FAQ - E-mode HEMTs

• What is recommended gate drive voltage?

+7V is recommended for gate drive. 

• What is the absolute maximum gate to source voltage rating?

+/-10V. 

• Can I use 5V for gate drive

7V gate drive voltage is recommended for the maximum efficiency point, where the Enhancement mode HEMT (E-HEMT) is fully enhanced and reaches its optimal efficiency point. 5V gate drive can be used safely but may result in a lower operating efficiency. 

• Do I have to use negative gate voltage for turning it off?

No. Inherently GaN Systems E-HEMT does NOT require negative gate bias to turn off. Negative gate bias insures safe operation against the voltage spike on the gate but at same time it increases the reverse conduction loss. For more details please refer the gate driver application note GN001 How to Drive GaN Enhancement Mode Power Switching Transistors. 

• How can I control the device slew rate?

Similar to a silicon MOSFET, external gate resistor can be used to control the switching speed and slew rate. 

• What is the recommended gate resistance to start with?

It is recommended to start with a gate resistor in the range between 20Ω to 47Ω, then adjust the resistor to achieve the desired slew rate. Lower turn-off gate resistance is recommended for better immunity to cross conduction. Please see gate driver application note (GN001) for more details. 

• What is the Source Sense (SS) pad and how to use it?

The source sense pad is a kelvin connection to the source. It is designed to be used by gate drive circuit to exclude the common source inductance from gate drive loop. To make proper use of the source-sense connection, the gate drive power supply (either isolated or shared with control circuit) ground return must be referenced to SS pad using star point connection. Unlike power devices that use wire bonds internally for connections, our GaNPX™ packaging utilizes no wire bonds so our source connection is already very low inductance. Utilizing the Source Sense pad can improve drive performance but may not be necessary in all systems. 

• Can I use standard MOSFET gate driver?

Yes, standard MOSET driver can be used as long as it supports 7V for gate drive and its ULVO is suitable for 7V operation. Gate drivers with low impedance and high peak current are recommended for fast speed switching. 

• Can I save money on my gate driver?

Yes, GaN Systems E-HEMTs have significantly lower QG when compared to equally sized (RDSON) MOSFETs so high speed can be reached with smaller and lower cost gate drivers. 

• Can I use standard half bridge MOSFET/IGBT driver for high side gate drive?

Many half bridge MOSFET drivers are not compatible with 7V gate drive for GaN enhancement mode HEMT due to their high under-voltage lockout threshold. Also simple boostrap method for high side gate drive may not be able to provide tight tolerance on the gate voltage. Therefore special care should be taken when you select and use the half bridge drivers. Alternatively isolated drivers can be used for high side device. Please see gate driver application note GN001 for more details. 

• TI has a special half bridge driver designed for GaN enhancement mode FET (LM5113), can I use it?

LM5113 is not recommended since it does not support 7V gate drive voltage and internally limits gate bias to 5.2V. This driver was created to overcome severe limitations of earlier gallium nitride devices. Since our devices can tolerate +/-10V and is optimized at 7V, it’s highly likely that the existing gate drive circuits can be easily modified to operate at 7V. 

• Do you have a design example for the gate driver?

Please see gate driver application note GN001 section “Gate driver example”. 

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• What is the substrate pad (pin 5) and how do I use it?

The pin 5 is connected to the die substrate and used as thermal pad for cooling. It is electrically insulated from drain or source connection on the package. In the circuit always connect substrate (thermal pad) to the source for best device performance. 

• What is most effective way to cool the device?

For GSxxxxP/B bottom side cooling with a heat sink below the PCB and copper-filled vias underneath the part is the most effective cooling method. Additional cooling ca nbe achieved by heat sinks on both the top and bottom-side of the parts. Please see application note GN005 for details. GSxxxxT parts are designed to be cooled by a heat sink on top of the part. 

• What is the recommended PCB layout for optimized thermal performance?

See thermal PCB layout application note GN005. 

• Can I attach a heat sink on the top side for cooling?

