Fig. 1. (Color online) Atomic unit cell of β-Ga₂O₃ with lattice constant and angle marked.

Fig. 2. Band structure of β-Ga₂O₃ with Fermi energy aligned to zero. Reprint from Appl Phys Lett, 88, 261904 (2006). Copyright 2006 American Institute of Physics.

Fig. 3. Photograph of 4-inch-diameter β-Ga₂O₃ wafer. Reprinted from Higashiwaki et al., J Phy D, 50 (2017). Copyright 2017 IOP Publishing^[30].

Fig. 4. (Color online) β-Ga₂O₃ crystals grown by the VB method in either (a) a full-diameter crucible (a’), and (b) or in a conical crucible (b’). Reprinted from Hoshikawa et al., J Cryst Growth 447, 36 (2016). Copyright 2016 Elsevier^[31].

Fig. 5. (Color online) (a) HRTEM image of an undoped homoepitaxial β-Ga₂O₃ layer from MOVPE method. (b) Surface morphologies of 60-nm-thick β-Ga₂O₃ (010) layers grown on Sn-doped β-Ga₂O₃ (010) substrates at a growth rate of 1 nm/min under slightly Ga-rich conditions at the growth temperatures of 500, 700, 800, and 900 °C, respectively. NDIC microscopy images of β-Ga₂O₃ surfaces after HVPE growth on (001) β-Ga₂O₃ substrates for 1 h at (c) 800 and (d) 1000 °C. Reprinted from Refs. [33, 37, 41].

Fig. 6. (Color online) (a) Electron mobility as a function of carrier concentration. (b) I–V characteristics of Pt/β-Ga₂O₃ Schottky barrier diodes. The inset shows the schematic structure of the β-Ga₂O₃ SBD. Reprinted from Sasaki et al., Appl Phys Express, 5, 035502 (2012)^[52]. Copyright of 2012 Japanese Society of Applied Physics.

Fig. 7. (Color online) (a) Forward and (b) reverse J–V characteristics of β-Ga₂O₃ FP-SBD at room temperature. Inset shows the cross-section device schematic of the vertical SBD. Reprinted from Appl Phys Lett, 110, 103506 (2017). Copyright 2017 American Institute of Physics^[53].

Fig. 8. (Color online) (a) Reverse I–V characteristics of 3 diodes with two different diameters, showing a diameter dependence of the BV, and (b) forward and reverse I–V characteristics from a 20 μm diameter diode. Reprinted from Appl Phys Lett, 110, 192101 (2017) and IEEE Electron Device Lett. Copyrights 2017 American Institute of Physics and 2017 IEEE^{[54, 55]}.

Fig. 9. (Color online) (a) DC output characteristics of β-Ga₂O₃ MESFET and (b) transfer characteristics of the same MESFET at V_DS = 40 V. Reprinted from Appl Phys Lett, 100, 013504 (2012). Copyrights 2012 American Institute of Physics^[57].

Fig. 10. (Color online) (a) Schematic cross section of a β-Ga₂O₃ MOSFET with a gate-connected FP structure and (b) Output characteristics of the MOSFET with maximum I_D of 78 mA/mm and BV of 750 V. Reprinted from IEEE Electron Device Lett, 37, 2 (2016). Copyright of 2016 IEEE^[58].

Fig. 11. (Color online) (a) Cross-section view and (b) optical image of the fabricated field-plated Ga₂O₃ MOSFET with SOG S/D doping. (c) Three-terminal breakdown measurement of the field-plated Ga₂O₃ MOSFET and a record-high BV of 1850 V is demonstrated. Reprinted from IEEE Electron Device Lett, 39, 9 (2018). Copyright of 2018 IEEE^[59].

Fig. 12. (Color online) I_D–V_GS characteristics of (a) Zeng’s and (b) Tadjer’s E-mode MOSFET. Reprinted from IEEE Device Research Conf and ECS J, Solid State Sci, Technol, 39, 9 (2018). Copyright of 2016 IEEE^[60] and 2016 ECS^[5].

Fig. 13. (Color online) (a) Tilted false-colored SEM image of a fabricated E-mode β-Ga₂O₃ Fin-MOSFET and (b) The breakdown characteristics of β-Ga₂O₃ Fin-MOSFET with L_G = 2 μm and L_GD = 16, 21 μm at V_GS = 0 V. The inset shows the transfer characteristics of the same device indicating a V_TH = 0.8 V. Reprinted from Appl Phys Lett, 109, 213501 (2016). Copyrights 2016 American Institute of Physics^[4].

Fig. 14. (Color online) (a) Schematic cross-section and (b) SEM image of a vertical E-mode β-Ga₂O₃ Fin-MOSFET, (c) transfer characteristics of the device with a V_TH = 2.2 V, on/off ratio of 108, and on-current of 400 A/cm². (d) The breakdown characteristics of β-Ga₂O₃ Fin-MOSFET with channel width of 330 nm. Reprinted from IEEE Electron Device Lett, 39, 869, 2018. Copyright of 2018 IEEE^[61].

