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As an ultra-wide bandgap semiconductor, $\beta $ -Ga2O3 has attracted great attention for high-power, high-voltage, and optoelectronic applications. However, until now, high-frequency performance of gallium oxide devices has been limited to relatively low current gain cutoff frequencies below 5 GHz. Here, we show that highly localized delta-doping designs can enable high-sheet-charge density to enable devices with short gate lengths that allow high-frequency operation. Field-effect transistors with a gate length of 120 nm on such delta-doped $\beta $ -Ga2O3 are reported here with extrinsic unity current gain frequency of 27 GHz. The device has a peak drain current of 260 mA/mm, transconductance (gm) of 44 mS/mm, and three-terminal off-state breakdown voltage of 150 V. These results demonstrate that the potential of $\beta $ -Ga2O3 for future RF and millimeter-wave device applications.

We report on the high-field transport characteristics and saturation velocity in a modulation-doped $\beta $ -(AlxGa1–x)2O3/Ga2O3 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 $^{2}/\text{V}\cdot \text{s}$ at room temperature to 1520 cm $^{2}/\text{V}\cdot \text{s}$ 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 small-signal radio frequency (RF) measurements. The measured velocity–field profile suggested a saturation velocity above $1.1\times 10^{7}$ 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 ( $\text{f}_{\text {t}}$ ) and maximum oscillation frequency ( $\text{f}_{\text {max}}$ ) showed significant increases from 4.0/11.8 GHz at room temperature to 17.4/40.8 GHz at 50 K for the device with gate length of $\text{L}_{{\textsf {G}}} = 0.61\,\,\mu \text{m}$ . The analysis of the low temperature $\text{f}_{\text {t}}$ based on device simulations indicated a peak velocity of $1.2\times 10^{7}$ 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 $\beta $ -Ga2O3 make it a promising candidate for high-power and high-frequency device applications.

Threshold voltage instability was observed on $\beta $ -Ga2O3 transistors using double-pulsed current–voltage and constant drain current deep level transient spectroscopy (DLTS) measurements. A total instability of 0.78 V was attributed to two distinct trap levels, at ${E}_{C}$ -0.70 and ${E}_{C}$ -0.77 eV, which need to be mitigated for future applications. The traps are likely located near the gate–drain edge and below the delta-doped layer, which is determined through the DLTS technique and an understanding of the fill and empty biasing conditions. The trap modulation was consistent with a gate leakage-based trap filling mechanism, which was demonstrated. It is likely that Fe is playing a role in the observed dispersion due to the close proximity of the Fe substrate.

The influence of growth temperature on the distribution (concentrations and energy levels) of individual defect states in metal organic chemical vapor deposition-grown, Si-doped β-Ga2O3 is investigated. A combination of deep level thermal transient/optical spectroscopies and admittance spectroscopy (AS) was used to quantitatively monitor the evolution of trap states throughout the ∼4.8 eV bandgap. States are observed at EC-0.12 eV by AS; at EC-0.4 eV by deep level transient spectroscopy; and at EC-1.2 eV, EC-2.0 eV, and EC-4.4 eV by deep level optical spectroscopy, and showed different dependencies on growth temperatures ranging from 800 °C to 920 °C. The EC-0.4 eV and EC-4.4 eV states both displayed a strong reduction in its concentration with increasing growth temperature, whereas no consistent trends were seen for the states at EC-1.2 eV and 2.0 eV over the temperature range studied. In contrast, the concentration of the EC-0.12 eV trap monotonically increased over the same range of increasing growth temperature, which tracked a slight, monotonic increase in overall Si concentration measured by secondary ion mass spectroscopy with growth temperature. The opposing trends in concentrations for some of these states shifted the dominant deep level in the bandgap from the EC-4.4 eV state at the lowest growth temperature explored here to the EC-0.12 eV state at the highest growth temperature. The shifting dominance of various bandgap states can have important ramifications on β-Ga2O3 device behavior, and the different trends for these deep levels cannot only guide further growth optimization but also advance the identification of their physical sources.

This report discusses the design and demonstration of β-(Al0.17Ga0.83)2O3/Ga2O3 modulation doped heterostructures to achieve high sheet charge density. The use of a thin spacer layer between the Si delta-doping and the heterojunction interface was investigated in a β-(AlGa)2O3/Ga2O3 modulation doped structure. It is shown that this strategy enables a higher two-dimensional electron gas (2DEG) sheet charge density up to 4.7 × 1012 cm−2 with an effective mobility of 150 cm2/V s. The presence of a degenerate 2DEG channel was confirmed by the measurement of a low temperature effective mobility of 375 cm2/V s and the lack of carrier freeze out from low temperature capacitance voltage measurements. The electron density of 4.7 × 1012 cm−2 is the highest reported 2DEG density obtained without parallel conducting channels in a β-(AlxGa(1−x))2O3/Ga2O3 heterostructure system.

The results of a detailed investigation of electrically active defects in metal-organic chemical vapor deposition (MOCVD)-grown β-Ga2O3 (010) epitaxial layers are described. A combination of deep level optical spectroscopy (DLOS), deep level transient (thermal) spectroscopy (DLTS), and admittance spectroscopy (AS) is used to quantitatively map the energy levels, cross sections, and concentrations of traps across the entire ∼4.8 eV bandgap. States are observed at EC-0.12 eV by AS; at EC-0.4 eV by DLTS; and at EC-1.2 eV, EC-2.0 eV, and EC-4.4 eV by DLOS. While each of these states have been reported for β-Ga2O3 grown by molecular-beam epitaxy (MBE) and edge-defined film fed grown (EFG), with the exception of the EC-0.4 eV trap, there is both a significantly different distribution in the concentration of these states and an overall ∼10× reduction in the total trap concentration. This reduction is consistent with the high mobility and low background compensating acceptor concentrations that have been reported for MOCVD-grown (010) β-Ga2O3. Here, it is observed that the EC-0.12 eV state dominates the overall trap concentration, in marked contrast with prior studies of EFG and MBE material where the state at EC-4.4 eV has dominated the trap spectrum. This sheds light on possible physical sources for this ubiquitous DLOS feature in β-Ga2O3. The substantial reduction in trap concentration for MOCVD material implies great promise for future high performance MOCVD-grown β-Ga2O3 devices.

