What is the typical bias sequence for a GaN MMIC to avoid damage?
GaN Bias Sequencing
Depletion-mode GaN HEMTs are normally-on devices: with Vgs = 0V, the channel is fully open and the transistor draws maximum current (Idss). This is fundamentally different from silicon MOSFETs and enhancement-mode GaAs devices which are normally-off. The depletion-mode behavior creates a potential damage scenario if the drain voltage is applied before the gate is negatively biased.
| 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 |
- Performance verification: confirm specifications against the application requirements before finalizing the design
- Environmental factors: temperature range, humidity, and vibration affect long-term reliability and parameter drift
- Cost vs. performance: evaluate whether the application demands premium components or standard commercial grades
- Interface compatibility: verify impedance, connector type, and mechanical form factor match the system architecture
Frequently Asked Questions
What happens if I get the sequence wrong?
Applying Vds before Vgs: the transistor draws maximum current (Idss), potentially causing thermal damage or electromigration failure. Power dissipation = Vds × Idss can be 10-50× the normal operating power. Even brief exposure (milliseconds) can degrade or destroy the device.
Do all GaN devices need sequencing?
Depletion-mode GaN HEMTs (the majority): yes, always gate-first. Enhancement-mode GaN (E-mode, normally-off): no sequencing needed because the channel is off with Vgs = 0V. E-mode GaN is used in some power switching applications but is less common in RF.
How do I implement sequencing?
Use a sequencing IC (TI LM3880) to control the enable pins of the gate and drain power supplies. Gate supply enables first, stabilizes, then drain supply enables. On power-down, the drain supply disable assertion triggers before the gate supply. Add a monitor circuit that shuts down the drain if the gate supply fails.