# Measurement Techniques of Threshold Voltage Shift and Recovery in GaN e-HEMTs

Burhan Etoz, Jose Ortiz Gonzalez, Arkadeep Deb, Olayiwola Alatise

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Burhan Etoz, Jose Ortiz Gonzalez, Arkadeep Deb, Olayiwola Alatise. Measurement Techniques of Threshold Voltage Shift and Recovery in GaN e-HEMTs. The 26th European Conference on Power Electronics and Applications, GDR SEEDS France & EPE Association, Mar 2025, Paris, France. 10.34746/epe2025-0258 . hal-05085522

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# Measurement Techniques of Threshold Voltage Shift and Recovery in GaN e-HEMTs

Burhan Etoz, Jose Ortiz Gonzalez, Arkadeep Deb and Olayiwola Alatise

School of Engineering
University of Warwick
Coventry UK

E-Mail: burhan.etoz@warwick.ac.uk, J.A.Ortiz-Gonzalez@warwick.ac.uk, Arkadeep.deb@warwick.ac.uk, O.Alatise@warwick.ac.uk

## Acknowledgements

This work has been partly funded by the Royal Society through the Royal Society Industry Fellowship.

## Keywords

«GaN e-HEMT», «Threshold Voltage», «Third Quadrant»

## Abstract

This paper presents two different techniques of threshold voltage shift ($\Delta V_{TH}$) and recovery characterisation in GaN e-HEMTs. The first technique applies a stressing gate voltage ($V_{GS}$) of defined magnitude and duration and then measures a $\Delta V_{TH}$ at different time intervals by shorting the gate-to-drain terminals of the e-HEMT while passing a sensing current trough the drain-source. In the second technique, after the $V_{GS}$ stress, a sensing current through the 3rd quadrant of the GaN e-HEMT is used to measure the 3rd quadrant voltage as an indicator of $\Delta V_{TH}$. Both measurements show the same trends. With positive $V_{GS}$ stress, $\Delta V_{TH}$ is positive (due to electron capture at the p-GaN/AlGaN interface) and then becomes negative over time due to faster electron release than hole release. The magnitude of the initial positive $\Delta V_{TH}$ is proportional to the $V_{GS}$ stress while the recovery rate is inversely proportional of $V_{GS}$ stress.

## Introduction

Threshold voltage ($V_{TH}$) shift in GaN HEMTs is more complicated than that in traditional silicon and SiC devices with traditional MOS interfaces. This is due to the fact that devices with MOS gates do not have hetero-junctions, hence, threshold voltage shift is due to charge trapping in the gate dielectric and in the semiconductor-to-dielectric interface [1-4]. Hence, positive gate

voltage ($V_{GS}$) stress in MOS gates results in negative charge trapping and positive $V_{TH}$ shifting. Likewise, negative $V_{GS}$ stress leads to positive charge trapping in the dielectric interface thereby leading to a negative $V_{TH}$ shift. However, commercially available normally OFF Schottky-Gated GaN HEMTs comprise of metal, p-GaN layer, AlGaN layer, GaN buffer layer all on a multi-layer substrate on silicon as shown below in Fig. 1. This figure shows that when the gate of the device is subjected to a positive voltage, electron injection into the p-GaN layer and hole injection into the AlGaN layer can occur with opposing effects on threshold voltage shifting. This means both positive and negative charge trapping can occur at the different interfaces of the complicated multi-layer GaN structure [5-11].

<figure><img src="image_2.png" /><figcaption>Fig. 1. Schematic picture of GaN HEMT and band diagram of the gate system</figcaption></figure>

## Threshold Voltage Characterization with 1st Quadrant Current

In this section of the paper, $V_{GS}$ stress and $V_{TH}$ measurement is performed using a Source-Measurement-Unit (SMU) from Keithley model 2602B. By using a relay and programming the SMU using LabVIEW, the configuration of the circuit can change from stress mode to measurement mode. Fig. 2(a). shows a circuit schematic of the measurement system in stress mode while Fig. 2(b). shows the schematic in measurement mode. In stress mode, the gate driver applies the stress voltage onto the DUT with the drain-source terminals of the DUT
shorted. The $V_{TH}$ is measured by shorting the gate and drain terminals and measuring the voltage drop across the drain-source while a current of 1mA is forced through the device. The picture of the set-up is shown in Fig. 2(c). where the gate driver board is shown alongside the DUT, the national instrument controller and the relays used to switch between $V_{GS}$ stress mode and $V_{TH}$ measurement mode. The recovery time (time between $V_{GS}$ stress and $V_{TH}$ measurement) is set by the SMU. Fig. 2(d). shows the entire test set-up including the SMU, the test PCB, gate driver. The device under test (DUT) is a 650 V enhancement mode GaN HEMT from GaN Systems with datasheet reference GS66508T. The device was mounted and soldered onto a custom designed interface board for interfacing with the SMU.

