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A systematic investigation of the electrical characteristics of \b{eta}-Ga2O3 Schottky barrier diodes (SBDs) has been conducted under high-dose 60Co gamma radiation, with total cumulative doses reaching up to 5 Mrad (Si). Initial exposure of the diodes to 1 Mrad resulted in a significant decrease in on-current and an increase in on-resistance compared to the pre-radiation condition, likely due to the generation of radiation-induced deep-level acceptor traps. However, upon exposure to higher gamma radiation doses of 3 and 5 Mrad, partial recovery of the device performance occurred, attributed to a radiation annealing effect. The capacitance-voltage (C-V) characterization revealed that the net carrier concentration in the $\beta$-Ga$_2$O$_3$ drift layer reduced from $\sim$3.19 $\times$ 10$^{16}$ cm$^{-3}$ to $\sim$3.05 $\times$ 10$^{16}$ cm$^{-3}$ after 5 Mrad (Si) irradiation. Temperature-dependent I-V characteristics showed that irradiation leads to a reduction in both forward and reverse current across all investigated temperatures ranging from 25 to 250$^\circ$C, accompanied by slight increases in on-resistance, ideality factors, and Schottky barrier heights. The reverse breakdown characteristics of the $\beta$-Ga$_2$O$_3$ SBDs showed a slight increase of the breakdown voltage after radiation. Overall, $\beta$-Ga$_2$O$_3$ Schottky diode exhibits high resilience to gamma irradiation, with performance degradation mitigated by radiation-induced self-recovery, highlighting its potential for radiation-hardened electronic applications in extreme environments.

To investigate the transport properties in random alloys, it is important to model the alloy disorder using supercells. Though traditional methods like Virtual Crystal Approximation (VCA) are computationally efficient, the local disorder in the system is not accurately captured as artificial translational symmetry is imposed on the system. However, in the case of supercells, the error introduced by self-image interaction between the impurities is reduced and translational symmetry is explicitly imposed over larger length scales. In this work, we have investigated the Effective Phonon Dispersion (EPD) and transport properties, from first principle calculations using supercells in AlxGa1-xN alloy systems. Using our in-house developed code, the EPD of AlGaN is obtained and the individual modes are identified. Next, we discuss our in-house developed method to calculate low-field transport properties in supercells. First to validate our methods we have solved the Boltzmann Transport Equation using Rode method to compare the phonon limited mobility in the 4 atom GaN primitive cell and 12 atom GaN supercell. Using the same technique, we have investigated the low field transport in random AlxGa1-xN alloy systems. Our calculations show that along with alloy scattering, electron-phonon scattering may also play an important role at room temperature and high-temperature device operation. This technique opens up the path for calculating phonon-limited transport properties in random alloy systems.

In this work, the Effective Phonon Dispersion of $\beta$- (Al$_x$Ga$_{1-x}$)$_2$O$_3$ alloy is investigated using the Phonon unfolding formalism. To capture the true randomness of the system, supercells are designed using the technique of special quasirandom structues. The phonon unfolding technique is then used to obtain the effective phonon dispersion (EPD) for aluminium fractions of 18.75 \% and 37.5 \% with respect to gallium atoms. The impact of disorder on the high frequency and DC dielectric constant is also investigated. The unfolding procedure was also applied to the dipole moment where the projection probability was used to obtain the IR spectra of the disordered system. The calculated properties gives us further insights to calculate the transport properties in these alloy semiconductors while capturing the true randomness of the system.

This letter reports a highly scaled 90 nm gate length beta-Ga2O3 T-gate MOSFET with no current collapse and record power gain cut off frequency (fMAX). The epitaxial stack of 60 nm thin channel MOSFET was grown by Molecular Beam Epitaxy (MBE) and highly doped (n++) contact regrowth was carried out by Metal Organic Chemical Vapour Deposition (MOCVD) in the source/drain region. Maximum on current (IDS, MAX) of 160 mA/mm and transconductance (gm) around 36 mS/mm was measured at VDS= 10 V for LSD= 1.5 micrometer channel length. Transconductance is limited by higher channel sheet resistance (Rsheet). We observed no current collapse for both drain and gate lag measurement even at higher VDG,Q quiescent bias points. This is the first report of Ga2O3 FET showing no current collapse without any external passivation. Breakdown voltage around 125 V was reported for LGD= 1.2 micrometer. We extracted 27 GHz current gain cut off frequency (fT) and 55 GHz fMAX for 20 V drain bias. fMAX value mentioned here is the highest for Ga2O3 and the first demonstration of 55 GHz operation. fT. VBR product of 3.375 THz.V has been calculated which is comparable with state-of-art GaN HEMT. This letter suggests that Ga2O3 can be a suitable candidate for X-band application.