We report on the origin of high Si flux observed during the use of Si as a doping source in plasma assisted MBE growth of β-Ga2O3. We show on the basis of secondary ion mass spectroscopy analysis that Si flux is not limited by the vapor pressure of Si but by the formation of volatile SiO. The low sublimation energy of SiO leads to a weak dependence of the SiO flux of Si cell temperature and a strong dependence on the background oxygen pressure. Extended exposure to activated oxygen results in reduction of SiO flux due to the formation of SiO2 on the Si surface. The work reported provides key understanding for incorporating Si into future oxide-based semiconductor heterostructure and device MBE growth.

Two buffer traps at EC-0.7 eV and EC-0.8 eV have been individually identified as causing threshold voltage and on-resistance instabilities in β-Ga2O3 Si ∂-doped transistors grown by plasma-assisted molecular beam epitaxy (PAMBE) on semi-insulating Fe doped β-Ga2O3 substrates. The instabilities are characterized using double-pulsed current-voltage and isothermal constant drain current deep level transient spectroscopy. The defect spectra are compared between transistors grown using two different unintentionally doped buffer layer thicknesses of 100 nm and 600 nm. The EC-0.8 eV trap was not seen using the thicker buffer and is shown to correlate with the presence of residual Fe in thePAMBE buffer layer. The EC-0.7 eV trap was unchanged in concentration and is revealed as the dominating source of the threshold voltage instability. This trap is consistent with the characteristics of a previously reported intrinsic point defect [Ingebrigtsen et al., APL Mater. 7, 022510 (2019)]. The EC-0.7 eV trap is responsible for ∼70% of the total threshold voltage shift in the 100 nm thick buffer transistor and 100% in the 600 nm thick buffer transistor, which indicates growth optimization is needed to improve β-Ga2O3 transistor stability.Two buffer traps at EC-0.7 eV and EC-0.8 eV have been individually identified as causing threshold voltage and on-resistance instabilities in β-Ga2O3 Si ∂-doped transistors grown by plasma-assisted molecular beam epitaxy (PAMBE) on semi-insulating Fe doped β-Ga2O3 substrates. The instabilities are characterized using double-pulsed current-voltage and isothermal constant drain current deep level transient spectroscopy. The defect spectra are compared between transistors grown using two different unintentionally doped buffer layer thicknesses of 100 nm and 600 nm. The EC-0.8 eV trap was not seen using the thicker buffer and is shown to correlate with the presence of residual Fe in thePAMBE buffer layer. The EC-0.7 eV trap was unchanged in concentration and is revealed as the dominating source of the threshold voltage instability. This trap is consistent with the characteristics of a previously reported intrinsic point defect [Ingebrigtsen et al., APL Mater. 7, 022510 (2019)]. The EC-0.7 eV trap is respons...

BaSnO_{3}, a high mobility perovskite oxide, is an attractive material for oxide-based electronic devices. However, in addition to low-field mobility, high-field transport properties such as the saturation velocity of carriers play a major role in determining device performance. We report on the experimental measurement of electron saturation velocity in La-doped BaSnO_{3} thin films for a range of doping densities. Predicted saturation velocities based on a simple LO-phonon emission model using an effective LO phonon energy of 120 meV show good agreement with measurements of velocity saturation in La-doped BaSnO_{3} films.. Density-dependent saturation velocity in the range of 1.6x10^{7} cm/s reducing to 2x10^{6} cm/s is predicted for {\delta}-doped BaSnO3 channels with carrier densities ranging from 10^{13} cm^{-2} to 2x10^{14} cm^{-2} respectively. These results are expected to aid the informed design of BaSnO3 as the active material for high-charge density electronic transistors., Comment: 23 pages, 10 figures, 2 tables

Modulation-doped heterostructures are a key enabler for realizing high mobility and better scaling properties for high performance transistors. We report the realization of a modulation-doped two-dimensional electron gas (2DEG) at the beta-(Al0.2Ga0.8)(2)O-3/Ga2O3 heterojunction by silicon delta doping. The formation of a 2DEG was confirmed using capacitance voltage measurements. A modulation-doped 2DEG channel was used to realize a modulation-doped field-effect transistor. The demonstration of modulation doping in the beta-(Al0.2Ga0.8)(2)O-3/Ga2O3 material system could enable heterojunction devices for high performance electronics. Published by AIP Publishing.

We report on the effect of iron (Fe)-doped semi-insulating buffers on the electron transport and DC-RF dispersion in Si delta (δ)-doped β-Ga2O3 metal-semiconductor field effect transistors. The effect of the distance between the 2-dimensional electron gas and the Fe-doped region was investigated, and Fe doping in the buffer was found to have a significant effect on the transport properties. It was found that buffers thicker than 600 nm can enable better transport and dispersion properties for field effect transistors, while maintaining relatively low parasitic buffer leakage. This work can provide guidance for the use of Fe-doped insulating buffers for future Ga2O3 based electronics.