Andrew J. Green

Enhancement-mode Ga2O3 wrap-gate fin field-effect transistors on native (100) β-Ga2O3 substrate with high breakdown voltage

Sn-doped gallium oxide (Ga2O3) wrap-gate fin-array field-effect transistors (finFETs) were formed by top-down BCl3 plasma etching on a native semi-insulating Mg-doped (100) β-Ga2O3 substrate. The fin channels have a triangular cross-section and are approximately 300 nm wide and 200 nm tall. FinFETs, with 20 nm Al2O3 gate dielectric and ∼2 μm wrap-gate, demonstrate normally-off operation with a threshold voltage between 0 and +1 V during high-voltage operation. The ION/IOFF ratio is greater than 105 and is mainly limited by high on-resistance that can be significantly improved. At VG = 0, a finFET with 21 μm gate-drain spacing achieved a three-terminal breakdown voltage exceeding 600 V without a field-plate.

Increasing efficiency, speed, and responsivity of vanadium dioxide based photothermally driven actuators using single-wall carbon nanotube thin-films.

Vanadium dioxide (VO2)-based actuators have demonstrated great performance in terms of strain energy density, speed, reversible actuation, programming capabilities, and large deflection. The relative low phase transition temperature of VO2 (∼68 °C) gives this technology an additional advantage over typical thermal actuators in terms of power consumption. However, this advantage can be further improved if light absorption is enhanced. Here we report a VO2-based actuator technology that incorporates single-wall carbon nanotubes (SWNTs) as an effective light absorber to reduce the amount of photothermal energy required for actuation. It is demonstrated that the chemistry involved in the process of integrating the SWNT film with the VO2-based actuators does not alter the quality of the VO2 film, and that the addition of such film enhances the actuator performance in terms of speed and responsivity. More importantly, the results show that the combination of VO2 and SWNT thin films is an effective approach to increase the photothermal efficiency of VO2-based actuators. The integration of SWNT films in VO2 devices can be easily applied to other VO2-based phototransducers as well as to similar devices based on other phase-change materials. While adding a sufficiently thick layer of some arbitrary material with high absorption for the light used for actuation (λ = 650 nm wavelength in this case) could have improved conversion of light to heat in the device, it could also have impeded actuation by increasing its stiffness. It is noted, however, that the low effective Youngs modulus of SWNT film coating used in this work does not impair the actuation range.

High pulsed current density β-Ga2O3 MOSFETs verified by an analytical model corrected for interface charge

We report on Sn-doped β-Ga2O3 MOSFETs grown by molecular beam epitaxy with as-grown carrier concentrations from 0.7 × 1018 to 1.6 × 1018 cm−3 and a fixed channel thickness of 200 nm. A pulsed current density of >450 mA/mm was achieved on the sample with the lowest sheet resistance and a gate length of 2  μm. Our results are explained using a simple analytical model with a measured gate voltage correction factor based on interface charges that accurately predict the electrical performance for all doping variations.

Highly conductive homoepitaxial Si-doped Ga2O3 films on (010) β-Ga2O3 by pulsed laser deposition

Si-doped Ga2O3 thin films were fabricated by pulsed laser deposition on semi-insulating (010) β-Ga2O3 and (0001) Al2O3 substrates. Films deposited on β-Ga2O3 showed single crystal, homoepitaxial growth as determined by high resolution transmission electron microscopy and x-ray diffraction. Corresponding films deposited on Al2O3 were mostly single phase, polycrystalline β-Ga2O3 with a preferred (20 1 ¯ ) orientation. An average conductivity of 732 S cm−1 with a mobility of 26.5 cm2 V−1 s−1 and a carrier concentration of 1.74 × 1020 cm−3 was achieved for films deposited at 550 °C on β-Ga2O3 substrates as determined by Hall-Effect measurements. Two orders of magnitude improvement in conductivity were measured using native substrates versus Al2O3. A high activation efficiency was obtained in the as-deposited condition. The high carrier concentration Ga2O3 thin films achieved by pulsed laser deposition enable application as a low resistance ohmic contact layer in β-Ga2O3 devices.

Si content variation and influence of deposition atmosphere in homoepitaxial Si-doped β-Ga2O3 films by pulsed laser deposition

Carrier concentration control by impurity dopants in epitaxial Ga2O3 thin films is progressing to deliver high mobility films for device structures. Si-doped Ga2O3 thin films were fabricated by pulsed laser deposition on (010) β-Ga2O3 substrates from Ga2O3 targets with 0.01–1 wt. % SiO2 yielding films with an electron mobility range consistent with other vapor growth techniques. Single crystal, homoepitaxial growth as determined by high resolution transmission electron microscopy and x-ray diffraction was observed, with a high Si dopant level causing film tensile strain as indicated by both techniques. The influence of oxygen on conductivity using different O2 pressures during deposition and O2/Ar mixtures with a fixed working pressure of 1.33 Pa was determined. With this optimized deposition pressure and atmosphere condition, a carrier concentration and mobility range of 3.25 × 1019 cm−3–1.75 × 1020 cm−3 and 20 cm2/V s–27 cm2/V s was achieved in films from Ga2O3-0.025 wt. % SiO2 and Ga2O3-1 wt. % SiO2 targets, respectively. A highest conductivity of 798 S cm−1 was achieved in films deposited at 550 °C–590 °C with targets of 0.05–1 wt. % SiO2. The electrically active and chemical Si content in films deposited at 550 °C was found to exceed the expected Si ablation target composition in all cases except the highest 1 wt. % SiO2 target attributed to imprecise target manufacturer compositional control at low SiO2 doping levels. Diminished electrical and structural quality films resulted from all targets at a 450 °C deposition temperature.Carrier concentration control by impurity dopants in epitaxial Ga2O3 thin films is progressing to deliver high mobility films for device structures. Si-doped Ga2O3 thin films were fabricated by pulsed laser deposition on (010) β-Ga2O3 substrates from Ga2O3 targets with 0.01–1 wt. % SiO2 yielding films with an electron mobility range consistent with other vapor growth techniques. Single crystal, homoepitaxial growth as determined by high resolution transmission electron microscopy and x-ray diffraction was observed, with a high Si dopant level causing film tensile strain as indicated by both techniques. The influence of oxygen on conductivity using different O2 pressures during deposition and O2/Ar mixtures with a fixed working pressure of 1.33 Pa was determined. With this optimized deposition pressure and atmosphere condition, a carrier concentration and mobility range of 3.25 × 1019 cm−3–1.75 × 1020 cm−3 and 20 cm2/V s–27 cm2/V s was achieved in films from Ga2O3-0.025 wt. % SiO2 and Ga2O3-1 wt. % SiO2 ta...

