Power-impedance-frequency limitations of IMPATT oscillators calculated from a scaling approximation

The obtainable CW power of silicon IMPATT oscillators, as a function of frequency, is calculated by scaling from reference results. The analysis differs from previous treatments in that the microwave circuit impedance limitation, as observed experimentally, is utilized simultaneously with thermal impedance limitations to uniquely determine device diameter, operating currents, and output power. Results are presented for single and multiple (parallel) units on copper and diamond mounting studs, and for both single (p⁺-n-n⁺) and double-drift-region (p⁺-p-n-n⁺) structures. Obtainable power falls off essentially as 1/f until an ultimate (nonthermal) space-charge-limited current density is reached. Beyond this point the obtainable power varies as f^-2.14. The calculated results on single-drift-region structures are in agreement with experimental observations over the range of frequencies from 13 to 55 GHz, and the analysis predicts an obtainable power of 300 mW at 110 GHz for a double-drift-region structure with 10 percent conversion efficiency.