A 2.8 kV Breakdown Voltage α-Ga2O3 MOSFET with Hybrid Schottky Drain Contact

Among various polymorphic phases of gallium oxide (Ga2O3), α-phase Ga2O3 has clear advantages such as its heteroepitaxial growth as well as wide bandgap, which is promising for use in power devices. In this work, we demonstrate α-Ga2O3 MOSFETs with hybrid Schottky drain (HSD) contact, comprising both Ohmic and Schottky electrode regions. In comparison with conventional Ohmic drain (OD) contact, a lower on-resistance (Ron) of 2.1 kΩ mm is achieved for variable channel lengths. Physics-based TCAD simulation is performed to validate the turn-on characteristics of the Schottky electrode region and the improved Ron. Electric-field analysis in the off-state is conducted for both the OD and HSD devices. Furthermore, a record breakdown voltage (BV) of 2.8 kV is achieved, which is superior to the 1.7 kV of the compared OD device. Our results show that the proposed HSD contact with a further optimized design can be a promising drain electrode scheme for α-Ga2O3 power MOSFETs.


Introduction
Gallium oxide (Ga 2 O 3 ) semiconductors have been rigorously studied for use in highperformance power devices owing to their ultra-wide bandgap (E g ) and to the high-quality Ga 2 O 3 substrates obtained from a bulk single crystal using melt-growth methods [1][2][3][4].Interestingly, Ga 2 O 3 crystalizes into five polymorphic phases, and the thermodynamically stable β-Ga 2 O 3 has stood at the center of research.On the other hand, the second most well-known α-Ga 2 O 3 has certain advantages because of its epitaxial compatibility with commercially available, large-size sapphire wafers.Furthermore, the heteroepitaxy of highquality α-Ga 2 O 3 epilayers on sapphire will allow better thermal conductivity compared with a native substrate [5][6][7][8][9].It is to be noted that α-Ga 2 O 3 has the highest E g of ~5.3 eV among Ga 2 O 3 phases and a correspondingly competitive figure of merits.Thus, α-Ga 2 O 3 deserves to be extensively investigated further.
Attributed to its high E g , α-Ga 2 O 3 has received great interest for ultraviolet (UV) light detectors and specifically deep-UV or UV C-band light (<280 nm) [10,11].Among various materials with a wide E g , including aluminum gallium nitride (AlGaN) and zinc oxide (ZnO), α-Ga 2 O 3 is particularly appropriate for large-area processes because of its relatively low growth temperature of about 500 • C.This relatively low temperature of growth or deposition allows not only the use of conventional large-area process methods but also novel Ga 2 O 3 film deposition approaches via thermal oxidation of Ga-based wafers [12][13][14].
These unique advantages of α-Ga 2 O 3 along with large-area film growth techniques make α-Ga 2 O 3 promising for use in high-performance UVC and flame photodetectors.
Power devices based on α-Ga 2 O 3 represent another important application.Indeed, various α-Ga 2 O 3 -based device structures have been reported.For example, Schottky barrier diodes using mist-CVD-grown α-Ga 2 O 3 films and α-Ir 2 O 3 /α-Ga 2 O 3 hetero p-n junction diodes have been demonstrated [15,16].Metal-semiconductor field-effect transistors based on a mist-CVD-grown α-Ga 2 O 3 layer on sapphire were also proposed [17].Jeong et al. reported an α-Ga 2 O 3 MOSFET with a breakdown voltage (BV) of 2.3 kV grown via halide vapor-phase epitaxy (HVPE) [18].Considering the demonstrated α-Ga 2 O 3 devices' structures, further enhancement of on-resistance (R on ) and breakdown characteristics can be achieved not only through a vertical structure but also through additional fabrication process steps such as surface passivation and field-plate structures [19][20][21][22][23][24].Alternatively, the hybrid Schottky-Ohmic drain (HSD) scheme utilizing charge trapping in the Schottky extension region can be considered because of the relatively inferior crystal quality of heteroepitaxial α-Ga 2 O 3 compared to the homoepitaxial Ga 2 O 3 layer [25][26][27][28].Indeed, the use of HSD on β-Ga 2 O 3 FETs has been reported to alleviate degradation under thermal stress conditions [29].
In this work, for the first time, we propose α-Ga 2 O 3 MOSFETs with HSD electrodes.The heteroepitaxial α-Ga 2 O 3 device layer was grown on a sapphire substrate via HVPE.The HSD is composed of Ti/Al/Ni/Au Ohmic and Ni Schottky electrodes, and both contact regions are analyzed using high-resolution transmission electron microscopy (HR-TEM).The electrical characteristics of the proposed HSD device are compared with a conventional Ohmic drain (OD) device for variable source-drain lengths (L SD ).Moreover, physics-based TCAD simulation is performed to elucidate the turn-on characteristics of the proposed HSD device.Electric (E)-field simulation in the off-state is also performed to compare the OD and HSD devices.Finally, the off-state BV of 2.8 kV is measured for the HSD device.The performance is compared with recently reported results.