Gate-recessed, laterally-scaled β-Ga 2 O 3 MOSFETs with high-voltage enhancement-mode operation

Beta-phase gallium oxide (Ga2O3) is a promising wide bandgap semiconductor possessing a larger bandgap (∼4.8 eV) and critical electric field strength (∼8 MV/cm) than GaN and SiC [1]. Early metal-oxide-semiconductor field-effect transistors (MOSFETs) have shown promising depletion-mode operation with high critical field strength [2], high-current density [3] and high breakdown voltage [4]. On native substrates, enhancement-mode operation has been achieved with a gated unintentional doped channel [5] and fin-channels [6], the latter achieving 600-V normally-off breakdown voltage. However, both are limited to ∼1 mA/mm or less. Here, we report a new enhancement-mode device achieved by gate recess with improved drain-current > 20 mA/mm and near 200-V breakdown for laterally scaled 3-μm source-drain distance (Lsd).

Toward realization of Ga 2 O 3 for power electronics applications

As a transparent conducting oxide with a large bandgap of ∼4.9 eV and associated large estimated critical electric field (Ec) strength of 8 MV/cm, β-Ga2O3 (BGO) has been touted for its tremendous potential as a power switch. Power switch metrics such as Baligas figure of merit (BFOM) estimating dc conduction losses and Huangs material figure of merit (HMFOM) incorporating dynamic switching losses are functions of EC3 and EC respectively [1, 2]. BFOM for BGO is expected to exceed that of GaN by 400% and HMFOM for BGO is expected to be comparable to GaN. It can also be shown that for a given power loss during switching, the switch frequency (f) varies as EC2 suggesting the potential of BGO power conversion in the GHz regime [1, 2]. The manufacturability and cost value proposition of BGO based on large area native substrate availability as shown by Huangs chip area manufacturing FOM (HCAFOM) indicate a disruptive cost advantage over GaN (330%). Additionally, the Johnson figure of merit (JFOM) representing the power-frequency product for RF amplification for BGO is similar to that of GaN indicating potential for integration of power conversion and RF applications in the same platform. Finally, Huangs high temperature figure of merit (HTFOM) shows that BGO has the lowest metric for all semiconductors compared here due to low thermal conductivity and high field strength. However, all wide bandgap power semiconductors, including diamond, face significant thermal engineering challenges relative to Si because of the inherent high energy densities of materials with large Ec values. A summary of unipolar FET figure-of-merit comparisons for power semiconductors is shown in Table 1.

Heterogeneous integration of low-temperature metal-oxide TFTs

The breadth of circuit fabrication opportunities enabled by metal-oxide thin-film transistors (MO-TFTs) is unprecedented. Large-area deposition techniques and high electron mobility are behind their adoption in the display industry, and substrate agnosticism and low process temperatures enabled the present wave of flexible electronics research. Reports of circuits involving complementaryMO-TFTs, oxide-organic hybrid combinations, and even MO-TFTs integrated onto Si LSI back end of line interconnects demonstrate this technology’s utility in 2D and 3D monolithic heterogeneous integration (HI). In addition to a brief literature review focused on functional HI between MO-TFTs and a variety of dissimilar active devices, we share progress toward integrating MO-TFTs with compound semiconductor devices, namely GaN HEMTs. A monolithically integrated cascode topology was used to couple a HEMT’s >200 V breakdown characteristic with the gate driving characteristic of an IGZO TFT, effectively shifting the HEMT threshold voltage from -3 V to +1 V.

Scalability of zinc oxide thin-film transistors for RF amplifiers and DC switch applications

Summary form only given. In this work, we characterize the scalability of ZnO TFTs for RF transistor and DC switching applications through variations in channel length, total device periphery, and parasitic gate overlap capacitance. For simple device test structures that have not been optimized for either RF transistors or DC switches, we show drain current density (I_DS) and R_on scalability for channel lengths (L_C) from 10 μm down to 150 nm achieving I_DS = 840 mA/mm. Separately, R_on·Q_G values as low as 359 mΩ-nC were measured for devices with mobility (μ_e) = 28 cm²/V·s and contact resistance (ρ_c) = 1 Ω·mm. R_on·Q_G values were observed to scale well for total device peripheries (W) ranging from 50 μm to 2 mm. Significant improvements in I_DS, Ron, and R_on·Q_G are projected through ohmic contact and mobility optimization, as well as parasitic gate overlap capacitance scaling.