used to reproduce J-V, C-V, and G-V data measured at different temperatures and frequencies by means of physics-based simulations relying on an accurate description of carrier-defect interactions. The proposed defect spectroscopy technique is applied to ZrO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> -based metal-insulator-metal structures of different compositions for dynamic random-access memory capacitor applications. The origin of the observed voltage, temperature, and frequency dependencies of the I-V, C-V, and G-V data is understood, and the atomic structure of the relevant stack defects is identified.

Activation Energy Distribution of Dynamical Structural Defects in RuO2 Films

Ruthenium dioxide (RuO$_2$) is an important metal widely used in nanoelectronic devices. It plays indispensable roles in the applications as catalyst and supercapacitors. A good understanding of the origin of the flicker or 1/$f$ noise in RuO$_2$ will advance the design and efficiency of these applications. We demonstrate in a series of sputtered RuO$_2$ polycrystalline films that the 1/$f$ noise originates from fluctuating oxygen vacancies which act as dynamical structural defects, i.e., moving scattering centers. Reducing the number of oxygen vacancies by adjusting thermal annealing conditions significantly reduces the noise magnitude $\gamma$, the Hooge parameter. We quantify the activation energy distribution function, $g(E)$, and calculate the oxygen vacancy density, $n_{TLS}$, from the measured $\gamma$ value. We show that $g(E)$ can be explicitly expressed in terms of $\gamma(T)$ and the electronic parameters of the metal, where $T$ denotes temperature. The inferred $n_{TLS}$ value is in line with the oxygen content determined from the x-ray photoelectron spectroscopy studies.

Band alignment of Cd-free (Zn, Mg)O layer with Cu2ZnSn(S,Se)4 and its effect on the photovoltaic properties

Cu2ZnSn(S,Se)4 (CZTSSe) is an interesting absorber material for thin film solar cells. However, one of the key challenges for the kesterite-based solar cells is to improve the open-circuit voltage (Voc) deficit, which is resultant of recombination at the interface of buffer/absorber. In this work, Cd-free n-type buffer layers with two different Mg-doped ZnO layers (Mg0.26Zn0.74O, Mg0.30Zn0.70O) have been examined using ultraviolet photoelectron spectroscopy. The most important electronic properties which are essential for the band offset study, ie. fermi level location, valence and conduction band offsets at the interface in the CZTSSe substrate, have been determined. The conduction band offset values for Mg0.26Zn0.74O, Mg0.30Zn0.70O buffer layers has been calculated experimentally. We have also established the correlation between device parameters and performances for dual ion beam sputtered ZnO buffer/CZTSSe-based heterojunction solar cells as a function of conduction band offset and the energy distribution of interface defects, to gain deeper understanding about the Voc-deficit behavior from a high recombination rate at the buffer/kesterite interface using simulation study. From the simulation study, the values of the solar cell efficiency with Mg0.26Zn0.74O and Mg0.30Zn0.70O buffer layers are 10.18 and 10.25%, respectively, which are higher in comparison to those obtained by using conventional CdS buffer layer.

Defects and Low-Frequency Noise in Irradiated Black Phosphorus MOSFETs With HfO2 Gate Dielectrics

We have evaluated radiation-induced charge trapping and low-frequency noise in passivated black phosphorus (BP) MOSFETs with HfO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> gate dielectrics. Thinning the gate dielectric reduces total ionizing dose-induced threshold voltage shifts. The defect-energy distribution estimated from low-frequency noise measurements performed as a function of temperature decreases with increasing energy in as-processed devices. Local maxima in noise magnitude are observed in irradiated devices at activation energies of ~0.2 and ~0.5 eV. Larger defect-related peaks in noise magnitude in the range of 0.35-0.5 eV are observed after biased post-irradiation annealing, and/or vacuum storage of the devices after irradiation and annealing. Density functional theory calculations strongly support significant roles for O vacancies in HfO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> and H+ transport and reactions near the BP/HfO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> interface in the observed radiation response and low-frequency noise.