Experimental Section
Heteroepitaxial α-Ga 2 O 3 films were deposited on a 2-inch (0001) sapphire substrate using the HVPE method with GaCl and O 2 precursors at a growth temperature of 470 • C. Undoped (~900 nm) and Si-doped α-Ga 2 O 3 (~300 nm) layers were grown for 10 min each, with SiH 4 gas supplied as an n-type dopant during the growth of the Si-doped layer.Donor concentration, Hall mobility, and sheet carrier density of the Si-doped α-Ga 2 O 3 epitaxial layer were measured to be 6 × 10 17 cm −3 , 98.7 cm 2 /V•s, and 1.8 × 10 13 cm −3 , respectively.Surface cleaning was performed for 10 min on each layer using acetone and IPA.The MESA isolation structure was then patterned using BCl 3 /Ar gas in the appropriate ratio with ICP RIE.The source/drain electrodes were deposited using an e-beam evaporator with a Ti/Al/Ni/Au (25/140/40/100 nm) metal stack and patterned using conventional photolithography and lift-off processes.Rapid thermal annealing was carried out in a N 2 ambient at 470 • C for 1 min to improve contact properties.The Schottky drain electrode of Ni (100 nm) is deposited by a DC sputtering system for the HSD device fabrication process.A 20 nm thick HfO 2 gate dielectric layer was deposited as the gate oxide using atomic layer deposition at 350 • C with a deposition rate of 1 Å/cycle using Tetrakis (ethylmethy lamino) hafnium precursor and ozone reactant.Finally, the Ni (100 nm) gate electrode was deposited by sputtering, followed by opening contact holes via ICP RIE.
Scanning electron microscopy images and cross-sectional TEM samples were prepared using a dual-beam focused ion beam (FIB-SEM, Helios 460F1).The TEM images and energy dispersive X-ray spectroscopy (EDS) elemental maps were obtained using a JEOL JEM-2100F with a probe Cs corrector.All electrical measurements were conducted using a semiconductor parameter analyzer (Keithley 4200A-SCS) in a dark box.The off-state breakdown measurement was conducted using in-house test equipment composed of a 5 kV power supply (Tektronix model 2290-5), picoammeter (Tektronix model 6485), source-measurement unit (Tektronix model 2440), and protection circuit.Physics-based TCAD (Synopsys, Inc., Mountain View, CA, USA) simulation was performed by elaborately reflecting parameters of the fabricated devices.

