Explain why GaAs and GaN suit UHF and microwaves, what MMICs are, and why surface mount beats leaded packages at RF.
Syllabus: Semiconductor materials and packages for RF use
Practise the 12 pool questions for this group. Answers are shuffled, and every one comes with a one-line "why".
Electron mobility is how quickly electrons move through the crystal. Gallium arsenide (GaAs) has higher electron mobility than silicon, so it is useful at UHF and above. In MMICs, gallium nitride (GaN) supports the highest frequency of the choices. Silicon dioxide and silicon nitride are insulating layers, not the active material.
RememberHigher electron mobility: GaAs at UHF and up. Highest MMIC frequency: gallium nitride.
A monolithic microwave integrated circuit (MMIC) is a ready-made amplifier with controlled gain, low noise figure and constant 50 Ω input and output impedance. It connects with microstrip lines. The common type is powered through a resistor and/or RF choke on the output lead. Noise figure is in dB, and a good low-noise UHF preamp is about 0.5 dB.
RememberMMIC: 50 Ω in and out, microstrip, bias through resistor/choke on the output lead. Good preamp: 0.5 dB noise figure.
DIP means dual in-line package: two rows of pins on opposite sides, passing through holes in the board. PLCC, BGA and SOT are surface-mount packages.
RememberDIP = two rows of pins, through-hole. PLCC, BGA, SOT = surface mount.
Every lead adds stray inductance and capacitance. At UHF and up even a few millimetres matter. Surface-mount parts have tiny leads, so less parasitic inductance and capacitance, plus smaller area and shorter traces. DIP ICs are avoided at UHF and higher because of excessive lead length.
RememberSurface mount: shortest leads, least parasitics. DIP: too much lead length for UHF.