The need for high power in the VHF, UHF, and microwave bands has led to transistors that can easily supply tens to hundreds of watts at RF frequencies to 10 GHz and beyond. Most of these devices are made with gallium arsenide (GaAs) or gallium nitride (GaN).
Table Of Contents
- Semiconductor Types
- Transistor Types
- GaAs RF Power Amplifers
- GaN RF Power Amplifiers
GaAs and GaN are compound semiconductors. Known as III-V semiconductors, they fall into categories of elements with either three or five valence electrons. Boron, aluminum, gallium, indium, and thallium are Category III elements. Nitrogen, phosphorus, arsenic, antimony, and bismuth are Category V elements.
Combining a Category III element with a Category V element produces a covalent bond with eight electrons, yielding a unique semiconductor. Such semiconductors have higher electron mobility than silicon, so they’re more useful at higher frequencies. GaAs and GaN have been found particularly useful for microwave power amplifiers.
You can use GaAs or GaN to make any type of transistor including the most popular, like bipolar junction transistors (BJTs) and enhancement-mode MOSFETs. But other types have emerged, such as heterojunction bipolar transistors (HBTs), metal-semiconductor FETs (MESFETs), high electron mobility transistors (HEMTs), and laterally diffused MOS (LDMOS). These transistors take advantage of the materials to produce the best amplifying and power handling capability.
HBTs use the standard BJT configuration but use different materials for the base and emitter. One popular combination, a GaAs emitter and an aluminum-gallium-arsenide (AlGaAs) base, yields very high gain at microwave frequencies out to 150 GHz.
A MESFET is essentially a JFET with a metal gate that’s used to form a Schottky junction with the main conducting channel. The operation is depletion mode, where the device is normally on and is turned off by a applying a negative gate voltage. MESFETs are usually made with GaAs and have high gain at microwave frequencies.
The HEMT is a variation of the MESFET. Also called a hetrostructure FET (HFET) or modulation-doped FET (MODFET), it is usually made with GaAs or GaN with extra layers and a Schottky junction (Fig. 1). Depletion mode is the most common configuration. The pseudomorphic or pHEMT version improves performance by using extra layers of indium to further speed electron movement. These devices work at frequencies up to 20 GHz or so.
1. The substrate in a GaN HEMT is usually sapphire or silicon carbide for best het reduction, although silicon can be used. The 2DEG means two-dimensional electron gas, a layer of gas made of electrons that can move in any direction but vertical.
The LDMOS transistor is simply a special version of the standard enhancement-mode MOSFET. Designed for high power dissipation, it can achieve hundreds of watts at frequencies to about 6 GHz. This format is used primarily with silicon devices.
Most GaAs products are ICs, specifically monolithic microwave ICs (MMICs). These MMICs are mostly low-signal-level gain blocks. Such amplifier blocks use HBTs or MESFETs, but some use pHEMTs. Frequency ranges up to 30 GHz are available. The actual high-frequency cutoff (ft) or the unity gain-bandwidth for GaAs devices is in the range of 150 GHz, although not many devices or IC amplifiers are available with that reach. Such MMICs are widely used in most microwave equipment designs including radios, satellites, radar, and electronic warfare products.
Cable TV (CATV) systems represent a major application for GaAs amplifiers. CATV systems use a mix of fiber and coax to carry digital TV signals and high-speed Internet service over a bandwidth from about 40 MHz to well over 1 GHz. The coax line feed amplifiers used along the distribution routes of long coax runs use GaAs amplifiers for their high gain and low noise characteristics.
GaAs power amplifiers are available with power levels up to about 5 W. That’s roughly the upper power limit for GaAs devices, since they cannot withstand the high voltages, currents, and heat levels of silicon or GaN. But by using multiple devices in push-pull or parallel or combining amplifier outputs in transformers or networks, power levels up to about 20 to 40 W are possible.
The most widely used GaAs power amplifiers are those for cell phones. They’re made in a variety of frequency ranges to cover the various fixed cellular bands. The power level ranges from 27.5 to 28 dBm (0.4 to 0.5 W). They’re class AB linear amplifiers with maximum power-added efficiencies (PAEs) in the 20% to 50% range.
Note: PAE is the more common measure of RF power amplifiers today. Accounting for the input power to drive the amplifier, it is the ratio of the RF power output less input power to total dc power consumed expressed as a percentage:
PAE = 100 (Pout – Pin)/Pdc
The Skyworks SKY77707/9 LTE power amplifier uses a GaAs substrate (Fig. 2). However, the transistors are HBTs made with indium-gallium-phosphide (InGaP) (Fig. 3). This is a common combination.