Results and Discussion
Figure 1a,b show the cross-sectional schematics and a top view of the representative HSD device, respectively.The blue box represents the drain electrode of the OD or HSD.The fabricated α-Ga 2 O 3 MOSFETs have a channel-width (W) = 96 µm with a variable L SD of 20, 28, and 40 µm.The gate length (L G ) and source-gate length (L SG ) are 2 and 4 µm, respectively.The width of the OD electrode is 96 µm.For the HSD, the Schottky and Ohmic contact regions are designed to be 34 and 62 µm, respectively.The Schottky contact metal is overlaid onto the Ohmic metal by 5 µm.The dashed line shown in Figure 1b indicates the position where TEM images were obtained.Figure 2 compares the TEM and EDS analyses for the Schottky contact (α-Ga 2 O 3 /Ni junction) and Ohmic contact (α-Ga 2 O 3 /Ti/Al/Ni/Au) regions.Based on the reported band alignment analysis, the Schottky contact between Ni and α-Ga 2 O 3 is predicted.The following equation is used to calculate E C -E F and N C [30]: (2) where ) is 3.62 eV [32,33].Therefore, the barrier height formed in Ni/α-Ga 2 O 3 is calculated to be The interface at the Schottky contact shows a smooth and sharp transition.Although we cannot elaborate it quantitively, the ALD temperature does not have a significant impact on the Schottky contact, as observed from electrical characteristics at a low drain bias.It is reported that interdiffusion is marginal under an annealing temperature of 400 • C and that the Ni content was only 5% in the Ni-Au alloy [34].The Ohmic contact exhibits a relatively mixed interface due to the formation of a Ti-Al inter-metallic phase, and oxygen vacancies facilitated the Ohmic contact by reducing contact resistance [35].
source-measurement unit (Tektronix model 2440), and protection circuit.Physics-based TCAD (Synopsys, Inc., Mountain View, CA, USA) simulation was performed by elaborately reflecting parameters of the fabricated devices.