Fig. 15. (Color online) (a) A device cross section schematic is shown for the β-Ga₂O₃ MOSFET under test. (b) A focused ion beam (FIB) cross sectional image of the device. (c) Extrinsic small signal RF gain performance recorded at V_GS = −3.5 V (peak gm) and V_DS = 40 V (d) 800 MHz Class-A power sweep of a 2 × 50 μm β-Ga₂O₃ gate recessed MOSFET. Reprinted from IEEE Electron Device Lett, 38, 790, 2017. Copyright of 2017 IEEE^[62].

Fig. 16. (Color online) (a) DC output characteristics and pulsed I–V from a quiescent bias of V_GS = V_DS = 0 V up to V_DS = 80 V with 1 μs pulse length and 1 ms period (b) Pulsed and CW large signal measurements at 1 GHz with input available power sweep up to 22 dBm, measured at V_DS = 40 V with I_DS = 5 mA. Reprinted from IEEE Electron Device Lett, 39, 1572, 2018. Copyright of 2018 IEEE^[63].

Fig. 17. (Color online) Transfer characteristics of the thin-channel BGO MOSFET with T-gate. The G_M reaches 25 mS/mm with current density over 275 mA/mm, and the inset shows a good I_ON/I_OFF ratio greater than 10⁸. (b) Small signal gain at V_DS = 15 V of the thin-channel BGO MOSFET with f_t/f_max = 5.1/17.1 GHz. Reprinted from IEEE IMWS-AMP, 2018. Copyright of 2018 IEEE^[64].

Fig. 18. (Color online) (a) Schematic-view and (b) top-view microscopy image of a fabricated lateral β-Ga₂O₃ SBD on sapphire substrate. (c) Log-scale forward characteristics of β-Ga₂O₃ SBD with L_{Schottky-Ohmic} = 15 μm at the temperature range from 30 to 150 °C with 20 °C as a step. (d) Linear-scale forward I–V characteristics of β-Ga₂O₃ SBDs with various L_{Schottky−Ohmic}. (e) Reverse I–V characteristics of lateral β-Ga₂O₃ SBDs with various L_{Schottky−Ohmic}. Reprinted from IEEE JEDS 6, 815, 2018. Copyright of 2018 IEEE^[66].

Fig. 19. (Color online) (a) Tilted 3-D schematic-view and (b) top-view microscopy image of a fabricated lateral field-plated β-Ga₂O₃ SBD on Sapphire substrate. (c) Forward I–V and differential R_on–V characteristics in linear scale. The inset shows as-measured current dependence on the width of the SBD at a similar L_AC of 14–18 μm. (d) Reverse I–V characteristics of lateral SBDs with field plate structure and various L_AC. (e) DC R_ON,SP versus BV of some both lateral and vertical β-Ga₂O₃ SBDs. Reprinted from IEEE Electron Device Lett, 39, 1564, 2018. Copyright of 2018 IEEE^[51].

Fig. 20. (Color online) (a) Schematic view of a GOOI FET with a 300 nm SiO₂ layer on Si substrate and (b) AFM image of the atomic flat β-Ga₂O₃ surface after cleavage. Reprinted from IEEE Electron Device Lett, 38, 103, 2017. Copyright of 2017 IEEE^[68].

Fig. 21. (Color online) (a) and (c) are I_D–V_DS output characteristics of D- and E-mode GOOI FETs with 3.0 × 10¹⁸ and 8.0 × 10¹⁸ cm⁻³ doping channel, respectively. (b) and (d) are I_D–g_m–V_GS transfer characteristics of D-mode and E-mode GOOI FETs with 8.0 × 10¹⁸ cm⁻³ doping channel, respectively. Record high I_DMAX of 1.5 and 1.0 A/mm are demonstrated for D/E mode devices. Both D and E-mode devices have high on/off ratio of 10¹⁰ and low SS of 150–165 mV/dec for 300 nm SiO₂. Reprinted from Appl Phys Lett, 111, 092102, 2017. Copyright of 2017 AIP^[67].

Fig. 22. (Color online) (a) Thickness dependent I_D–V_GS plots of various GOOI FETs from D-mode of thicker β-Ga₂O₃ to E-mode of thinner β-Ga₂O₃. (b) Thickness dependent V_T extracted at V_DS = 1 V of 15 devices. Reprinted from IEEE Electron Device Lett, 38, 103, 2017. Copyright of 2017 IEEE^[68].

Fig. 23. (Color online) (a) I_D–V_GS comparison between GOOI FETs with and without ALD passivation for β-Ga₂O₃ nano-membrane with doping concentration of 3.0 × 10¹⁸ cm⁻³, (b) simulated C–V curve for E-mode GOOI FET at a β-Ga₂O₃ nano-membrane thickness of 80 nm and doping concentration of 3.0 × 10¹⁸ cm⁻³ after considering the top and bottom negative surface charge (n_s = 1.2 × 10¹³ cm²) depletion effect. Band diagram and electron density distribution of E-mode GOOI FETs with surface negative charge depletion on (c) lower doping (n_s = 1.2 × 10¹³ cm⁻²) and (d) high doping (n_s = 2.2 × 10¹³ cm⁻²) β-Ga₂O₃ nano-membrane channels at V_GS = 0 V. Reprinted from Appl Phys Lett, 111, 092102, 2017. Copyright of 2017 AIP^[67].

