Cascode GaN FET Dynamic Characterization

Cascode GaN FET Dynamic Characterization

As we’ve discussed in a previous article, dynamic characterization of Gallium Nitride (GaN) power semiconductor field effect transistors (FET) present very difficult challenges because of their higher frequency operation and multiple variations of technology. One of the popular GaN device types is Cascode GaN FET, which provides even more difficult challenges with its oscillation prone device behavior. In this article, we specifically discuss how we overcome the challenges associated with Cascode GaN FET measurements.

By Ryo Takeda, Takamasa Arai, Bernhard Holzinger, Michael Zimmermann and Mike Hawes at Keysight Technologies

Challenges for Cascode GaN FET dynamic testing

Cascode GaN FET came into marketplace earlier than the other types of GaN power devices because it can provide normally off operation and has a wider gate drive voltage range. However, circuit designers found the device is not so easy to use in an actual circuit, because it is prone to oscillate and its device characterization is very difficult to measure and get repeatable extractions. Many designers must slow down the device operation with a large gate resistor when using it in their circuit, which reduces the benefit of using a fast speed GaN power device.

Key considerations when evaluating Cascode GaN devices

There are three important components to avoid oscillation when using Cascode GaN devices. One is a snubber circuit, the second is a ferrite bead and the third is the gate resistor (Rg) dependency. The RC snubber circuit consists of a resistor and a capacitor connected in series (i.e. a simple low pass filter).  If it is attached between the drain and the source of the power FET, it can reduce or eliminate the sharp voltage rise when the FET turns off. It is important to use the RC snubber circuit to avoid oscillation. It is also important to find the right combination of resistor and capacitor by performing the double pulse test as it will be used in the actual application.

The Ferrite bead reflects and dampens high-frequency noise with its inductive behavior. If an appropriately sized ferrite bead is used, based on the noise frequency on Vgs during the double pulse test, the noise is cleanly removed. There is a chip type ferrite bead that has a spiral structure in the horizontal direction which minimizes stray capacitance and therefore, works effectively to suppress oscillation.  Application notes published from Cascode GaN FET manufacturers recommend and describe the use of ferrite beads.

Another key characteristic to evaluate is the Rg dependency. Rg limits the current flowing into gate and therefore, controls the ramp speed of Vgs. The dependency is important when designing power circuits.  However, exchanging Rg is not convenient, because the gate resistor for GaN power circuits is usually a SMD type to minimize stray inductance. Therefore, Rg should be soldered and unsoldered to measure the gate resistor dependency.

All three of the considerations described are very important for power electronics circuit design. It is desirable to have a double pulse test system that allows characterization of a device, including the ability to see effects of different RC snubber circuits, ferrite beads and Rgs.