Results and Discussion
Figure 1a,b show the cross-sectional schematics and a top view of the representative HSD device, respectively.The blue box represents the drain electrode of the OD or HSD.The fabricated α-Ga2O3 MOSFETs have a channel-width (W) = 96 µm with a variable LSD of 20, 28, and 40 µm.The gate length (LG) and source-gate length (LSG) are 2 and 4 µm, respectively.The width of the OD electrode is 96 µm.For the HSD, the Schottky and Ohmic contact regions are designed to be 34 and 62 µm, respectively.The Schottky contact metal is overlaid onto the Ohmic metal by 5 µm.The dashed line shown in Figure 1b indicates the position where TEM images were obtained.Figure 2 compares the TEM and EDS analyses for the Schottky contact (α-Ga2O3/Ni junction) and Ohmic contact (α-Ga2O3/Ti/Al/Ni/Au) regions.Based on the reported band alignment analysis, the Schottky contact between Ni and α-Ga2O3 is predicted.The following equation is used to calculate EC-EF and NC [30]: where NC, ND, m0, and m* are the effective density of states of the conduction band, electron density of α-Ga2O3, electron mass, and electron effective mass of α-Ga2O3, respectively.Haiying et al. reported that the value of m* for α-Ga2O3 is 0.276 m0 [31].The calculated NC is 3.65 × 10 18 cm −3 , and the extracted EC-EF is 0.05 eV.The work function of Ni (ΦNi) is 5.01 eV, and the electron affinity of α-Ga2O3 (χα-Ga2o3) is 3.62 eV [32,33].Therefore, the barrier height formed in Ni/α-Ga2O3 is calculated to be ΦNi − χα-Ga2o3 − (EC-EF) = 1.34 eV.The interface at the Schottky contact shows a smooth and sharp transition.Although we cannot elaborate it quantitively, the ALD temperature does not have a significant impact on the Schottky contact, as observed from electrical characteristics at a low drain bias.It is reported that interdiffusion is marginal under an annealing temperature of 400 °C and that the Ni content was only 5% in the Ni-Au alloy [34].The Ohmic contact exhibits a relatively mixed interface due to the formation of a Ti-Al inter-metallic phase, and oxygen vacancies facilitated the Ohmic contact by reducing contact resistance [35].Figure 3a shows transfer characteristics of the representative OD and HSD devices with LSD = 20 µm.The on/off ratios of the devices are 10 6 at VDS = 10 V.The larger VDS is, the higher the observed off-current is, because the undoped α-Ga2O3 buffer layer is unintentionally an n-doped layer due to the presence of oxygen vacancies.Therefore, even in the off-state, the source-drain current can flow through the buffer layer (i.e., undoped α-Ga2O3), which depends on VDS. Figure 3b presents the output characteristics of the OD and HSD devices.The Ron values of the OD and HSD devices (Ron.OD, Ron.HSD) are extracted to  Figure 3a shows transfer characteristics of the representative OD and HSD devices with LSD = 20 µm.The on/off ratios of the devices are 10 6 at VDS = 10 V.The larger VDS is, the higher the observed off-current is, because the undoped α-Ga2O3 buffer layer is unintentionally an n-doped layer due to the presence of oxygen vacancies.Therefore, even in the off-state, the source-drain current can flow through the buffer layer (i.e., undoped α-Ga2O3), which depends on VDS. Figure 3b presents the output characteristics of the OD and HSD devices.The Ron values of the OD and HSD devices (Ron.OD, Ron.HSD) are extracted to Figure 3a shows transfer characteristics of the representative OD and HSD devices with L SD = 20 µm.The on/off ratios of the devices are 10 6 at V DS = 10 V.The larger V DS is, the higher the observed off-current is, because the undoped α-Ga 2 O 3 buffer layer is unintentionally an n-doped layer due to the presence of oxygen vacancies.Therefore, even in the off-state, the source-drain current can flow through the buffer layer (i.e., undoped α-Ga 2 O 3 ), which depends on V DS .Figure 3b presents the output characteristics of the OD and HSD devices.The R on values of the OD and HSD devices (R on.OD , R on.HSD ) are extracted to be 2.9 and 2.1 kΩ mm, respectively.The varying slopes at low-V DS regions indicate that the current path forms under the Ohmic and then Schottky regions in turn.In low-drain-bias operation, the HSD affects carrier concentration and increases L GD because the Schottky contact is off.The Schottky and Ohmic contact regions are designed to be 34 and 62 µm, respectively.This significant portion of the Schottky contact region results in a longer L GD (i.e., L SD + the width of Schottky contact) under the low-drain-bias regime because the current flows only via the Ohmic region.Therefore, lower drain currents were observed in the low-drain-bias region of output characteristics.Once the Schottky contact region turns on under high drain bias, however, the current drastically increases because of the current flow via the Schottky contact region because the width of the Schottky contact is larger than the transfer length.Thus, a lower R on is extracted in comparison with the conventional Ohmic drain contact.Figure 3c compares extracted field-effect mobility (µ FE ) of the OD and HSD devices for variable L SD at V DS = 1 V and 40 V.The µ FE is calculated by in the saturation region.The oxide capacitance (C OX ) is calculated to be 712 nF/cm 2 .The µ FE of the HSD device is lower than the OD device at V DS = 1 V because the α-Ga 2 O 3 /Ni Schottky junction was not turned on completely, and this can affect the current's flow toward the drain [29].In the saturation region, a high V DS of 40 V induces current paths beneath the Schottky region, and thus similar levels of µ FE were obtained.Figure 3d also compares extracted R on.OD and R on.HSD , and R on.HSD is lower than R on.OD regardless of L SD .
Micromachines 2024, 15, x FOR PEER REVIEW be 2.9 and 2.1 kΩ mm, respectively.The varying slopes at low-VDS regions indic the current path forms under the Ohmic and then Schottky regions in turn.In low bias operation, the HSD affects carrier concentration and increases LGD beca Schottky contact is off.The Schottky and Ohmic contact regions are designed to be 62 µm, respectively.This significant portion of the Schottky contact region resu longer LGD (i.e., LSD + the width of Schottky contact) under the low-drain-bias reg cause the current flows only via the Ohmic region.Therefore, lower drain curren observed in the low-drain-bias region of output characteristics.Once the Schottky region turns on under high drain bias, however, the current drastically increases of the current flow via the Schottky contact region because the width of the Schott tact is larger than the transfer length.Thus, a lower Ron is extracted in comparis the conventional Ohmic drain contact.Figure 3c [36,37].Simulation was conducted taking the bias conditions of V GS = 8 V and V DS = 1 and 10 V. Figure 4a,b depict the E-current density distributions in the vicinity of the HSD contact edge at V DS = 1 and 10 V, respectively.Figure 4a shows that the current flow to the Schottky contact region (I HSD.Schottky ) is blocked by the depletion of the Schottky region at V DS < V ON , and most of the current flows through the Ohmic contact region (I HSD.Ohmic ).As shown in Figure 4b, however, I HSD.Schottky takes the form of V DS > V ON , indicating two parallel current paths.Figure 4c presents E-current density line profiles along the channel, and clearly shows that I HSD.Schottky formed at a V DS of 10 V.The output curves of the simulated HSD and OD devices are presented in Figure 4d.Until a certain point of V DS , which is 2.5 V in this simulation, the drain current of the HSD device is mostly composed of I HSD.Ohmic .As V DS > 2.5 V, I HSD.Schottky starts to flow and R on.HSD becomes lower than R on.OD .
