International Journal of Microwave Science and Technology

Volume 2016, Article ID 4370345, 9 pages

http://dx.doi.org/10.1155/2016/4370345

## Comparative Assessment of GaN as a Microwave Source with Si and SiC for Mixed Mode Operation at Submillimetre Wave Band of Frequency

Electron Devices Group, School of Physics, Sambalpur University, Jyoti Vihar, Burla, Sambalpur, Odisha 768019, India

Received 9 September 2015; Accepted 11 January 2016

Academic Editor: Salvador Sales Maicas

Copyright © 2016 Pranati Panda et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

#### Abstract

The potentials of GaN, SiC, and Si for application as microwave sources in mixed tunnelling avalanche transit time mode operation at submillimetre wave (sub-mm wave) frequency around 0.35 terahertz (THz) are investigated using some computer simulation methods. Design criteria to choose width, doping concentration, and area are highlighted. From the results of our simulation we observed that the Si diode produces the least power output of 41 mW followed by the GaN diode with 760 mW and the SiC diode with 2.89 W. In addition, the GaN diode has more noise than the SiC diode (by 5 dB) as well as the Si diode (by 10 dB). The drastically different performance between the GaN and the SiC diode is attributed to the incorporation of disparate carrier velocity in GaN which were not being used by other authors. In spite of the low power and high noise of the GaN compared to the SiC diode, the presence of several peaks in the mean square noise voltage curves and the existence of several minima in the noise measure curves would open a new direction in the design of GaN low-noise ATT diodes capable of multifrequency tuning like a DAR diode.

#### 1. Introduction

The potentials of GaN for avalanche transit time (ATT) devices have been explored by several authors [1–3]. But they are based on simulation results of symmetric diode structures where the hole saturation velocity is assumed to be the same as the electron saturation velocity. This assumption is however incorrect in view of reports [4, 5]. In report [4], Albrecht et al. have used Monte Carlo simulations of electron transport based upon an analytical representation of the lowest conduction bands of bulk, wurtzite phase GaN to develop a set of transport parameters for devices with electron conduction in GaN. On the other hand, in report [5], Oǧuzman et al. have calculated the hole saturation velocity using an ensemble Monte Carlo simulator, including the full details of the band structure, and numerically determined phonon scattering rate based on empirical pseudopotential method. They found that the average hole energies are significantly lower than the corresponding electron energies believed to be due to the drastic difference in curvature between the uppermost valence bands and the lowest conduction band [5]. The relatively flat valence band is responsible for hole heating, leading to low average hole energy and drastically low hole velocity compared to that for electrons. Thus there is a substantial difference in electron and hole velocities reported by the two groups. We for the first time used such disparate carrier velocities for the simulation of microwave properties of GaN MITATT (Mixed Tunnelling Avalanche Transit Time) diodes [6] and reported some interesting results from our preliminary study. The purpose of this paper is to substantiate our earlier work by extending the study and compare the results with those of the industry leader Si and the wide band gap rival SiC for operation as MITATT diodes in the same sub-mm wave band of frequency.

In avalanche transit time (ATT) diodes carrier velocities play an important role in generating the transit time phase delay which together with the avalanche phase delay produces the microwave negative resistance responsible for power production from the device. When the carrier velocities are equal, the electron and hole currents maintain the same phase leading the total current to preserve the required phase relationship with the voltage. This is the case with Si and SiC avalanche transit time diodes where the carrier velocities are nearly equal. But such phase relationship between the voltage and current is disturbed when the electron and hole currents develop different amount of transit time phase delays from the diode active region due to disparate carrier velocities in materials like GaN. This has an adverse impact on the performance of the GaN ATT diode. We thus feel that comparing the microwave properties of the MITATT diodes based on the three materials will not only reveal their relative merits but also uncover the effect of disparate carrier velocity on the performance of the device. To start with we present the design methods for the diodes in the next section. A brief description of the simulation method is presented in Section 3 followed by results and discussion in Section 4. Finally we conclude our paper in Section 5.

#### 2. Design Considerations

Four diode structures, three DDR (Double Drift Region) diodes based, respectively, on GaN, Si, and SiC and one SDR (single drift region) diode based on GaN, were designed for operation in sub-mm wave band at a frequency around 0.35 THz. The basic design parameters of the diode include the width, doping, and the area of cross section. The methods used to determine them are explained in the following subsections.

##### 2.1. Width of the Active Region

For the determination of width, two criteria are generally followed. In one of them the thrust is to maximise the efficiency while the other aims at maximising the diode negative resistance. It has been seen that the efficiency is maximum when the IMPATT mode transit angle function is the maximum [7]. A little amount of algebra will show that will be maximum when (the appendix contains definition of symbols)where is the transit time across the diode width, is the design frequency, and is the saturation drift velocity of charge carriers. With this, (1) can be manipulated to get an expression for the diode width as

Now we come to the second criterion for width determination. We know that to maximise the negative resistance the phase delay between the RF voltage and RF current, , should be equal to . Using this condition the expression for width becomesOnce the design frequency is decided, (2) or (3) can be used to determine the required width of the diode from the knowledge of experimental values of carrier velocities for the semiconductor under consideration. Since we have chosen same frequency of operation, and since Si and SiC have equal carrier velocities, these two materials have symmetric diode structures. But, as a result of disparate carrier velocities the structures of GaN DDR diode have become asymmetric (Table 1).