Fig. 24. (Color online) Cross-section schematic view of a top-gate GOOI FET on (a) SiO₂/Si and (b) sapphire and diamond substrates with different κ marked. 15 nm of Al₂O₃ is used as the gate dielectric, Ti/Al/Au (15/60/50 nm) is used as the source/drain electrodes, and Ni/Au (30/50 nm) is used the gate electrode. (c) False-colored SEM top-view of a GOOI FET with L_G = 1 μm and L_SD = 6 μm. Reprinted from ACS Omega 2,11, 2017. Copyright of 2017 ACS^[71].

Fig. 25. (Color online) I_D–V_DS characteristics of GOOI FETs on (a) SiO₂/Si, (b) sapphire and (c) diamond substrates with L_SD = 6–6.5 μm and L_G = 1 μm. A high I_DMAX = 960 mA/mm is demonstrated on top-gate β-Ga₂O₃ GOOI FETs. Comparison of (d) I_D−V_DS (e) log-scale I_D−V_GS and (f) linear-scale g_m−V_GS of β-Ga₂O₃ FETs on a diamond, sapphire and SiO₂/Si substrates. I_D–V_GS transfer characteristics of GOOI FETs on three substrates with high on/off ratio of 10⁹ and low SS of 65 mV/dec, yielding a low D_it of 2.6 × 10¹¹ eV⁻¹·cm⁻². Higher g_m shows higher electron μ on diamond due to the reduced device temperature. Reprinted from ACS Omega 2,11, 2017. Copyright of 2017 ACS^[71] and 2018 IEEE^[72].

Fig. 26. (Color online) TR and charge-coupled device (CCD) camera merged images of GOOI FET on (a) SiO₂/Si, (b) sapphire and (c) diamond substrates when device P is increased by increasing V_DS at V_GS = 0 V. (d) Comparison of measured or simulated ΔT versus P (W/mm²) characteristics of top-gate GOOI FETs on a diamond substrate using TR imaging, Raman thermography and the thermal simulations. (e) Measured by TR method and simulated ΔT versus P characteristics of top-gate GOOI FETs on different substrates. The ΔT of GOOI FET on the SiO₂/Si substrate is more than 3 and 8 times of that on the sapphire and diamond substrates at the same P. As a result, the R_T of GOOI FET on the sapphire substrate is 4.62 × 10⁻² and 1.71 × 10⁻² mm²·K/W, which is less than 1/3 and 1/8 of that on the SiO₂/Si substrate. Reprinted from ACS Omega 2,11, 2017. Copyright of 2017 ACS^[71].

Fig. 27. (Color online) (a) Schematic view of β-Ga₂O₃ FE-FETs. The gate stack includes a heavily n-doped Si as the gate electrode, 20 nm HZO as the ferroelectric insulator, 3 nm Al₂O₃ as the capping layer. Ti/Au (30/60 nm) is used as the source/drain electrodes. Sn-doped n-type β-Ga₂O₃ (86 nm) is used as the channel. (b) Top-view false-color SEM image of representative β-Ga₂O₃ FE-FETs on the same membrane with different channel lengths. (c) Cross-sectional view of the HZO/Al₂O₃ gate stack, capturing the polycrystalline HZO and the amorphous Al₂O₃. Reprinted from ACS Omega 2,10, 2017. Copyright of 2017 ACS^[74].

Fig. 28. (Color online) (a) I_D–V_GS characteristics in the log scale of a β-Ga₂O₃ FE-FET. This device has a channel length of 0.5 μm and a channel thickness of 86 nm. SS versus I_D characteristics of the same device in (a) at (b) V_DS = 0.1, (c) V_DS = 0.5, and (d) V_DS = 0.9 V. SS less than 60 mV/dec at room temperature is demonstrated for both forward and reverse V_GS sweeps. Reprinted from ACS Omega 2,10, 2017. Copyright of 2017 ACS^[74].

Fig. 29. (Color online) (a) I_D−V_GS characteristics in the linear scale of the same β-Ga₂O₃ FE-FET as in Fig. 9. (b) I_D−V_DS characteristics of the same β-Ga₂O₃ FE-FET as in Fig. 9. Reprinted from ACS Omega 2,10, 2017. Copyright of 2017 ACS^[74].

Table 1. Properties of β-Ga₂O₃ relative to some other major semiconductors used for power electronics applications, considering their different kinds of FOM.

Table 2. Comparison of CW and pulse large signal measurements performed at two different operating power levels of 0.4 and 0.8 W/mm. larger differences in performance between cw and pulsed modes can been seen with increasing operating power. Reprinted from IEEE Electron Device Lett, 39, 1572, 2018. Copyright of 2018 IEEE.