It is well known that trapping effects can limit the output power performance of microwave field-effect transistors (FETs

The two deep traps responsible for current collapse in AlGaN/GaN high electron mobility transistors grown by metalorganic vapor-phase epitaxy have been studied by photoionization spectroscopy. Varying the growth pressure of the high resistivity GaN buffer layer results in a change in the deep trap incorporation that is reflected in the observed current collapse. Variations in the measured trap concentrations with growth pressure and carbon incorporation indicate that the deepest trap is a carbon-related defect, while the mid-gap trap may be associated with grain boundaries or dislocations.

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The effects of measurement technique and measurement conditions (e.g., injection level, temperature) on measured carrier lifetimes in n− 4H-SiC epilayers are investigated both experimentally and through detailed carrier dynamics simulations to better understand differences between reported lifetimes. Three common, optically based techniques are compared: time resolved photoluminescence, transient free carrier absorption, and microwave photoconductivity decay. From the details of these measurement techniques it is shown from both theory and experiment that for the limits of high or low injection, these techniques can reflect very different lifetimes. The effect of measurement conditions on the carrier lifetime was approached by simulating the carrier dynamics assuming a dominant Z1/Z2 defect in order to calculate the evolution of the lifetimes and the carrier and defect charge state concentrations for arbitrary injection level or temperature, as a closed-form solution to this problem does not exist. The simulated behavior was found to be in reasonable agreement with experiment and the resulting values for the electron and hole capture cross sections for the 0/+ transition of Z1/Z2 were found to be σn2≈(2−4)×10−15 cm2 and σp2≈(1−2)×10−14 cm2, respectively. The simulations provide insight into the dominant processes controlling the lifetime and identify four distinct stages of decay. A simple expression for the ratio of high- to low-injection lifetimes is presented which compares well with experiment. The temperature dependence of the lifetime is found to be relatively weak below 500 K and thermally activated immediately above this temperature due to electron emission from the Z0 state. Electron emission from Z− and hole emission become important only at higher temperatures. Simulations with both Z1/Z2 and EH6/EH7 defects suggest the latter does not contribute significantly to the lifetime in as-grown epilayers, due primarily to a small capture cross section for holes.

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Low-injection minority carrier lifetimes ͑MCLs͒ and deep trap spectra have been investigated in n − 4H-SiC epilayers of varying layer thicknesses, in order to enable the separation of bulk lifetimes from surface recombination effects. From the linear dependence of the inverse bulk MCL on the concentration of Z1/Z2 defects and from the behavior of the deep trap spectra in 4H-SiC p-i-n diodes under forward bias, we conclude that it is Z1/Z2 alone that controls the MCL in this material.

Nonequilibrium carrier recombination in highly excited epitaxial layers of 4H–SiC and free standing 3C–SiC was analyzed numerically and studied experimentally by the time-resolved free carrier absorption (FCA) technique. The measurement setup combined interband carrier excitation by a picosecond laser pulse and probing of carrier dynamics at excess carrier densities in the ΔN=1017–1020 cm−3 range by optically or electronically delayed probe pulses, thus providing temporal resolution of 10 ps and 10 ns, respectively. FCA decay kinetics at different excitation levels and subsequent numerical modeling were used to determine the bulk lifetime, surface recombination velocity, and bimolecular (B) and Auger recombination (C) coefficients at 300 K. Bulk lifetimes of ∼800 ns and ∼65 ns were determined in 4H and 3C epitaxial layers, respectively. The numerical fitting of FCA kinetics in the 4H layer provided values of B=(1.2±0.4)×10−12 cm3/s and C=(7±4)×10−31 cm6/s at lower excitations while the Auger coefficient decreased to C=(0.8±0.2)×10−31 cm6/s at ΔN∼1020 cm−3 due to screening of the Coulomb-enhanced Auger recombination. In 3C crystals, these values were measured to be B=(2.0±0.5)×10−12 cm3/s and C=(2.0±0.5)×10−32 cm6/s. The tendency for a strongly increased surface recombination rate in 3C at high excitation conditions was observed experimentally and associated with the screening of the surface potential by the high density carrier plasma.

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