<figure><img src="image_3.png" /><figcaption>(a)</figcaption></figure>

<figure><img src="image_4.png" /><figcaption>(b)</figcaption></figure>

<figure><img src="image_5.png" /><figcaption>(c)</figcaption></figure>

<figure><img src="image_6.png" /><figcaption>(d)</figcaption></figure>

<caption>Fig. 2. (a). $V_{GS}$ stress set-up for GaN e-HEMTs (b). $V_{TH}$ measurement set-up for GaN e-HEMTs (c).</caption>

driver board and Controller for HTGB Test of GaN e-HEMTs (d). Test set-up for HTGB test on GaN e-HEMTs.

Constant $V_{GS}$ stress and $V_{TH}$ measurements were performed using the set-up described in Fig. 2. Using the gate driver, the $V_{GS}$ stress is applied at different magnitudes (3 V, 6 V and 7.5 V) and for different durations (1, 10 and 100 seconds). $V_{TH}$ point measurements as described in Fig. 2(b). The SMU is programmed to perform $V_{TH}$ measurements at logarithmic intervals from 1 millisecond to 1000 seconds so that the $V_{TH}$ recovery profile can be determined. The minimum time interval between stress removal and $V_{TH}$ measurement is determined by the switching speed of the relay. This minimum time interval is 2.5 milliseconds. Fig. 3(a) shows the point measurements of $V_{TH}$ at different times after the removal of 3 $V_{GS}$ stress for 1, 10 and 100 second $V_{GS}$ pulses. After the stress voltage is removed, the trapped charges are released and the threshold voltage recovers to its pre-stress value, hence, this is called the $V_{TH}$ recovery transient. Fig. 3(b) shows similar measurements for a 6 V $V_{GS}$ stress while Fig. 3(c) shows similar measurements for the 7.5 V $V_{GS}$ stress. All measurements show a positive $V_{TH}$ shift at short recovery times (below 10 seconds) with the $V_{TH}$ shift becoming negative after 10 seconds.

<figure><img src="image_7.png" /><figcaption>(a)</figcaption></figure>
<figure><img src="image_8.png" /><figcaption>Fig. 3. $V_{TH}$ recovery transient in 650 V GaN e-HEMTs for 1, 10 and 100 seconds $V_{GS}$ stress at (a) $V_{GS}$= 3 V, (b) $V_{GS}$= 6 V, (c) $V_{GS}$= 7.5 V</figcaption></figure>

What is clear from all 3 measurements is that the rate of $V_{TH}$ recovery ($dV_{TH}/dt$) follows an inverse power law with time. Fig. 4 shows the measured $V_{TH}$ recovery rate as a function of recovery time for all 3 stress voltages. This relation is expressed as the equation below

$$ \frac{dV_{TH}}{dt} = At^{-k} $$

<figure><img src="image_9.png" /><figcaption>Fig. 4. Rate of initial $V_{TH}$ recovery as a function time</figcaption></figure>

## Threshold Voltage Characterization with 3rd Quadrant Current

In this section of the paper, experimental measurements using a different technique is used to measure the threshold voltage shift in GaN e-HEMTs. This technique, which was introduced in [1, 20] has the advantage of not using high-cost SMUs while also being capable of capturing not just the $V_{TH}$ shift but the transient recovery after stress. The technique is based on the use of a small sensing current (typically a few mA) to measure the 3rd quadrant voltage during $V_{GS}$ stress and $V_{TH}$ recovery. As GaN e-HEMTs do not have body diodes, third quadrant operation is enabled by self-commutated reverse conduction. The voltage drop across the GaN e-HEMT channel as the current flows from source to drain is sufficiently high to exceed the gate-drain threshold voltage, hence, the device conducts in the 3rd quadrant without a body diode [17, 18]. The equation relating the threshold voltage to the 3rd quadrant voltage is given by

$$ V_{SD} = V_{TH} + i_{DS}R_{DSON} $$

Hence, by measuring the 3rd quadrant voltage using a small sensing current, it is possible to infer the threshold voltage. The experimental set up depicted in [20], as illustrated in Fig. 5(a), includes a current source, a voltage source, and the DUT. Fig. 5(b) shows a circuit diagram of the set-up in $V_{GS}$ stress mode. Here, the gate driver imposes a static or dynamic $V_{GS}$ bias on the gate of the GaN e-HEMT. The current source supplies a 10-mA sensing current while measuring the 3rd quadrant voltage.

