A Unified Treatment of the Impedance of Transit-Time Devices

This paper proposes a simplified treatment of microwave transit-time devices by dividing the device operation into two regions: the injection and the drift regions. This division may simplify the mathematical formulation and bring out the physical mechanism of the operation more clearly; but it also imposes a limitation that only such devices in which this division can be drawn clearly may be analyzed. In other words, the carriers must be injected with a saturated velocity when entering the drift region. Fortunately, most microwave solid-state devices assume a saturated velocity. In fact, even in a vacuum diode in which the carrier velocity is constantly varying, we can still use this approach by modifying the formulation to include a continuous injection process. We assume an idealized simple two-terminal device. A simple formula for the impedance of the drift region due to the transit-time effect is derived. It includes a term to account for the injection phase of the operation. Different devices are differentiated by the injection mechanism which is characterized by its amplitude and phase. Examples for computing impedances in IMPATT, BARATT, TUNNETT, and vacuum diode devices are illustrated. This paper concludes with a discussion of a futuristic heterojunction device as a natural extension of the method of analysis. It is believed that the present approach will give students a clearer physical picture of the device operation without complicated mathematical derivations to obscure the essentials.