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Carrier transport in the drift region of read-type diodes

The transport properties of the drift region significantly impact the conversion efficiency of Read-type IMPATT diodes. A detailed numerical study has been undertaken to gain insight into the carrier transport under large-signal conditions and the roles of depletion-width modulation and the carrier-induced electric field. A phase delay in the peak of the avalanche current results from assuming a sinusoidal voltage rather than a sinusoidal field and is incorporated in the model. induced current waveforms, transport factors, diode admittances, and conversion efficieneies are presented as functions of RF level, bias current density, and the small-signal transit angle. A region of partial collection of the drifting carriers exists between regular IMPATT operation and the onset of the premature collection mode, which provides a smooth transition between these two modes. The effects of premature collection are most pronounced at long transit angles where the corresponding large increase in the transport factor at a high RF level will give high-efficiency operation, but also a large hysteresis in the tuning characteristic. The displacement current has an appreciable effect on the induced external current under these conditions. The presented results aid the understanding of tuning behavior and tradeoffs in design for Read diodes.