**1. Introduction**

The revolution of electronic device in 20th century is mostly based on silicon and is regarded as the first generation semiconductor. Before the beginning of 21st century gallium arsenide (GaAs) and indium phosphide (InP) have evolved as second generation semiconductors constituting the base for the wireless and information revolution. However, at the begin of the 21st century, silicon carbide (SiC) and gallium nitride (GaN) are emerge as the wide bandgap semiconductors can work at high temperature and at high voltage and they may be regarded as third generation semiconductors used in the electronic and optoelectronic industries. Moreover the superior properties of wide bandgap semiconductors and the recent rush of research on wide bandgap semiconductor based electronic devices; one might speculate that wide bandgap semiconductors like diamond, AlN, etc. may be the future generation semiconductors. However, all the semiconductors have their vital performance in the field of information and communication.

The tremendous growth in information and communication technology has resulted in demand for millions of channels simultaneously. In order to avoid the interference between individual communications, the frequency of operation has been increased to a high value (i.e. the microwave and millimeter wave range). The advancement in solid state devices has contributed significantly towards the feasibility of modern microwave and mm-wave systems. Among several solid state devices capable of producing millimeter wave, IMPATT (IMPact Avalanche Transit Time) diode is considered as a leading source of solid-state power. The high power

generating capability and high efficiency of IMPATT diode makes it attractive both at commercial and military sectors. The device has the dominant characteristics over other microwave and millimeter wave sources both with respect to the frequency coverage and output power. Later the report of first experiment of the microwave oscillation [1] the efficiency and output power has been increased with frequency. And some of the records making output from IMPATT diode are 42 W of pulsed power at 96 GHz [2], 520 mW at 217 GHz [3] and a continuous wave (CW) power 980 mW near 100 GHz and 50 mW at 220 GHz [4] have been reported. Again IMPATT diode being fabricated from any semiconductor, it has made itself an attractive device for both theoretical as well as experimental study.

IMPATT diodes are well recognized two terminal solid-state devices to deliver sufficiently high power at both microwave and mm-wave frequency bands [5]. Silicon is the most popular base material for IMPATT diodes from the point of view of its advanced process technology [6–10]. However, GaAs is a vibrant base semiconductor for IMPATT diodes at the both microwave and mm-wave frequencies. Since early seventies, several researchers have fabricated IMPATT diodes based on GaAs and obtained higher DC to RF conversion efficiency and better avalanche noise performance of those as compared to their conventional Si counterparts [11–16].

This chapter looks at the benefits of GaAs in power electronics applications, reviews the current state of the art, and shows how it can be a strong and feasible candidate for IMPATT. It is also well known that at a given frequency the microwave and millimeter wave power output of an IMPATT diode is proportional to the square of the product of semiconductor critical field and carrier saturation velocity. Again heat generation and dissipation in IMPATT diodes can severely limit the performance of IMPATT diodes. GaAs is, therefore, an ideal semiconductor for IMPATT diodes over Si, because it offers higher (i) critical electric field, (ii) carrier saturation velocity and (iii) thermal conductivity. These properties can lead to high-performance IMPATT diodes for microwave and millimeter wave applications. Some theoretical work using drift–diffusion methods for IMPATT device simulation confirmed that GaAs devices operated in the pulsed mode can offer very high power in the short-wavelength part of the millimeter range. So in this chapter, we have explored the device properties of GaAs IMPATT diode using a small signal model for mm-wave applications around the design operating frequencies of 94, 140, 220 and 300 GHz. The power performance and noise behavior of the diode is determined and compared with the Si base double drift region (DDR) IMPATT diode.
