Nanometer-Scale Strain Measurements in AlGaN/GaN High-Electron Mobility Transistors During Pulsed Operation

Abstract

Electric, thermal, and mechanical strain fields drive the degradation of AlGaN/GaN high-electron mobility transistors (HEMTs). The resulting mechanical strains within the devices are particularly important. However, a lack of high-resolution measurements of device deformation has limited progress in understanding the related phenomena. This paper presents the atomic force microscope measurements of thermomechanical deformation of AlGaN/GaN HEMT devices during pulsed operation. We investigate the devices with various operating conditions: drain–source voltage, $V_{\mathrm {\mathrm {DS}}}$ , of 0–50 V; drain–source power of 0–6 W/mm; and operating frequency of 55–400 kHz. As $V_{\mathrm {\mathrm {DS}}}$ increases, thermomechanical deformation decreases, especially in the region above the gate. An electrothermomechanical model closely matches with and helps to explain the measurements. According to the model, the maximum periodic tensile thermal stress, which occurs at the drain-side edge of the gate footprint, is 55% larger for $V_{\mathrm {\mathrm {DS}}} = 10$ V than for $V_{\mathrm {\mathrm {DS}}} = 48$ V for the same device power. The maximum tensile thermal stress in the device depends on the gate temperature and not the maximum device temperature. As $V_{\mathrm {\mathrm {DS}}}$ increases, the hotspot moves away from the gate, leading to lower gate temperature rise and lower tensile thermal stress.