conditions of VGS = 8 V and VDS = 1 and 10 V. Figure 4a,b depict the E-current distributions in the vicinity of the HSD contact edge at VDS = 1 and 10 V, respe Figure 4a shows that the current flow to the Schottky contact region (IHSD.Schottky) is by the depletion of the Schottky region at VDS < VON, and most of the current flows the Ohmic contact region (IHSD.Ohmic).As shown in Figure 4b, however, IHSD.Schottky t form of VDS > VON, indicating two parallel current paths.Figure 4c presents E  To analyze off-state characteristics, additional simulation of the E-field analy conducted for both the HSD and OD devices at VGS = −25 V and VDS = 40 V. Fi compares the simulated E-field distribution below the gate electrode of the HSD device.The OD device exhibits a higher peak E-field at the edge of the gate electro the HSD device.Figure 5b,c compare the E-field distributions and corresponding l files under the drain region, respectively.The HSD device shows a slightly higher field at the edge of the drain electrode in comparison with the OD device.The di at the edge of the gate electrode is significant.Figure 5d,e show line profiles of the under the gate electrode and its corresponding α-Ga2O3/UID interface, respectiv peak E-field of the HSD device is lower due to the Schottky-contact-induced sur tential change.Additionally, the HSD device exhibits a lower peak E-field at th sponding region of the α-Ga2O3/UID interface.Considering the undoped α-Ga2O layer, which is an unintentionally n-doped layer due to the presence of oxygen va To analyze off-state characteristics, additional simulation of the E-field analysis was conducted for both the HSD and OD devices at V GS = −25 V and V DS = 40 V. Figure 5a compares the simulated E-field distribution below the gate electrode of the HSD and OD device.The OD device exhibits a higher peak E-field at the edge of the gate electrode than the HSD device.Figure 5b,c compare the E-field distributions and corresponding line profiles under the drain region, respectively.The HSD device shows a slightly higher peak E-field at the edge of the drain electrode in comparison with the OD device.The difference at the edge of the gate electrode is significant.Figure 5d,e show line profiles of the E-field under the gate electrode and its corresponding α-Ga 2 O 3 /UID interface, respectively.The peak E-field of the HSD device is lower due to the Schottky-contactinduced surface potential change.Additionally, the HSD device exhibits a lower peak E-field at the corresponding region of the α-Ga 2 O 3 /UID interface.Considering the undoped α-Ga 2 O 3 buffer layer, which is an unintentionally n-doped layer due to the presence of oxygen vacancies, suppressing the peak E-field at the UID layer might contribute to the breakdown characteristics of the α-Ga 2 O 3 MOSFET [38].The simulation results foresee enhanced breakdown characteristic from the HSD device, as demonstrated in the following section.
suppressing the peak E-field at the UID layer might contribute to the breakdown characteristics of the α-Ga2O3 MOSFET [38].The simulation results foresee enhanced breakdown characteristic from the HSD device, as demonstrated in the following section.Figure 6 shows the three-terminal off-state breakdown measurement results for the OD and HSD devices with LSD = 40 µm (gate-to-drain distance is 34 µm).The devices were submerged in a fluorinert liquid (FC-40) to avoid the effects of air-arcing.VGS was set to −16 V to maintain the off-state.The off-state leakage current level was set by our highcurrent, high-voltage measurement set-up.VDS was swept until the abrupt increase in IDS was observed, which is indicative of a hard breakdown and defined as BV in this work.The BV of the OD and HSD devices is 1.7 and 2.8 kV, respectively.As discussed above, the Schottky contact region with a smooth interface at the edge of drain electrode functions as a drain field-plate, contributing to electric-field distribution and thus an improved BV [26].The lateral figures of merit (LFOMs) of the OD and HSD devices are calculated as LFOM = BV 2 /Ron-sp, where Ron-sp is 1.2 and 1 Ω cm 2 , respectively.The LFOMs of the OD and HSD devices are 2.4 and 7.8 MW/cm 2 , respectively.The electrical characteristics of the heteroepitaxial Ga2O3 MOSFETs are summarized in Table 1 [18,[38][39][40][41][42].Some parameters are missing because they were not discussed in the published results.A record BV of 2.8 kV is achieved for the HSD device, although the Ron-sp and LFOM are inferior to those in the report by Jeong et al. [18].Run-to-run epi growth variations are inevitable on account Figure 6 shows the three-terminal off-state breakdown measurement results for the OD and HSD devices with L SD = 40 µm (gate-to-drain distance is 34 µm).The devices were submerged in a fluorinert liquid (FC-40) to avoid the effects of air-arcing.V GS was set to −16 V to maintain the off-state.The off-state leakage current level was set by our high-current, high-voltage measurement set-up.V DS was swept until the abrupt increase in I DS was observed, which is indicative of a hard breakdown and defined as BV in this work.The BV of the OD and HSD devices is 1.7 and 2.8 kV, respectively.As discussed above, the Schottky contact region with a smooth interface at the edge of drain electrode functions as a drain field-plate, contributing to electric-field distribution and thus an improved BV [26].The lateral figures of merit (LFOMs) of the OD and HSD devices are calculated as LFOM = BV 2 /R on-sp , where R on-sp is 1.2 and 1 Ω cm 2 , respectively.The LFOMs of the OD and HSD devices are 2.4 and 7.8 MW/cm 2 , respectively.The electrical characteristics of the heteroepitaxial Ga 2 O 3 MOSFETs are summarized in Table 1 [18,[38][39][40][41][42].Some parameters are missing because they were not discussed in the published results.A record BV of 2.8 kV is achieved for the HSD device, although the R on-sp and LFOM are inferior to those in the report by Jeong et al. [18].Run-to-run epi growth variations are inevitable on account of the growing conditions of HVPE, and the reproducibility needs to be improved.Still, these results fairly demonstrate that the proposed HSD device can be a promising approach to improve BV as well as R on-sp in comparison with the fabricated OD device.We believe the BV can be further improved by adopting a conventional dielectric passivation layer and a field-plate design.
of the growing conditions of HVPE, and the reproducibility needs to be improved.Still, these results fairly demonstrate that the proposed HSD device can be a promising approach to improve BV as well as Ron-sp in comparison with the fabricated OD device.We believe the BV can be further improved by adopting a conventional dielectric passivation layer and a field-plate design.

