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X band IMPATT diode chips have been efficiently combined in parallel, in series on diamond heat sinks, and in series-parallel on diamond heat sinks. This paper summarizes experimental and analytical work performed over a four-year period and places some of the results in perspective with respect to practical applications. Several device types have shown to be compatible with series geometries. Analysis has shown that some device types, when connected in series, form a combination which is neither open-circuit nor short-circuit stable, an intrinsically unstable condition. It has been shown further that capacitors of practical size can be placed in parallel with each chip of such an assembly to prevent the occurrence of the unstable condition. Thus unique problems reported earlier with some types of IMPATT's are understood and can be prevented. These experimental and analytical results appear to eliminate any hypothetical barrier to the routine series power combining of at least several types of IMPATT device chips.

The design and performance of broad-band TRAPATT amplifiers in X band are described along with a discussion of critical circuit parameters. Bandwidths of 10 percent and peak powers up to 38 W have been achieved using coaxial circuits with single and multichip diodes.

The results of experiments using TRAPATT diodes connected in series at 0.5, 2, and 8 GHz are described. These experiments demonstrate that successful series operation of TRAPATT's at frequencies up to at least 2 GHz can be achieved in a configuration suitable for long pulsewidth or CW operation. An 8-GHz operation of series stacked TRAPATT'S yielded high power outputs at the same efficiency achieved with single devices but no well heat sunk configuration was found that yielded good efficiency.

This correspondence presents a simple circuit which allows direct combining of the power obtainable from several avalanche diodes. The circuit does not require extremely close matching of dc or RF diode characteristics, and no particular isolating networks such as hybrid combiners are necessary. CW power output exceeding 4 watts at 7 GHz and 3 watts at 9 GHz has been demonstrated in a device employing five diodes. The number of diodes which can be combined using this technique is limited by geometric and heat sink considerations. Descriptions of typical single-diode and multiple-diode oscillators are given along with equivalent circuits. The circuits employ resistors or resistor networks to suppress low-frequency oscillations and undesired resonances which occur when several oscillators are coupled together. Negligible insertion loss is incurred at the design output frequency. Measured performance is given on similar single- and multiple-diode oscillators. Data include frequency, power, AM noise, FM noise, temperature, and loaded Q.

Experiments with four-mesa silicon p+-n-n+ IMPATT diodes have shown power saturation and reduced efficiency when connected and packaged in electrically asymmetrical configurations. The need for electrical symmetry is illustrated by experiments wherein seemingly trivial asymmetries caused severe saturation of the power output.

Gallium-arsenide Schottky low-high-low IMPATT-diode chips have been successfully combined in series. A maximum c.w. power output of 22.5 W has been repeatedly obtained in X-band by using six 4-W 0.2-mm-diameter mesa GaAs devices. After careful preselection of GaAs chips (yield ≃ 20%), the GaAs devices combined more readily than did silicon p+-n-n+ impatt chips.

TRAPATT diodes for X band c.w. operation have been made using both double-epitaxy and diffusion techniques. These p+nn+ devices are fabricated in ring configurations and mounted on type-IIA diamond heat spreaders. A c.w. TRAPATT performance of 3.3 W and 18.5% efficiency has been achieved at 8 GHz using these diodes.