Influence of LDD Spacers and H+Transport on the Total-Ionizing-Dose Response of 65-nm MOSFETs Irradiated to Ultrahigh Doses

The degradation induced by ultrahigh total ionizing dose in 65-nm MOS transistors is strongly gate-length dependent. The current drive decreases during irradiation, and the threshold voltage often shifts significantly during irradiation and/or high-temperature annealing, depending on transistor polarity, applied field, and irradiation/annealing temperature. Ionization in the spacer oxide and overlying silicon nitride layers above the lightly doped drain extensions leads to charge buildup as well as the ionization and/or release of hydrogen. Charge trapped in the spacer oxide or at its interface modifies the parasitic series resistance, reducing the drive current. The released hydrogen transports as H+ with an activation energy of ~0.92 eV. If the direction of the electric field is suitable, the H+ can reach the gate oxide interface and depassivate Si-H bonds, leading to threshold voltage shifts. Newly created interface traps are most prominent near the source or drain. The resulting transistor responses and defect-energy distributions often vary strongly in space and energy as a result, as demonstrated through current–voltage, charge-pumping, and low-frequency noise measurements.

Radiation-Induced Charge Trapping and Low-Frequency Noise of Graphene Transistors

We have performed a detailed evaluation of radiation-induced charge trapping and low-frequency noise for back-gated graphene transistors fabricated on a thermal SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> layer, with Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> or hexagonal boron nitride passivation over-layers. Irradiation with positive or 0 V back-gate bias leads to negative shifts of the charge neutral point (CNP) of the graphene transistors; irradiation under negative back-gate bias leads to positive CNP shifts. The low-frequency noise increases with irradiation and decreases with 400 K postirradiation annealing. The temperature dependence of the noise is described well by the Dutta-Horn model of low-frequency noise. Peaks in effective defect-energy distributions of irradiated devices at ~0.4 and ~0.7 eV are identified via measurements of the temperature dependence of the low-frequency noise. The noise of as-processed devices stored in room ambient also decreases with baking, but does not show the clear peaks observed after irradiation. Density functional theory calculations suggest that OH <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-</sup> and H <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">+</sup> at or near the graphene/dielectric interfaces likely play key roles in both the irradiation and baking response. Low-frequency noise and CNP voltage shifts during switched-bias postirradiation annealing at room temperature also suggest significant roles for O vacancies in the near interfacial SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> and/or passivation layers.

Reduction in the Interfacial Trap Density of Mechanochemically Synthesized MAPbI3.

Organo-lead halide perovskites have emerged as promising light harvesting materials for solar cells. The ability to prepare high quality films with a low concentration of defects is essential for obtaining high device performance. Here, we advance the procedure for the fabrication of efficient perovskite solar cells (PSCs) based on mechanochemically synthesized MAPbI3. The use of mechano-perovskite for the thin film formation provides a high degree of control of the stoichiometry and allows for the growth of relatively large crystalline grains. The best device achieved a maximum PCE of 17.5% from a current-voltage scan (J-V), which stabilized at 16.8% after 60 s of maximum power point tracking. Strikingly, PSCs based on MAPbI3 mechanoperovskite exhibit lower "hysteretic" behavior in comparison to that comprising MAPbI3 obtained from the conventional solvothermal reaction between PbI2 and MAI. To gain a better understanding of the difference in J-V hysteresis, we analyze the charge/ion accumulation mechanism and identify the defect energy distribution in the resulting MAPbI3 based devices. These results indicate that the use of mechanochemically synthesized perovskites provides a promising strategy for the formation of crystalline films demonstrating slow charge recombination and low trap density.

Worst-Case Bias for Proton and 10-keV X-Ray Irradiation of AlGaN/GaN HEMTs

Responses to 1.8 MeV proton irradiation and 10-keV X-ray irradiation under typical bias conditions are investigated for AlGaN/GaN HEMTs fabricated with different types of process technologies. We find that, in contrast to previous generations of process technologies, total ionizing dose effects can be significant in these devices. For proton irradiation, worst-case bias for transconductance degradation for GaN-on-SiC substrate devices is ON bias, and for devices built on free-standing GaN substrates, the worst-case bias condition is semi-ON bias. Low-frequency noise measurements demonstrate that these differences result from differences in defect types and energy distributions for the different types of devices, both before and after irradiation. These results emphasize the need to test devices under a wide range of conditions during characterization and qualification testing.