ABSTRACT
For power amplifiers, different technologies are being used or being considered: Si-lateral-diffused metal-oxide semiconductor (LDMOS) and bipolar transistors, GaAs heterojunction bipolar transistors (HBTs), SiC (silicon carbide) metal-semiconductor fieldeffect transistors (MESFETs), and GaN HEMTs. The relevant materials properties of competing materials, including GaN, are shown in Table 23.1. Because of high bandgap, high breakdown voltage, and low on resistance, the power density can be very high. The high power per unit width translates into smaller devices that not only are easier tofabricate but also offer much higher impedance. High breakdown voltage allows these to be used at high voltage. Use of high drain voltages eliminates or at least reduces the need for voltage conversion. High output impedance enables easier low loss matching. Commercial systems (e.g., wireless base stations) operate at 28 V and a low-voltage technology would need a voltage stepdown from 28 V to the required voltage. GaN devices can easily operate at 28 V. The higher efficiency that results from this high operating voltage reduces power requirements. High power and gain can allow replacement of multiple stages of amplifiers in a module by just one power amplifier die. Higher thermal conductivity (see Table 23.1) of the substrate allows high-temperature operation and simplifies cooling, an important advantage for reducing the cost and weight of cooling systems. WBG materials offer a rugged and reliable technology capable of high voltage-high temperature operation and industrial, automotive, and aircraft and defense applications. Crystal and epilayer growth, device types, and fabrication processes, as well as reliability of GaN devices and circuits, are discussed in this chapter. 23.2 Bulk Crystal Growth [2]Gallium nitride substrates are available only in a very small size. Phosphorus-or arsenic-based III-V semiconductors can be grown from melt with relative ease due to a melting point of 1238°C with an arsenic dissociation pressure of about 1 atm. At temperatures needed for GaN growth, nitrogen has very low solubility in gallium; therefore
a very high vapor pressure of N2 is needed to grow GaN. GaN has a melting point of 2500°C with a dissociation pressure of nitrogen estimated at around 45,000 atm., making it virtually unattractive for liquid-phase growth. Heteroepitaxyon substrates like sapphire and silicon was tried historically. Growth on foreign substrates has disadvantages. These are presence of defects, strain, and thus lower performance and reliability. Bulk-grown boules are a necessity for large-volume production of high-quality devices. Even epigrowth can be simplified if a native substrate is used, because elaborate and expensive buffer layers would not be needed. Potential advantages of bulk crystal availability would be low defect density and smooth morphology of the epidevice layers. Thermal conductivity of GaN is higher than that of sapphire. And electrically conductive substrates could be used for devices that can be fabricated with ohmic contacts on the backside, for example, Schottky diodes or LEDs. Since large-volume boule growth, like that used for GaAs or InP cannot be used, pseudobulk growth methods are described below. 23.2.1 Hydride Vapor-Phase Epitaxy [2, 3]Hydride vapor-phase epitaxy (HVPE) is a practical method that can be used to grow layers thick enough to be separated from the substrate. The earliest GaN substrate was grown by halogen transport vapor-phase epitaxy, which for historical reasons is more commonly called HVPE from reactions of GaCl and NH3. The main advantages of this process are user-friendly growth conditions, low pressure, relatively low growth temperature, and high growth rate. The halide epigrowth process was discussed in Chapter 4, where the source and the substrate are held at different temperatures. Figure 23.1 shows a schematic diagram of such a system for GaN. Two temperatures are shown, but more controlled temperature zones are used in practice. HCl is passed over Ga metal held at 860°C to form GaCl. GaCl flows over to the substrate zone and is directed by a showerhead to the substrate held at a higher temperature of 950°C-1050°C. Ammonia is fed through separately, and H2 is used as the carrier gas. GaCl + NH3 Æ GaN + HCl + H2
A growth rate of greater than 100 mm/hr can be achieved. Layers thicker than 200 mm can be grown, which could be separated to make a freestanding substrate for epigrowth. Semi-insulating, n-or p-type layers have been grown. Unintentional doping by silicon and oxygen from the quartz-ware is unavoidable, and that results in n-type material. Intentional Si doping can be done by using SiH4or Si2H6. For p-type doping, the standard III-V dopants can be used, namely Zn, Cd, Be, and Mg.
