The output power from 12 IMPATT diodes has been combined in a single-cavity multiple-device oscillator. The oscillator is free from the well-known moding problem of multiple-device oscillators. The objective of this paper is to present the oscillator-circuit theory, which clearly indicates why this particular circuit configuration can give a stable operation free from moding problems. To handle the formidable equations necessary for the analysis, the eigenfunction approach is extensively used. The condition for stable operation, the noise performance, and the locking behavior of the oscillator are all discussed. The noise performance is similar and the locking behavior identical to those of single-device oscillators.

The Single-Cavity Multiple-Device Oscillator

This paper summarizes the current state of the art of silicon CW millimeter-wave IMPATT diodes and oscillators in the frequency range from 30 to 250 GHz. Design procedures, fabrication, and packaging technology are reviewed, and the current performance of diode oscillators is reported. A brief account of present device reliability is also presented. The contrast between maturing device technology below 100 GHz and largely laboratory-based technology at higher frequencies is discussed. Finally, a prognosis of future developments is offered.

Millimeter-Wave CW IMPATT Diodes and Oscillators

Microwave avalanche diodes of various types (IMPATT, TRAPATT, etc.) can generate power sufficient for microwave receivers and some transmitters. This brief review summarizes mechanisms of operation, power output, efficiency, noise, and some important features of design and fabrication.

Microwave avalanche diodes

This paper utilizes a simplified physical model to describe TRAPATT (TRApped Plasma Avalanche Triggered Transit) operation. By yielding on computational accuracy, a complete high-efficiency device design is generated and the dependence of operation on physical parameters is elucidated. The extreme complexity of the precise differential equations describing TRAPATT operation has made the calculation of a single diode-circuit configuration a tour de force. However, by observing the important features of such a solution, a simplified approach giving realistic answers has been evolved. A theoretical device design has been evolved. This design provides device width and impurity density as a function of TRAPATT frequency, and indicates a decreasing degree of "reach through" with increasing frequency. In addition, the explicit dependence of width and impurity density on the diode's reverse saturation current has been obtained. The launching of the avalanche zone through the diode, and, in particular, the limitations implicit in the recovery to a swept-out state, are of broad significance in other types of diodes, particularly p-i-n switches and "snap" diodes.

Device physics of TRAPATT oscillators

This paper describes and analyzes the circuits which have been used successfully for TRAPATT oscillator studies. The results lead to a better understanding of the TRAPATT oscillator and yield a simple model of the oscillator which is useful for circuit design. The circuit characteristics of an experimental TRAPATT oscillator are determined from measurements on the circuits and from equivalent circuit model calculations. The following conclusions can be drawn from the analysis. First, the avalanche diode requires sufficient capacitance near the diode to sustain the high-current state required for TRAPATT operation. Secondly, at a distance from the diode corresponding to approximately one half-wavelength at the TRAPATT frequency the transmission line containing the diode should be terminated by a low-pass filter. The function of the filter is to pass the TRAPATT frequency and to provide a shorting plane for the harmonics of that frequency. Finally, on the load side of the filter, tuning for the TRAPATT frequency is required. The model of the circuit described above suggests a simple explanation of the diode-circuit interaction in a TRAPATT oscillator. Simplified waveforms suggested by the model have been used to calculate power out-put, efficiency, dc voltage change, and RF impedance for the oscillator. The results agree within a few percent with those obtained for an experimental oscillator. An important conclusion of the analysis is that the high-efficiency operation of avalanche diodes at frequencies in the UHF range can be explained by the TRAPATT theory, even though the trapped-plasma or low-voltage state may last only 1/20th of the oscillation period.

Circuits for High Efficiency Avalanche Diode Oscillators

Pulsed operation of Ge avalanche diodes has produced oscillations with efficiencies exceeding 40 percent at frequencies of 2-3 GHz. Computer simulation of diode and circuit behavior has provided an explanation for the observed results.

High-efficiency oscillations in germanium avalanche diodes below the transit-time frequency

The advantages of a double-drift-region avalanche diode oscillator are discussed. Conventional structures (p+nn+or n+pp+) are essentially single-drift-region devices in that transit-time delay (for IMPATT mode) and zone transit (for TRAPATT mode) occur in a single region of one impurity type. The proposed structure (p+pnn+) has two drift regions and is essentially two complementary avalanche diode oscillators in series.

Double-drift-region (p + pnn + ) avalanche diode oscillators

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.

D.L. Scharfetter

Papers

Device physics of TRAPATT oscillators

High-efficiency oscillations in germanium avalanche diodes below the transit-time frequency

Double-drift-region (p + pnn + ) avalanche diode oscillators

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

Computer simulation of low-frequency high-efficiency oscillations in germanium IMPATT diodes