Beschreibung:
<jats:p> Beta-gallium oxide (β-Ga<jats:sub>2</jats:sub>O<jats:sub>3</jats:sub>) 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-Ga<jats:sub>2</jats:sub>O<jats:sub>3</jats:sub> 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-Ga<jats:sub>2</jats:sub>O<jats:sub>3</jats:sub> MOSFETs. The Al<jats:sub>2</jats:sub>O<jats:sub>3</jats:sub>-β-Ga<jats:sub>2</jats:sub>O<jats:sub>3</jats:sub> interface is primarily important in the performance of β-Ga<jats:sub>2</jats:sub>O<jats:sub>3</jats:sub> 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.</jats:p>
<jats:p>Here, we report on using a modified transfer length method (TLM) to determine the contact, series, and channel resistances from planar, depletion-mode β-Ga<jats:sub>2</jats:sub>O<jats:sub>3</jats:sub> FETs. The results are compared with respect to conventional TLM structures fabricated on the same wafer. The β-Ga<jats:sub>2</jats:sub>O<jats:sub>3</jats:sub> FETs and TLM structures are fabricated on a (010) semi-insulating β-Ga<jats:sub>2</jats:sub>O<jats:sub>3</jats:sub> substrate. The channel layer is a 50 nm Si-doped epi-layer with a target concentration of 2.4 x 10<jats:sup>18</jats:sup> cm<jats:sup>-3</jats:sup>. 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 (Al<jats:sub>2</jats:sub>O<jats:sub>3</jats:sub>, 20 nm). The structures have a constant gate length, L<jats:sub>G</jats:sub>, of 1.94 μm, while the total source-drain spacing, L<jats:sub>SD</jats:sub>, varies as 3μm, 8 μm, and 13 μm. Transfer characteristics were measured at room temperature and low drain-source voltage, V<jats:sub>DS</jats:sub>, of 0.01 V. The threshold voltage, V<jats:sub>TH</jats:sub>, was determined to be » -4V for all MOSFET devices. Measurements were performed in the dark or under UV (265 nm, 2.8 W/cm<jats:sup>2</jats:sup>) exposure.</jats:p>
<jats:p>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-Ga<jats:sub>2</jats:sub>O<jats:sub>3</jats:sub>. We also performed I-V measurements of the FETs in the dark and observed hysteresis behavior with respect to gate-source voltage, V<jats:sub>GS</jats:sub>, which we attribute to trapped charge at the Al<jats:sub>2</jats:sub>O<jats:sub>3</jats:sub>-β-Ga<jats:sub>2</jats:sub>O<jats:sub>3</jats:sub> interface. Using the TLM method, we developed a methodology to extract the β-Ga<jats:sub>2</jats:sub>O<jats:sub>3</jats:sub> MOSFET channel sheet resistances within the gated (R<jats:sub>sh,G</jats:sub>) and ungated (R<jats:sub>sh</jats:sub>) regions as a function of V<jats:sub>GS</jats:sub>. We observe a significant decrease in R<jats:sub>sh,G</jats:sub> from 288 ± 14 kΩ sq<jats:sup>-1</jats:sup> to 7.6 ± 0.38 kΩ sq<jats:sup>-1</jats:sup> as V<jats:sub>GS</jats:sub> varies from -3 V to 3 V. The sheet resistance in the ungated regions, R<jats:sub>sh</jats:sub>, shows a negligible change from 28 ± 1.4 kΩ sq<jats:sup>-1</jats:sup> above a V<jats:sub>GS</jats:sub> of -1.8 V, comparable to that of the conventional TLM structures, but begins to increase with more negative V<jats:sub>GS</jats:sub>, reaching 90 ± 4.5 kΩ sq<jats:sup>-1</jats:sup> at a V<jats:sub>GS</jats:sub> of -3 V. The V<jats:sub>GS</jats:sub> dependence of R<jats:sub>sh</jats:sub> 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 Al<jats:sub>2</jats:sub>O<jats:sub>3</jats:sub>-β-Ga<jats:sub>2</jats:sub>O<jats:sub>3</jats:sub> interface has on R<jats:sub>sh</jats:sub> and R<jats:sub>sh,G</jats:sub>. </jats:p>