Piezotronic effect in a normally off p-GaN/AlGaN/GaN HEMT toward highly sensitive pressure sensor

Abstract

We report the effect of stress or strain on the electronic characteristics of a normally off AlGaN/GaN high electron mobility transistor (HEMT) and demonstrate its role as a highly sensitive pressure sensor. We observe that the HEMT drain current exhibits a linear change of 2.5%/bar upon the application of pressure, which is translated to a strain sensitivity of 1250 ppm . This is the highest strain sensitivity ever reported on HEMTs and many other conventional strain sensing configurations. The relative change of drain current is largest when the gate bias is near-threshold and drain bias is slightly larger than the saturation bias. The electron sheet density and mobility changes in the AlGaN/GaN heterointerface under the applied pressure or mechanical strain are explained qualitatively. The spontaneous and piezoelectricpolarization-induced surface and interface charges in the AlGaN/GaN heterojunction can be used to develop very sensitive and robust pressure sensors. The results demonstrate a considerable potential of normally off AlGaN/GaN HEMTs for highly sensitive and reliable mechanical sensing applications with low energy consumption. Published under an exclusive license by AIP Publishing. https://doi.org/10.1063/5.0053701 In recent years, gallium nitride (GaN) has become a popular semiconductor material for advanced electronic and power switching devices. Compared to silicon (Si), GaN has several superior properties such as widebandgap, high critical breakdown electric field, high thermal conductivity, and high electronic saturation velocity for power electronics. Moreover, GaN-based materials possess many advantages such as high biocompatibility, high thermal and mechanical stability, and excellent irradiation resistance, which are critical for applications in harsh environments. Another unique property of GaN-based materials is the possibility of bandgap engineering by tailoring the strain effect in GaN-related alloys and heterostructures. In particular, a two-dimensional electron gas (2DEG) with a high sheet carrier density and mobility is generated at the AlGaN/GaN heterojunction, owing to the spontaneous and piezoelectric polarization, and the AlGaN surface state acting as an electron source. The spontaneous polarization (PSP) is caused by the asymmetry of the wurtzite crystal structure of III-nitrides, while the piezoelectric polarization (PPE) originates from the strained AlGaN barrier layer due to lattice mismatch between AlGaN and GaN. Thanks to the strong PPE effects, obvious changes in the conductivity of the 2DEG channel can be seen when applying external stress onto an AlGaN/GaN heterostructure, which promises applications in stress/strain sensors and pressure sensors. AlGaN/GaN pressure sensors based on simple piezoresistors and gated piezoresistors have been reported earlier, but they suffer from either low sensitivity or a complex fabrication process. Recently, the use of a high electron mobility transistor (HEMT) as the sensing element has led to a significant increase in sensitivity along with potential for the integration of sensors with conditioning circuits. Generally, high electron mobility transistors (HEMTs) are in the normally on (depletion-mode or d-mode) state, with typical threshold voltages to switch off devices ranging from 6 to 2V. Compared to normally on counterparts, normally off (depletion-mode or D-mode) HEMTs have attracted significant attention due to significant reduction in power consumption during the standby state and enhanced reliability. Moreover, the configuration of normally off HEMT devices allows for simpler power amplifier circuits and high-power switches by eliminating the requirement for negative-polarity voltage supply, thus significantly reduces the circuit complexity and system cost. Among technologies such as recessed gate and fluorine plasma Appl. Phys. Lett. 118, 242104 (2021); doi: 10.1063/5.0053701 118, 242104-1 Published under an exclusive license by AIP Publishing Applied Physics Letters ARTICLE scitation.org/journal/apl