<figure><img src="image_10.png" /></figure>
<figure><img src="image_11.png" /><figcaption>Fig. 5(a). Experimental set-up of $V_{TH}$ shift measurements in GaN e-HEMTs using 3rd quadrant characteristics (b). Third quadrant technique circuit diagram in $V_{GS}$ stress mode</figcaption></figure>

Fig. 6 shows the gate-source voltage as well as the measured 3rd quadrant voltage ($V_{SD}$) for a 650 V GaN e-HEMT from GaN Systems with datasheet reference GS66508T. The $V_{GS}$ stress pulse is 3 V for 10 seconds applied at 5 seconds into the experiment. It can be seen that $V_{SD}$ is equal to 1.6 V pre-stress (closed channel) and falls to a very low voltage during the stress (open channel) and rises back to its original voltage after stress. However, it can be noticed that at 15 seconds, $V_{SD}$ rises beyond 1.6 V indicating a $V_{TH}$ rise after stress. Moreover, there is $V_{TH}$ recovery indicated by the reduction of $V_{SD}$ back to its pre-stress value. The positive $V_{TH}$ shift indicates electron trapping dominating at short times.

Using the technique described, $V_{GS}$ stress has been applied for different time durations (1, 10 and 100 seconds) and different magnitudes (3 V, 5.5 V and 7.5 V). Fig. 7(a), (b) and (c) shows the $V_{SD}/V_{TH}$ recovery for 3 V, 5.5 V and 7.5 V respectively. The $V_{SD}/V_{TH}$ in the figures have been normalized to the pre-stress voltage. Each of the stresses are applied for different time durations as described above. It can be seen from Fig. 7 that regardless of the stress voltage, $V_{TH}$ shift is initially positive (indicating more/faster electron than hole trapping) and as the recovery time increases, the shift becomes negative (indicating more/faster electron release). These trends are evident for $V_{GS} = 3$ V (Fig. 7(a)) and $V_{GS} = 7.5$ V (Fig. 7(c)) which is below and above the rated $V_{GS}$ drive voltage of the GaN e-HEMT (5.5 to 6V). At low $V_{GS}$ stress (3.3 V), the positive $V_{TH}$ shift is lower (5% in Fig. 7(a)), the recovery is slower and the peak negative shift during recovery is smaller. At high $V_{GS}$ stress (7.5 V), the peak positive $V_{TH}$ shift is larger (15% in Fig. 7(c)), the recovery is faster and the peak negative shift during recovery is higher. When the

GaN e-HEMT is stressed at the rated voltage (5.5 V), there is a strong dependency of the $V_{TH}$ shift and recovery on the time duration of the stress. As the stress duration is increased from 1 to 100 seconds, the shift becomes more negative.

<figure><img src="image_12.png" /><figcaption>Fig. 6. Measured $V_{GS}$ and $V_{SD}$ for 650 V GaN e-HEMT with $V_{GS}$ stress and $V_{TH}$ recovery</figcaption></figure>

<figure><img src="image_13.png" /></figure>

<figure><img src="image_14.png" /></figure>

<figure><img src="image_15.png" /><figcaption>Fig. 7. $V_{TH}/V_{SD}$ recovery in 650 V GaN e-HEMTs for (a) $V_{GS} = 3$ V (b) $V_{GS} = 5.5$ V (c) $V_{GS} = 7.5$ V and 1-, 10- and 100-seconds stress time</figcaption></figure>
Conclusions

This paper has demonstrated that two different
methods of measuring threshold voltage shift and
recovery in GaN e-HEMTs yield the same trends.
The first method measures the threshold voltage
in the first quadrant using a forced current with
the gate and drain shorted, while the second
method uses the third quadrant voltage as an
indicator of threshold voltage. Both techniques
show that the V<sub>TH</sub> shift under positive V<sub>GS</sub> stress is
initially positive and then reduces over time as
trapped charge is released.

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