Microwave semiconductors devices

Abstract

The field of microwave semiconductor devices has expanded greatly in recent years, both in tHe functions that may be performed and the range of devices available for any function. Detector and mixer diodes have been available for many years, but the period of rapid growth began with the introduction of two terminal negative differential resistance diodes in the late 1950s and 1960s. The upper frequency limits of transistor operation have also marched steadily upwards, with millimetre-wave (above 30 GHz) operation being achieved in many laboratories before 1980.The stringent demands of microwave applications have made the field of microwave semiconductors a forcing house for the industry. Microwave requirements lead to fine geometries, accurately controlled doping and the use of different materials to perform different functions, sometimes within the same device. On the other hand, microwave monolithic integrated circuits are not very complex or, as yet, highly developed, most microwave integrated circuits'being of hybrid construction (discrete devices mounted in a passive circuit).The highest operating frequencies are achieved by the simplest devices ; mixer and detector diodes can work at over 1 THz, negative resistance diodes at a few hundred GHz, while transistors are limited to under 100 GHz at present. Future developments will exploit the properties of heterostructures in which different parts of the device are made from different semiconductor materials.The aim of this review is to provide an introduction for those new to microwave semiconductors and an update for those who have not been closely involved for some time. Following the introduction is a section on carrier transport, which is a topic of great importance in the understanding of device operation. The individual families of devices are discussed in the succeeding sections, starting with varistor and varactor diodes in which the variation of resistance or reactance is the important feature. The next section introduces transit-time phenomena and the devices designed to take advantage of them. Transit-time semiconductor devices are not widely used outside the microwave bands whereas transistors, the subjects of the next two sections, are used in virtually every electronic assembly. The appendix serves to briefly describe fabrication technology, which has a great influence on the performance and cost of microwave semiconductor devices.