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Current collapse is a performance degradation phenomenon unique to gallium nitride (GaN) devices (especially AlGaN/GaN HEMTs) under high-voltage stress. It manifests as a decrease in device output current and an increase in on-resistance, severely affecting power output and reliability.
Its core mechanism is as follows: when the device is subjected to high-voltage stress between the drain and gate, hot electrons in the channel tunnel to the surface of the barrier layer and are captured by surface electron traps in the gate-drain region. These trapped negative charges form a "virtual gate," causing the gate depletion region to expand laterally and reducing the concentration of high-mobility two-dimensional electron gas (2DEG) in the channel, ultimately degrading the device's current output capability.
This effect typically occurs in the high drain voltage region (e.g., around 20V) and is closely related to the sweep method and the voltage stress threshold. In RF or high-frequency applications, the charging and discharging speed of trap charges cannot keep up with the signal frequency, further exacerbating current collapse and leading to dynamic on-resistance degradation.
How can the current collapse characteristics of GaN devices be verified through measurement?
Users can measure the current collapse waveform using the double-pulse test method, which is the most commonly used electrical parameter test method in the industry. The specific procedure is as follows:
Set the double-pulse waveform parameters
Use a "static bias - test bias" double-pulse sequence:Static bias phase: Set the device gate voltage (Vg Q) to -5V (off state) and the drain voltage (Vd Q) to 20V (simulating high-voltage stress), with a duration of approximately 10⁻⁷ seconds, allowing surface traps to fully capture electrons.
Test bias phase: Quickly switch to the on state (Vg=0V, Vd=5V), with a duration of approximately 10⁻⁴ seconds, and observe the dynamic change in drain current.
Measure the current transient response
During the test bias phase, record the drain current (Id) versus time curve in real time. If current collapse exists, the current will first remain at a lower initial value and then gradually recover to a stable value. The time constant of the recovery process is related to the release rate of trapped charges.Analyze the waveform characteristics
Current collapse magnitude: Calculate the percentage of the difference between the initial current and the stable current relative to the stable current (e.g., up to 13.6% for conventional structures, reducible to 0.8% for optimized structures).
Recovery time: Observe the time for the current to recover from the initial value to the stable value, which reflects the release speed of trapped charges. In RF applications, it is necessary to check whether this time matches the signal frequency.
In addition, dynamic on-resistance testing can be used as an auxiliary verification: quickly turn on the device after high-voltage stress and measure the transient change in on-resistance (Rdson). Current collapse causes Rdson to temporarily increase, and its variation trend is consistent with the current waveform. The IWATSU DS-8104_SS-350 dynamic on-voltage drop test system can clamp voltages in three automatic ranges of 1.5V, 2.5V, and 5.5V, demonstrating exceptional measurement capability even for extremely small voltage deviations of the device.
