Recently, the joint research team of Shanghai Second Polytechnic University and Tsinghua University has made important progress in the reliability research of wide band gap semiconductor devices. The results were published in IEEE Transactions on Electron Devices, an important international academic journal in the field of microelectronic devices. The team studied the evolution behavior and spatial distribution of trap states in AlGaN / GaN high electron mobility transistors (HEMT) under high-temperature and high-frequency off-state stress through an in-house optoelectronic joint measurement system. It provides the key experimental basis for the optimization design of high performance GaN power devices.

Figure 1. distribution of typical trap states in HEMT

Gan-based HEMT devices have become the core of the new generation of devices for high-frequency and high-power applications due to their high electron mobility, high saturation speed and excellent voltage resistance. However, with the increase of the main frequency, the HEMT tends to increase the calorific value, increase the current collapse, shift the threshold voltage and shift the safe operating boundary to the left, leading to significant performance degradation and lifetime reduction. The accretion of electron trap (trap state) is one of the main reasons for this phenomenon. Due to material growth and heterogeneous interface construction, the trap states are widely distributed in the HEMT device (see Figure 1), and show remarkable evolution and rising characteristics with the increase of the device operating time. Trap states have the ability to trap and release electrons, which is the key factor to interfere with carrier transport, resulting in the concentration of local electric and thermal stresses. It has been proved that the stress generated by the trap state causes the micro-cracks extending from the interface to the surface under the high frequency alternating action, which is the origin of the breakdown, and restricts the reliability and lifetime of the device.

Fig. 2 Photoelectric Joint Trap State Space-resolved Detection Platform

This research innovatively combines the transient current analysis technique with the Raman spectrum measurement method to realize the multi-dimensional localization observation of the trap time and space inside the device (see Fig. 2). It is found that under the off-state stress at high temperature (363 K) and high frequency (1 MHz), four different types of trap states in the device exhibit different responses, The interface trap states whose time constant is on the order of microsecond increase remarkably, while the surface trap states show abnormal instability. The team further determined the energy levels and spatial distribution of each trap state through temperature and voltage varying experiments and Arrhenius fitting analysis. It is observed that the intensity and time constant of the deep level trap state near the grid interface do not change with temperature, showing the trend of microcrack evolution. In addition, that study also observe the pre-filling phenomenon of the surface state traps unde the stress-free condition for the first time, which is the trap state effect with the equivalent energy level of 0.716 eV, resulting in the continuous decrease of the on-current of the device, And affect its dynamic response characteristics.

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Fig. 3. detection of trap state after high temperature stress: (A) interface state accretion and surface state instability; (b) in-channel Raman transverse scanning; (c) Bayesian deconvolution analytical temperature transient current curve; And (d) Arrhenius fitted surface equivalent trap energy levels

This research not only deepens the understanding of the degradation mechanism of GaN HEMT devices, but also provides technical and data support for device structure design, process optimization and reliability improvement in the future. The first authors are Li Fengyi, associate professor of integrated circuits at the School of Energy and Materials, and the corresponding authors are Professor Xing Zhang from Tsinghua University and Professor Huaqing Xie from Shanghai Second Polytechnic University. Relevant achievements were supported by the National Natural Science Foundation of China and other projects.

Paper information:

Fengyili et al., Effect of the High-Temperature and High-Frequency Off-StateStreams on the Evolution of Traps in AlGaN / GaN HEMTs, IEEE TRANSACTIONON ELECTRON DEVICES, 2025, doi: 10.1109 / TED.2025.3603135.