When should I select GaN over GaAs for a power amplifier design?
GaN vs GaAs PA Selection
The GaN vs GaAs decision is one of the most important technology choices in RF PA design. The answer depends on a complex interaction of performance, cost, and system requirements.
Frequently Asked Questions
Is GaN always more expensive than GaAs?
Currently: yes, for the same die size. GaN on SiC: wafer cost $2,000-$5,000 per 4-inch wafer (the SiC substrate is expensive). GaAs: wafer cost $200-$500 per 6-inch wafer (mature process, high volume). Per mm² of die: GaN is 5-15× more expensive than GaAs. However: GaN power density is 5-10× higher. So the die needed for a given power level is much smaller. Net cost per watt: GaN and GaAs are increasingly comparable for power levels > 5 W. Below 5 W: GaAs is cheaper (the die size is already small, and the GaN cost premium is not offset by the size reduction). Trend: GaN on Si (instead of GaN on SiC) is emerging as a lower-cost alternative. GaN on Si uses standard 8-inch silicon wafers (much cheaper). The thermal performance is worse (Si has 3× lower thermal conductivity than SiC), but for moderate-power applications (< 10 W), GaN on Si may become cost-competitive with GaAs.
Can I use GaN for receiving (LNA)?
GaN LNAs exist but have higher noise figure than GaAs: GaN HEMT NF at 10 GHz: 0.8-1.5 dB. GaAs pHEMT NF at 10 GHz: 0.3-0.5 dB. InP HEMT NF at 10 GHz: 0.2-0.3 dB. The higher NF of GaN is due to: higher electron temperature in the channel (the high-field transport that gives GaN high power also increases the noise temperature). Higher gate leakage current (the wider bandgap helps, but the AlGaN/GaN interface has higher defect density than AlGaAs/GaAs). GaN LNAs are used when: the LNA must survive high power levels (e.g., front-end LNA without a limiter in a radar system). GaN can handle > +30 dBm input power without damage (GaAs pHEMT damages at +20-25 dBm). The NF penalty (0.5-1 dB) is traded for the survivability. This is common in military radar receivers where a jammer may illuminate the antenna with high power.
What about GaN for 5G mmWave base stations?
GaN is increasingly used for 5G mmWave gNB PAs at 28 and 39 GHz: GaN on SiC at 28 GHz: PAE ≈ 25-35% (vs 15-25% for SiGe). P_sat per element: +23 to +28 dBm (vs +15 to +20 dBm for SiGe). The higher PAE reduces the heat load per element, which is critical in a 256-element array. The higher P_sat allows the same EIRP with fewer elements (cost saving). Challenge: cost. GaN at 28 GHz requires fine gate length (100-150 nm) on expensive SiC substrates. The cost per element is higher than SiGe. Trade-off: fewer GaN elements (each with higher power) vs more SiGe elements (each with lower power and lower cost). The optimal choice depends on the total system cost (including thermal management, which is cheaper with higher-efficiency GaN PAs).