1
Wide-Bandgap Semiconductor Device Technologies for High-Temperature and Harsh Environment Applications

Md. Rafiqul Islam1 Roisul H. Galib2 Montajar Sarkar1 and Shaestagir Chowdhury3

1Bangladesh University of Engineering and Technology (BUET), Department of Materials and Metallurgical Engineering, Old Academic Building, Zahir Raihan Road, Dhaka, 1000, Bangladesh

2University of California San Diego, Department of Mechanical and Aerospace Engineering, La Jolla, CA, 92093, USA

3Portland State University, Department of Mechanical and Materials Engineering, OR, 97291, USA

CHAPTER MENU

Introduction, 1

Crystal Structures and Fundamental Properties of DifferentWide-Bandgap Semiconductors, 3

Devices of Wide-Bandgap Semiconductors, 10

Conclusion, 25

1.1 Introduction

Silicon carbide () has become the preferred semiconductor material for harsh environment sensing applications, induction heating, photovoltaics, downhole oil development, and hybrid and electric vehicles because of its wide-bandgap energy (3.2 eV for 4H-SiC), excellent chemical and thermal stability, and high breakdown electric field strength (~2.2 MV cm-1) [1-3]. Particularly in sensors and electronic systems which can operate in the temperature range 300-600 °C, are required for in situ monitoring of fuel combustion and subsurface reservoirs (i.e. deep well drilling), and for outer space exploration [3]. The use of semiconductor devices that can operate properly at such high temperatures would not only minimize the need for expensive and large cooling systems but also provide for improved system reliability [4]. SiC also has gained popularity as a material for both unipolar and bipolar power device applications under high-power, high-frequency and high-temperature conditions. Besides, high-temperature pressure sensors have been proposed and implemented using SiC-based piezoresistive devices and have demonstrated sensing capabilities between 350 and 600 °C [5]. Piezoresistive sensors, however, exhibit strong temperature dependence and suffer from contact resistance variations at elevated temperatures. Moreover, SiC has a longer lifetime, since it is an indirect bandgap material. The high lifetime yields a long diffusion length, and thus a high base transport factor. SiC is replacing Si as a semiconductor since SiC has the capability to be used in high-temperature, high-speed, and high-voltage applications. Most current SiC-based electronic devices are fabricated using either 4H- or 6H-SiC due to the aforementioned shortcoming of 3C-SiC. Between 4H- and 6H-SiC, 4H-SiC has substantially higher carrier mobility, shallower dopant ionization energies, and low intrinsic carrier concentration. Thus, it is the most favorable polytype for high-power, high-frequency, and high-temperature device applications. In addition, 4H-SiC has an intrinsic advantage over 6H-SiC for vertical power device configurations because it does not exhibit electron mobility anisotropy, while 6H-SiC does [6]. Indeed, many SiC device fabrication efforts have shifted toward 4H-SiC as it has become more readily available. For example, the unipolar 4H-SiC junction field-effect transistor () and the metal semiconductor field-effect transistor () are seen as suitable structures for integrated circuit () development since they do not suffer from gate oxide degradation.

Apart from SiC, gallium nitride () has gained much interest since it is naturally a high bandgap emitter. GaN not only has a higher bandgap, 3.4 eV, than SiC but it also has a high thermal conductivity, 1.3 W cm-1 °C-1. GaN-based field-effect transistors (s) such as high-electron mobility transistors (s) and metal-oxide-semiconductor () channel HEMTs have shown outstanding properties in terms of achieving high breakdown voltage, low on resistance, and high switching frequency [7,8].

In the field of light emitting diode () devices, several trends are pushing research into new materials to improve their efficiency. LED efficiency is increasing by strain control of epitaxial films that compose the LED's active region structure [9]. Heterostructures of GaInN and GaN are used to produce a strain-relieving layer located beneath the active region [10]. Moreover, implementation of LED driver circuits using GaN-based FETs can potentially increase their efficiency and improve switching frequencies.

Wide-bandgap emitters are also bringing semiconductor technology to full color displays [11]. For the first time, all three primary colors can be generated using semiconductor technology, which promises to allow the reliability, compactness, and other desirable attributes of semiconductors to be applied to this important technological market [11]. Besides, diluted magnetic semiconductor () Ni2: ZnO are ferromagnetic at high temperatures, which is attributed to the increase in domain volumes and the generation of lattice defects upon aggregation [12].

In this chapter, we focus on the crystal structures of SiC, GaN, and AlN. Then, we correlate their structures with their applications in JFET, metal oxide semiconductor field-effect transistor (), MESFET, etc.

1.2 Crystal Structures and Fundamental Properties of Different Wide-Bandgap Semiconductors

1.2.1 Relevant Properties of GaN, SiC, and Si

Table 1.1 compares the relevant material properties of SiC and GaN with Si, the three most popular semiconductor device technologies for high-temperature applications. Most notable are the large thermal conductivities, breakdown voltages, and saturation velocities of SiC and GaN. The device maximum operating temperature parameter is calculated as the temperature at which the intrinsic carrier concentration equals 5 × 1015 cm-3 and is intended as a rough estimate of the bandgap limitation on device operation. More important for the eventual maximum operating temperature is the physical stability of the material.

Table 1.1 Comparison of important semiconductors properties for high-temperature electronics.

Figure 1.1 Two types of tetrahedrons forming the building blocks of all SiC crystals, with each tetrahedron consisting of one Si atom and four covalently bonded nearest-neighboring C atoms.

Figure 1.2 Three types (A, B, C) of Si-C double-atomic layer arrangement along the c-axis (stacking direction) through close-packed spheres. The c-axis is normal to the paper plane.

1.2.2 Structure of SiC

1.2.2.1 Polytypism in SiC

SiC is the most prominent of a family of close-packed materials that exhibit a one-dimensional polymorphism called polytypism. The SiC polytypes are differentiated by the stacking sequence of the tetrahedrally bonded Si-C bilayers, such that the individual bond lengths and local atomic environments are nearly identical, while the overall symmetry of the crystal is determined by the stacking periodicity. Similar to silicon, SiC is a covalently bonded semiconductor. In the crystalline form, each silicon atom is covalently bonded to four neighboring carbon atoms to form a tetrahedron (Figure 1.1) and vice versa. There are two types of tetrahedrons in the SiC crystal. The first type is obtained by rotating another tetrahedron along its c-axis by 180°, and one type of tetrahedron is the mirror image of the other when the c-axis is parallel to the mirror. The c-axis denotes the direction normal to the Si-C double-atomic layers. In each layer, the silicon (or carbon) atoms have a close-packed hexagonal () arrangement. There are three types of sites (named A, B, C) in arranging the Si-C double-atomic layers, and each layer is...