Cascode GaN
Understanding Cascode GaN
Normally-Off Hybrid GaN Devices
Gallium Nitride (GaN) high electron mobility transistors (HEMTs) are wide-bandgap semiconductors that offer high breakdown voltage, low on-resistance, and fast switching speeds. However, standard GaN HEMTs are depletion-mode (D-mode) devices, meaning they are normally-on when the gate-source voltage is zero. In power electronics and RF transmitter power supplies, normally-on devices represent a severe safety hazard during startup or fault conditions. A cascode GaN configuration resolves this issue by combining the D-mode GaN HEMT in series with a low-voltage, normally-off enhancement-mode (E-mode) silicon MOSFET.
The resulting hybrid device acts as a normally-off component. When the gate of the silicon MOSFET is at 0 V, the MOSFET is off, blocking current. This forces the source voltage of the GaN HEMT to rise relative to its gate, biasing the GaN HEMT off. When a positive drive voltage is applied to the silicon gate, the MOSFET turns on, grounding the GaN source and turning the GaN HEMT on.
Gate Drive and Switching Advantages
A primary benefit of the cascode GaN device is its compatibility with standard silicon gate drivers. Enhancement-mode GaN HEMTs have fragile gates that can only tolerate a narrow voltage range (typically 0 V to 6 V) and are sensitive to voltage spikes. The cascode GaN configuration shielding the GaN gate behind the silicon MOSFET, allowing designers to use robust, standard 0 V to 12 V gate drives. This simplifies circuit design and reduces cost. However, the co-packaged design introduces internal parasitic inductance between the MOSFET and the HEMT, which must be managed to prevent high-frequency oscillations during fast switching transients.
Key Mathematical Relations
Technical Specifications Comparison
| Transistor Type | Gate Drive Voltage Range | Gate Robustness | On-Resistance (RDS(on)) | Switching Frequency Limit | Reverse Recovery Charge (Qrr) |
|---|---|---|---|---|---|
| Silicon MOSFET | -20 V to +20 V | Very High | High | Low (< 500 kHz) | High (causes loss) |
| Enhancement GaN (E-mode) | 0 V to +6 V (Strict) | Low (spikes destroy gate) | Low | Very High (> 10 MHz) | Zero |
| Cascode GaN | -20 V to +20 V (standard MOS) | High | Low | High (1 MHz to 5 MHz) | Extremely Low |
Frequently Asked Questions
Why is a cascode configuration used for GaN transistors?
It is used to convert a normally-on depletion-mode GaN HEMT into a normally-off device. This is crucial for safety in power conversion systems, ensuring the device remains off by default when control power is lost.
What are the advantages of cascode GaN over enhancement-mode (E-mode) GaN?
Cascode GaN allows the use of standard, robust gate drivers designed for silicon MOSFETs (tolerating up to 20 V gate swings), whereas E-mode GaN requires specialized gate drivers with strict voltage limits to avoid gate oxide breakdown.
What package parasitics affect cascode GaN performance?
The package parasitics of concern are the stray inductances between the silicon MOSFET and the GaN HEMT. During rapid switching, these inductances can cause gate ringing, voltage spikes, and electromagnetic interference, requiring advanced co-packaging technologies to minimize loop area.