Can I determine if a QPA1015L GaN, or any other GaN transistor is undamaged with a multimeter?

RF Solutions RF Power Amplifiers
1

My QPA1015L is imbedded in an amplifier circuit. It has recently started drawing down the magnitude of Vg_pinchoff at -6.4v, and Vg_biasON at -2.8v, to Vgs = -0.64v. Ig is 5.7mA, which is causing this reduction in Vgs due to a drop in the gate current limiting resistor, and the input impedance measured with an ohmmeter on the de-embedded QPA reads around Rgs = 14ohms. A fresh new unused unit measures ~500kohm Rgs. Is my low 14 ohm measurement an indication that this unit has failed? Is the ohmeter a safe and valid test? in the old days we used curve tracers to verify biased transistor operation, but I haven’t seen one of these in many years. what is the correct way to test a GaN FET? David M

2

Yes you can check GaN devices with a multimeter, following ESD precautions whilst probing the device pins. This can be used as a basic check for functionality. Depends on the device size, but the gate should be high impedance to ground/source, from 100s of KOhm to several MOhm range. The drain to ground/source will be a low impedance in the Ohm range since these are depletion mode devices, and the channel will be open when unbiased.
As you are measuring only 14Ohms from gate to ground this does indicate that the QPD1015L has been damaged.

3

Thank you, Chris, this is valuable information. Is it possible to tell what caused this damage as evidenced by low Rgs? Could it be that my Vgs was 0v, causing full ON conduction with Vdd ON (28v with 100mA current limiting enabled in my case) and excessive Ids? David M

4

A number of failure mechanisms could cause low gate impedance, so hard to be certain on root cause. Did you measure the drain to ground impedance?

5

Chris, I did, in fact, measure Rds (Source is, of course, grounded):

QPA1015L status Rgs-+ Rsg± Rds-+ Rds±

new, uninstalled 3.8M 0.5M 1.8ohm 2.0ohm
apparently failed 16.5ohm17.5ohm >6M >6M
during power ON

note: in Rgs-+ measurement, negative ohmmeter lead on Gate, positive on Source.

David M

6

Thanks. Looks like the drain has failed open circuit, which can be due to high drain current. This can indicate incorrect bias sequencing.