Tera-Hertz Photodetectors

1. Introduction

    A metal-semiconductor-metal (MSM) photodetector consists of interdigitated metal fingers on a semiconductor,and it detects photons by collecting electric signals generated by photoexcited electrons and holes in the semiconductor which drift under the electrical field applied between the fingers. MSM photodetectors can be classified according to whether their speed is intrinsically limited by the carrier transit time between the

fingers or the carrier recombination time. Certainly, the speed of the detector will be limited by the RC time constant if it is larger than the transit time or the recombination time. Usually, transit-time-limited detectors are fabricated on high-quality semiconductors and have a sensitivity several orders of magnitude higher than that of recombination-time-limited MSM Photodetectors. Furthermore, their fabrication technology is very compatible to field effect transistor (FET) fabrication. However, in the past, the transit-time-limited MSM photodetectors generally were much slower than th e recombination-time-limited MSM photodetectors, because it used to be difficult to make the finger spacing small. In the recombination-time-limited MSM Photodetectors, shorter recombination time and therefore higher speed are achieved through introducin g high-density recombination centers into semiconductor that drastically lower
sensitivity and make their fabrication less compatible with FET IC fabrication.

    MSM photodetectors are very promising candidate for applications in many future optoelectronic systems, such as optical fiber communica