Strain Effect in III-Nitride Thin Film Based High Electron Mobility Transistors



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We investigate the strain effect in III-nitride (III-N) thin film based high-electronmobility transistors (HEMTs) by both numerical simulation and experimental demonstration. Based on simulation results, the strain induced piezoelectric polarization charge change can alter the density of 2-dimensional electron gas (2DEG) channel at the AlGaN/GaN interface and thus modify the output characteristics of HEMT. Moreover, by applying extreme compressive strain on the AlInN/GaN heterostructure, the original 2-DEG can be fully depleted and even shift into 2dimensional hole gas (2-DHG). The strain effect is firstly demonstrated by depositing single-crystalline BeO layer on top of AlGaN/GaN HEMT heterostructures. A 20-nm BeO layer with macroscopic polarization on top of the AlGaN barrier layer can increase the 2-DEG density at the interface of AlGaN/GaN heterostructure. Hall-effect measurement confirms that the HEMT with BeO forms a channel with a 14% increment of sheet carrier concentration as compared to conventional HEMT. An improved output performance is also observed in I-V characteristics which confirms the polarization modulation effect of BeO layer.
The strain effect is also demonstrated by mechanical bending. A flexible HEMT is fabricated by layer-transfer process and integrated with 150-μm Cu layer. The mechanical neutral plane of such structure is 80 μm from top surface and 0.57% and −0.59% strain is calculated from high resolution x-ray diffraction results at 4cm bend down and 4cm bend up condition respectively. Output characteristics show a current increment of about 3.4% at bend down condition and −4.3% at bend up condition. Transfer characteristics show a shift of threshold voltage which confirms the 2dimensional electron gas (2-DEG) channel modulation during bending. Computational simulation based on the same structure is also performed and similar current modulation effect (4.3%) and threshold voltage shift is demonstrated.
Both methods show the significance of polarization engineering by external strain in III-N thin film based transistors and offer great potential for electro-mechanical device and high power, high temperature applications.



2-DEG, Piezoelectric polarization, Spontaneous polarization, External strain, Bending, Flexible


Portions of this document appear in: Wang, Weijie, Seung Min Lee, Sara Pouladi, Jie Chen, Shahab Shervin, Seonno Yoon, Jung Hwan Yum et al. "Polarization modulation effect of BeO on AlGaN/GaN high-electron-mobility transistors." Applied Physics Letters 115, no. 10 (2019): 103502. And in: Wang, Weijie, Shahab Shervin, Seung Kyu Oh, Jie Chen, Yang Huai, Sara Pouladi, Hyunsoo Kim, Sung-Nam Lee, and Jae-Hyun Ryou. "Flexible AlGaInN/GaN Heterostructures for High-Hole-Mobility Transistors." IEEE Electron Device Letters 38, no. 8 (2017): 1086-1089.