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 RSH between 10 4 Ω / □ and 10 6 Ω / □ , the unbiased RSH is determined to be (3 ± 3) × 10 3 Ω / □ , in contrast to ∼ 450 Ω / □ 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 ± 2) × 10 5 Ω / □ 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.