Beta-gallium oxide (β-Ga₂O₃) is a promising ultra-wide bandgap semiconductor for high power electronic applications due to its high theoretical critical field of 8 MV/cm, and Baliga figure of merit (BFOM) of ~3300, 3 – 10 times larger than current state-of-the-art wide bandgap semiconductors. β-Ga₂O₃ field-effect transistors (FETs) have been explored for both high power and radio frequency (RF) operation, but with relatively high parasitic resistances. A method is developed to determine the contact, series, and channel resistances in depletion mode β-Ga₂O­₃ MOSFETs. The Al₂O­₃-β-Ga₂O₃ interface is primarily important in the performance of β-Ga₂O­₃ FETs due to charge trapping. Measurements will be performed with UV and visible light radiation to study the role of trap states on the FET resistances and device performance.

Here, we report on using a modified transfer length method (TLM) to determine the contact, series, and channel resistances from planar, depletion-mode β-Ga₂O₃ FETs. The results are compared with respect to conventional TLM structures fabricated on the same wafer. The β-Ga₂O₃ FETs and TLM structures are fabricated on a (010) semi-insulating β-Ga₂O₃ substrate. The channel layer is a 50 nm Si-doped epi-layer with a target concentration of 2.4 x 10¹⁸ cm^-3. A Ti/Al/Ni/Au metal stack is deposited and annealed to form Ohmic contacts. The gate dielectric is composed of atomic layer deposition-grown aluminum oxide (Al₂O₃, 20 nm). The structures have a constant gate length, L_G, of 1.94 μm, while the total source-drain spacing, L_SD, varies as 3μm, 8 μm, and 13 μm. Transfer characteristics were measured at room temperature and low drain-source voltage, V_DS, of 0.01 V. The threshold voltage, V_TH, was determined to be » -4V for all MOSFET devices. Measurements were performed in the dark or under UV (265 nm, 2.8 W/cm²) exposure.

Conventional TLM structures were measured in the dark and with UV exposure, and a 40% decrease in the sheet resistance was observed when exposed to UV. We attribute this change to an increase in the carrier concentration from photo-generated electrons due to trap states within the β-Ga₂O₃. We also performed I-V measurements of the FETs in the dark and observed hysteresis behavior with respect to gate-source voltage, V_GS, which we attribute to trapped charge at the Al₂O₃-β-Ga₂O₃ interface. Using the TLM method, we developed a methodology to extract the β-Ga₂O₃ MOSFET channel sheet resistances within the gated (R_sh,G) and ungated (R_sh) regions as a function of V_GS. We observe a significant decrease in R_sh,G from 288 ± 14 kΩ sq^-1 to 7.6 ± 0.38 kΩ sq^-1 as V_GS varies from -3 V to 3 V. The sheet resistance in the ungated regions, R_sh­, shows a negligible change from 28 ± 1.4 kΩ sq^-1 above a V_GS of -1.8 V, comparable to that of the conventional TLM structures, but begins to increase with more negative V­_GS, reaching 90 ± 4.5 kΩ sq^-1 at a V_GS of -3 V. The V_GS dependence of R_sh­ is due to complicated current flow mechanisms when the depletion region is large, which is not considered in this model. We will extend the study by performing I-V measurements with UV and visible light radiation to observe any effect of traps at the Al₂O₃-β-Ga₂O₃ interface has on R_sh and R_sh,G.