What is the thermal management strategy for a high power GaN amplifier?
GaN Thermal Design
GaN on SiC has the advantage of the SiC substrate's high thermal conductivity (370 W/m·K versus 150 W/m·K for silicon), which efficiently spreads heat from the active device to the package base. Despite this advantage, GaN PAs generate very high heat flux densities (up to 10 kW/cm²) at the gate finger level, requiring careful thermal design at every level of the packaging hierarchy.
| Parameter | LNA | Driver | Power Amplifier |
|---|---|---|---|
| Noise Figure | 0.3-2.0 dB | 3-8 dB | 5-15 dB (not specified) |
| Gain | 10-25 dB | 10-20 dB | 8-15 dB |
| P1dB | -10 to +10 dBm | +15 to +25 dBm | +30 to +50 dBm |
| OIP3 | +5 to +25 dBm | +25 to +40 dBm | +40 to +55 dBm |
| DC Power | 10-100 mW | 0.5-5 W | 5-500 W |
Frequently Asked Questions
How many thermal vias do I need?
For a surface-mount GaN package dissipating 10W: minimum 20 thermal vias (0.3 mm diameter, 1 oz copper filled) under the exposed pad. More vias improve thermal performance but with diminishing returns above 30-40 vias. Via-in-pad construction with copper filling provides the best thermal resistance.
What heat sink material?
Aluminum: lightweight, cheap, adequate for most applications (k = 170-220 W/m·K). Copper: best thermal conductivity (k = 390 W/m·K), heavy, expensive, used for high-performance applications. Copper-tungsten (CuW) or copper-molybdenum (CuMo): CTE-matched to GaN/SiC, used as die carriers for bare-die mounting.
When do I need liquid cooling?
When forced-air cooling cannot maintain the junction temperature below the target. Rule of thumb: above 100W total dissipation in a small enclosure, liquid cooling becomes necessary. Cold plates with circulating coolant achieve thermal resistances of 0.01-0.1°C/W, far better than air-cooled heat sinks.