Quantitative Scanning Microwave Microscopy for Transfer Characteristics of GaN High-Electron-Mobility Transistors

This article demonstrates the feasibility in using a scanning microwave microscope (SMM) to probe the transfer characteristics of an ungated GaN high-electron-mobility transistor (HEMT). To guide the experiment and to interpret the result, an equivalent circuit is proposed to model the probe-sample near-field interaction, and the model is validated by simulation and experimentation. In the experiment, the SMM probe with a dc bias voltage acts as a surrogate to locally modulate the 2-D electron gas (2DEG) at the GaN heterojunction. Because the present SMM is most sensitive to a 2DEG sheet resistance $R_{\text {SH}}$ between $10^{4}~\Omega/\square $ and $10^{6}~\Omega/\square $ , the unbiased $R_{\text {SH}}$ is determined to be ( $3~\pm ~3$ ) $\times 10^{3}~\Omega/\square $ , in contrast to $\sim 450~\Omega/\square $ determined by Hall measurements. However, with the bias decreasing from 0 to −8 V, the 2DEG is depleted and its resistance is increased to ( $5~\pm ~2$ ) $\times 10^{5}~\Omega/\square $ with an on/off ratio of 160, a peak transconductance around −5 V, and a threshold voltage of −6 V. These results agree with the dc-measured current-voltage characteristics on a gated HEMT after its fabrication is completed. This shows that the SMM could be a powerful tool for in-process monitoring and material/device correlation.