Conclusions
In summary, we demonstrated HSD contact for heteroepitaxial α-Ga 2 O 3 MOSFETs via HVPE on 2-inch (0001) sapphire wafers.The HSD device, comprising both Ti/Al/Ni/Au Ohmic and Ni Schottky electrode regions, exhibits lower R on and similar µ FE for variable channel lengths of 20, 28, and 40 µm in comparison with conventional OD contact.The Schottky and Ohmic regions were modeled as a parallel electrodes, and turn-on characteristics were elaborated through comprehensive TCAD simulation.Furthermore, off-state simulations for the OD and HSD devices were conducted for comparison and analysis of the E-field distribution, as influenced by the Schottky contact region.The HSD device (L SD = 40 µm) exhibits a record BV of 2.8 kV at a R on-sp of 1 Ω-cm 2 , which is superior to the 1.7 kV of the conventional OD device.Our results show that the proposed HSD electrode can be promising with further advanced device structures toward α-Ga 2 O 3 power devices.
N C , N D , m 0 , and m* are the effective density of states of the conduction band, electron density of α-Ga 2 O 3 , electron mass, and electron effective mass of α-Ga 2 O 3 , respectively.Haiying et al. reported that the value of m* for α-Ga 2 O 3 is 0.276 m 0 [31].The calculated N C is 3.65 × 10 18 cm −3 , and the extracted E C -E F is 0.05 eV.The work function of Ni (Φ Ni ) is 5.01 eV, and the electron affinity of α-Ga 2 O 3 (χ α-Ga2o3

