Evaluation of Low-Temperature Saturation Velocity in <inline-formula> <tex-math notation="LaTeX">$\beta$ </tex-math> </inline-formula>-(AlxGa1–x)2O3/Ga2O3 Modulation-Doped Field-Effect Transistors

We report on the high-field transport characteristics and saturation velocity in a modulation-doped β-(Al _x Ga _1-x ) ₂ O ₃ /Ga ₂ O ₃ heterostructure. The formation of a 2-D electron gas (2DEG) in the modulation-doped structure was confirmed from the Hall measurements, and the 2DEG channel mobility increased from 143 cm ² /V·s at room temperature to 1520 cm ² /V·s at 50 K. The high electron mobility at 50 K made it feasible to achieve velocity saturation inside the channel. The saturation velocity was estimated based on both pulsed current-voltage measurements and smallsignal radio frequency (RF) measurements. The measured velocity-field profile suggested a saturation velocity above 1.1 x 10 ⁷ cm/s at 50 K. The small-signal RF characteristics were measured for the fabricated modulation-doped field-effect transistors with a Pt-based Schottky contact. The current gain cutoff frequency (f _t ) and maximum oscillation frequency (f _max ) showed significant increases from 4.0/11.8GHz at room temperature to 17.4/40.8GHz at 50 K for the device with gate length of L _G = 0.61 μm. The analysis of the low temperature f _t based on device simulations indicated a peak velocity of 1.2 x 10 ⁷ cm/s. The three-terminal off-state breakdown measurement further suggested an average breakdown field of 3.22 MV/cm. The high saturation velocity and high breakdown field in β-Ga ₂ O ₃ make it a promising candidate for high-power and high-frequency device applications.