GaAs buffer layers grown by molecular-beam epitaxy (MBE) at low temperatures (200-300 degrees C) have been successfully used to reduce sidegating in both MESFETs and MODFETs. There are, however, high concentrations of defects in the low-temperature (LT) buffers that adversely affect the high-frequency performance of precision analog and certain digital circuits. In unoptimized structures, nanosecond and microsecond transients are as large as 85 and 15% of the total voltage swing, respectively. These transients cause various detrimental effects in circuits. These effects are described. Their origin is attributed to the outdiffusion of defects from the LT buffer, and a method for optimizing the device structure for minimum sidegating and maximum high-frequency performance is presented. >