Cooling through the thermal pad is recommended as it has the best heat transfer. The top side of the package as higher thermal resistance but it also help the overall thermal performance and can only be used as additional to the bottom side cooling if needed. Please note that top side of the device is covered by a layer of soldermask and silkscreen printed on the top. It has uneven surface and is not designed to withstand high voltage or provide safety insulation. If a heatsink is to be attached on the top case, a layer of interface material with HV insulation must be added between heatsink and device case to fill the gap and provide safety insulation. GaN Systems has just released products with top-side cooling. 

• Are the drain and source pads as thermally conductive as the thermal pad? Can thermal resistance be reduced by adding copper under drain and source pads?

Drain and source pads are not as thermally conductive as thermal pad. Adding more copper under these two pads may reduce the packaging temperature at the edge but it does not affect the total junction to ambient thermal resistance. 

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• Does the GaN enhancement mode HEMT have a body diode? How do the reverse conduction characteristics compare with the silicon MOSFET?

The GaN Systems enhancement mode HEMTs do not have intrinsic body diode and there is zero reverse recovery charge. The devices are naturally capable of reverse conduction and exhibit different characteristics depending on the gate voltage. At the system level the reverse conduction capability can be advantage compared to IGBTs because no anti-parallel diodes are required. 

On-state (VGS = +7V): 

The reverse conduction characteristics of a GaN Systems enhancement mode HEMT in the on-state is similar to that of a silicon MOSFET, with the IV curve symmetrical about the origin and it exhibits a channel resistance RDS(ON) similar to forward conduction. 

Off-state (VGS ≤ 0V): 

The reverse characteristics in the off-sate is different from silicon MOSFET as the GaN device has no body diode. In reverse direction, the device starts to conduct when the gate voltage in respect to the drain (VGD) exceeds the gate threshold voltage and then the device exhibits a channel resistance. It can be modeled as a “body diode” with slightly higher VF and no reverse recovery charge. If negative gate voltage is used in off-state, the source-drain voltage must be higher than Vth+VGS(OFF) in order to turn the device on. Therefore a negative gate voltage will add to the reverse voltage drop “VF” and hence increase the reverse conduction loss. 

 

• How much safety margin does the device have above the blocking voltage rating?

The blocking voltage rating BVDS is defined by the drain leakage current. The hard (unrecoverable) breakdown voltage is typical about 30% higher than rated BVDS, or 2x the system DC voltage in most cases. As a general practice, the maximum drain voltage should be de-rated in a similar manner as IGBTs or silicon MOSFETs. 

• Does the maximum drain-to-source voltage rating change with junction temperature?

The maximum VDS voltage has negative temperature coefficient. 

• What is the absolute maximum rating for Drain-to-Source voltage when negative voltage is applied to the Gate?

The absolute maximum drain-to-source rating is 650V and doesn’t change with negative gate voltage. 

• What is avalanche breakdown rating?

All E-HEMT GaN transistors do not avalanche and thus do not have an avalanche breakdown rating. GaN Systems’ transistors can withstand voltages higher than the rated voltage, typically 30% higher. 

• What is the temperature dependence of gate-source threshold voltage?

The GaN device exhibit a threshold voltage with a slightly positive temperature coefficient. 

• Can I parallel the GaN devices?

Yes. GaN enhancement mode HEMT has positive temperature coefficient on-state resistance which helps the current balance. However special care should be taken in the driver circuit and PCB layout since the device switches at very fast speed. It is recommended to have symmetric PCB layout and equal gate drive loop length on all paralleling devices to ensure balanced dynamic current sharing. 

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• What is the package material?

The package material is high temperature epoxy-based PCB material, which is similar to FR4 but has higher temperature rating, thus allowing our devices to be specified to 150°C. 

• How many reflow solder cycles can a device handle?

The device can handle at least 3 reflow cycles. 

• What is the recommended reflow temperature profile?

It is recommended to use reflow profile in IPC/JEDEC J-STD-020 REV D.1 (March 2008)

The basic temperature profiles for Pb-free (Sn-Ag-Cu) assembly:

• Preheat/Soak: 60-120 seconds Tmin = 150°C, Tmax = 200°C.
• Reflow: Ramp up rate 3°C/sec max. Peak temperature is 260C and time within 5°C of peak temperature is 30 seconds.
• Cool down: Ramp down rate 6°C/sec max.

For SnPb assembly preheat to 120-150°C and use peak temperature 235°C for reflow. 

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