Figure 1 .
Figure 1.(a) A cross-sectional schematic and (b) top-view scanning electron microscope image of the fabricated α-Ga2O3 MOSFET with HSD contact.The blue box represents the drain electrode of OD or HSD.Dashed lines (black) show the region of the focused-ion beam milling for HR-TEM analysis.

Figure 2 .
Figure 2. HR-TEM images and corresponding EDS elemental mapping images of the (a,b) Schottky and (c,d) Ohmic contact regions.

Figure 1 . 10 Figure 1 .
Figure 1.(a) A cross-sectional schematic and (b) top-view scanning electron microscope image of the fabricated α-Ga 2 O 3 MOSFET with HSD contact.The blue box represents the drain electrode of OD or HSD.Dashed lines (black) show the region of the focused-ion beam milling for HR-TEM analysis.

Figure 2 .
Figure 2. HR-TEM images and corresponding EDS elemental mapping images of the (a,b) Schottky and (c,d) Ohmic contact regions.

Figure 2 .
Figure 2. HR-TEM images and corresponding EDS elemental mapping images of the (a,b) Schottky and (c,d) Ohmic contact regions.
compares extracted field-effect m (µFE) of the OD and HSD devices for variable LSD at VDS = 1 V and 40 V.The µFE is cal by µFE = gm • LSD/(W • COX • VDS) in the linear region and µFE = 2 • slope 2 /(COX • (W/L saturation region.The oxide capacitance (COX) is calculated to be 712 nF/cm 2 .Th the HSD device is lower than the OD device at VDS = 1 V because the α-Ga2O3/Ni S junction was not turned on completely, and this can affect the current's flow tow drain[29].In the saturation region, a high VDS of 40 V induces current paths bene Schottky region, and thus similar levels of µFE were obtained.Figure3dalso co extracted Ron.OD and Ron.HSD, and Ron.HSD is lower than Ron.OD regardless of LSD.

