In smartphone radio circuits, the transceiver including the LNA, mixer and PLL is often integrated on a CMOS chip, while compound semiconductors such as GaAs HBTs are often used for the transmit PA. Which is the most appropriate main reason?

Answer: B. Scaled CMOS transistors have low breakdown voltages and have difficulty delivering large output power efficiently, whereas GaAs HBTs excel in breakdown voltage and in output and efficiency at high frequencies

Scaled CMOS can integrate many transistors cheaply and handle GHz signals, so it suits combining the LNA, mixer, PLL and even baseband processing on one chip. A PA, on the other hand, must deliver power of several hundred mW with high efficiency and linearity, and scaled CMOS with its low breakdown voltage has difficulty supporting large voltage swings, putting it at a disadvantage in efficiency and output. For this reason, GaAs HBTs are widely used for smartphone PAs, and GaN HEMTs, which are even better in breakdown voltage and heat dissipation, for high-power PAs such as those in base stations. Compound semiconductor wafers are generally smaller and more expensive than silicon, so they are used only where needed.

• A. CMOS can handle GHz signals, and most transceivers are made in CMOS.
• B. Correct. In breakdown voltage, output and efficiency, compound semiconductors have the advantage for PAs.
• C. Compound semiconductor wafers are generally smaller and more expensive than silicon, so silicon has the cost advantage.