The Elimination of Tuning-Induced Burnout and Bias-Circuit Oscillations in IMPATT Oscillators

Low-frequency instabilities including noise, bias-circuit oscillations, and diode burnout at low bias currents are often encountered when tuning high-power, high-efficiency IMPATT diodes. Gallium arsenide IMPATTs, which produce high power at higher efficiency and lower noise than either silicon or germanium IMPATTs, are particularly prone to these instabilities. Microwave tuning operations in GaAs often burn out diodes at bias currents of one-half to one-quarter their thermally expected values. This problem has been so serious that it has cast doubt on the practicality of using GaAs IMPATTs at their fullest potential power and efficiency. These instabilities also make the testing and evaluation of diodes a very tedious procedure and cause the operating conditions of the oscillator to be very sensitive to microwave and bias-circuit load conditions. The cause of these problems is an RF voltage-induced negative resistance in the IMPATT diode. This negative resistance has a lowpass frequency dependence, extending from essential^ zero frequency up through several tens, and perhaps hundreds, of megahertz. The upper frequency limit is determined by the bandwidth of the microwave circuit. The magnitude and frequency dependence of the induced negative resistance are dependent on the microwave circuit tuning. It is possible to tune through very high values of negative resistance,