Figure 3 .
Figure 3. (a) Transfer and (b) output curves of a representative α-Ga2O3 MOSFET with the HSD contacts.(c) Comparison of extracted µFE for variable LSD = 20, 28, 40 µm from linear V) to saturation (VDS = 40 V) region.(d) Comparison of extracted Ron for variable LSD = 20, 28Physics-based TCAD simulation was performed to elaborate the turn-on char tics of the HSD.The simulated OD and HSD devices have same dimensions, includ = 40 µm, and the Schottky barrier height of Ga2O3/Ni was calculated based on r parameters and was set to 1.2 eV[36,37].Simulation was conducted taking t

Figure 3 .
Figure 3. (a) Transfer and (b) output curves of a representative α-Ga 2 O 3 MOSFET with the OD and HSD contacts.(c) Comparison of extracted µ FE for variable L SD = 20, 28, 40 µm from linear (V DS = 1 V) to saturation (V DS = 40 V) region.(d) Comparison of extracted R on for variable L SD = 20, 28, 40 µm.Physics-based TCAD simulation was performed to elaborate the turn-on characteristics of the HSD.The simulated OD and HSD devices have same dimensions, including L SD = 40 µm, and the Schottky barrier height of Ga 2 O 3 /Ni was calculated based on reported parameters and was set to 1.2 eV[36,37].Simulation was conducted taking the bias conditions of V GS = 8 V and V DS = 1 and 10 V. Figure4a,b depict the E-current density distributions in the vicinity of the HSD contact edge at V DS = 1 and 10 V, respectively.Figure4ashows that the current flow to the Schottky contact region (I HSD.Schottky ) is blocked -curre sity line profiles along the channel, and clearly shows that IHSD.Schottky formed at a V V.The output curves of the simulated HSD and OD devices are presented in Fig Until a certain point of VDS, which is 2.5 V in this simulation, the drain current of t device is mostly composed of IHSD.Ohmic.As VDS > 2.5 V, IHSD.Schottky starts to flow and becomes lower than Ron.OD.

Figure 4 .
Figure 4. E-current density distribution of the α-Ga2O3 MOSFET with HSD at (a) VDS = 1 V 10 V. Dotted lines and circles are cut−line and edge of the Schottky drain, respectively.(c) E density line profiles for VDS = 1, 10 V.The E-current cut-off level indicates a minimum val color legend.(d) Simulated output curves of HSD and OD.Schematic illustrations of two components through Schottky and Ohmic contact regions are also presented.

Figure 4 .
Figure 4. E-current density distribution of the α-Ga 2 O 3 MOSFET with HSD at (a) V DS = 1 V and (b) 10 V. Dotted lines and circles are cut−line and edge of the Schottky drain, respectively.(c) Ecurrent density line profiles for V DS = 1, 10 V.The E-current cut-off level indicates a minimum value in the color legend.(d) Simulated output curves of HSD and OD.Schematic illustrations of two current components through Schottky and Ohmic contact regions are also presented.

Figure 5 .
Figure 5. E-field distribution of the α-Ga2O3 MOSFET with HSD and OD devices near the (a) gate and (b) drain region.(c) Drain E-field line profiles toward lateral axis for OD and HSD.E-field line profiles toward lateral axis for OD and HSD at (d) channel and (e) UID layer.

Figure 5 .
Figure 5. E-field distribution of the α-Ga 2 O 3 MOSFET with HSD and OD devices near the (a) gate and (b) drain region.(c) Drain E-field line profiles toward lateral axis for OD and HSD.E-field line profiles toward lateral axis for OD and HSD at (d) channel and (e) UID layer.

Figure 6 .
Figure 6.Three terminal off-state breakdown characteristics of α-Ga2O3 MOSFET with the HSD contact in comparison with the OD device.

Figure 6 .
Figure 6.Three terminal off-state breakdown characteristics of α-Ga 2 O 3 MOSFET with the HSD contact in comparison with the OD device.

Table 1 .
Comparison of electrical characteristics of heteroepitaxial Ga 2 O 3 MOSFETs on a sapphire substrate.

Table 1 .
Comparison of electrical characteristics of heteroepitaxial Ga2O3 MOSFETs on a sapphire substrate.