Inverted Scanning Microwave Microscopy of GaN/AlN High-Electron Mobility Transistors

In this paper, an inverted scanning microwave microscope (iSMM) is used to characterize the channel of a gateless GaN/AIN high-electron-mobility transistor (HEMT). Unlike conventional SMM, iSMM allows for 2-port measurements. Unlike conventional iSMM, the present iSMM probe is connected to Port 1 of a vector network analyzer with the HEMT drain and source remain on Port 2. Under different DC biases $V_{\mathrm{GS}}$ (applied through the iSMM probe) and $V_{\mathrm{DS}}$ (kept constant at 1 V), changes in both reflection coefficient $S_{11}$ and transmission coefficient $S_{21}$ are monitored as the iSMM probe scans along the width of the channel, revealing significant nonuniformity. Additionally, changes in $S_{11}$ and $S_{21}$ are significant when $V_{G S} \geq-4 \mathrm{~V}$, but insignificant when $V_{G S}=-8 \mathrm{~V}$, consistent with the measured threshold voltage at -6 V for a gated HEMT. These results confirm that iSMM can be used to locally modulate the channel conduction of a HEMT while monitoring its RF response, before the actual gate is added. In turn, the nonuniformity measured by the iSMM can be used to diagnose and improve HEMT materials and processes.