AC Transconductance (AC-Gm) Method for Spatial and Energy Profiling of Bulk Traps in GaN MESFETs

The AC transconductance ($\mathrm{AC}-\mathrm{G}_{\mathrm{m}}$) method, first proposed by T. P. Ma, measures oxide trap densities as a function of both space and energy in field effect transistors (FETs) lacking a body contact [1–3]. In MOSFETs, channel carriers tunnel into oxide traps. When an AC signal is superimposed on the DC gate voltage, traps with varying time constants respond according to the frequency of the AC signal. High frequencies capture fast traps near the channel, while lower frequencies detect slower, deeper traps, as shown in Fig. 1(a). However, this model does not apply to MESFETs, which lack an insulating oxide between the gate metal and channel. Fig. 1(b) shows the carrier trapping mechanism in a MESFET when the gate bias is varied along with superimposing an AC signal on the gate. In the MESFET device, the depletion width, hence the channel position shifts away from the gate metal to the substrate. Thus, the AC-Gm technique unleashes a powerful technique of obtaining the bulk trap distribution as a function of position, a capability unreported in prior applications of this technique. $\mathrm{AC}-\mathrm{G}_{\mathrm{m}}$ measurements also provide insight into the trap energy location, similar to the case of MOSFETs. In this work, we create a new model for extracting trap distribution with AC-Gm method. Measurements are performed on GaN MESFETs.