The lack of p-type doping has impeded the development of vertical gallium oxide (Ga2O3) devices. Current blocking layers (CBL) using implanted deep acceptors has been used to demonstrate vertical devices. This paper presents the first demonstration of in situ Mg-doped beta-Ga2O3 CBLs grown using metalorganic chemical vapor deposition. Device structures were designed with in-situ Mg doped layers with varied targeted Mg doping concentrations, which were calibrated by quantitative secondary ion mass spectroscopy (SIMS). The effectiveness of the CBL is characterized using temperature dependent current-voltage measurements using n-Mg-doped-n structures, providing crucial insight into the underlying mechanisms. To further validate the experimental results, a TCAD simulation is performed and the electrically active effective doping is found to be dependent on the Mg-doping density, offering a new perspective on the optimization of CBL performance. Breakdown measurements show a 3.4 MV/cm field strength. This study represents a significant step forward in the development of Ga2O3-based devices and paves the way for future advancements in this exciting field.

In this article, we investigate the Single Events Effects (SEE) leading to Single Event Burnout (SEB) in $\beta$-Ga$_2$O$_3$ MOSFETs. Using Silvaco TCAD, 2D simulations were performed to understand the mechanism behind the SEB mechanism in lateral Ga$_2$O$_3$ MOSFETS. The high electric fields in the channel played a critical role leading to high impact generation rates and eventual SEB. To reduce the electric field in the channel, radiation hardened designs are then proposed with rounded gates and the use of a combination of high permittivity (k) dielectric with SiO$_2$. With HfO$_2$-SiO$_2$ dielectric combination, the SEB threshold of 550V at LET=10 MeV/mg/cm$^2$ is seen. However, to operate under extreme radiation conditions, a combination of very high-k dielectric material BaTiO$_3$ with SiO$_2$ is proposed. Using the radiation hardened design, SEB thresholds up to 1000 V for LET=75 MeV/mg/cm$^2$ could be achieved which is higher than the state-of-the-art technology. The energy dissipated during the ion strike event is also calculated and it is observed that it is lower than that of SiC MOSFETs.

This letter reports high-performance $\mathrm{\beta} Ga2O3 thin channel MOSFETs with T-gate and degenerately doped source/drain contacts regrown by MOCVD. Gate length scaling (LG= 160-200 nm) leads to a peak drain current (ID,MAX) of 285 mA/mm and peak trans-conductance (gm) of 52 mS/mm at 10 V drain bias with 23.5 Ohm mm on resistance (Ron). A low metal/n+ contact resistance of 0.078 Ohm mm was extracted from TLM measurement. Ron is dominated by interface resistance between channel and regrown layer. A gate-to-drain breakdown voltage of 192 V is measured for LGD = 355 nm resulting in average breakdown field (E_AVG) of 5.4 MV/cm. This E_AVG is the highest reported among all sub-micron gate length lateral FETs. RF measurements on 200 nm Silicon Nitride (Si3N4) passivated device shows a current gain cut off frequency (f_T) of 11 GHz and record power gain cut off frequency (f_MAX) of 48 GHz. The f_T.V_Br product is 2.11 THz.V for 192 V breakdown voltage. The switching figure of merit exceeds that of silicon and is comparable to mature wide-band gap devices.

In this article, the electronic band structure $\beta-(Al_xGa_{1-x})_2O_3$ alloy system is calculated with $\beta-Ga_2O_3$ as the bulk crystal. The technique of band unfolding is implemented to obtain the effective bandstructure \textit{(EBS)} for aluminium fractions varying between 12.5\% and 62.5\% with respect to the gallium atoms. A 160 atom supercell is used to model the disordered system that is generated using the technique of special quasirandom structures which mimics the site correlation of a truly random alloy and reduces the configurational space that arises due to the vast enumeration of alloy occupation sites. The impact of the disorder is then evaluated on the electron effective mass and bandgap which is calculated under the generalized gradient approximation \textit{(GGA)}. The EBS of disordered systems gives an insight into the effect of the loss of translational symmetry on the band topology which manifests as band broadening and can be used to evaluate disorder induced scattering rates and electron lifetimes. This technique of band unfolding can be further extended to alloy phonon dispersion and subsequently phonon lifetimes can also be evaluated from the band broadening.

$\beta$-Gallium Oxide (Ga$_{2}$O$_{3}$) is an extensively investigated ultrawide-bandgap semiconductor for potential applications in power electronics and RF switching. The room temperature bulk electron mobility ($\sim$200 cm$^{2}$V$^{-1}$s$^{-1}$) is comparatively low and is limited by the 30 phonon modes originating from its 10-atom primitive cell. The theoretically calculated saturation velocity is 1-2$\times$10$^{7}$ cms$^{-1}$ which is comparable to GaN. The high field electron transport in the 2DEG is explored in this work based on the first principles calculated parameters. A self-consistent calculation on a given heterostructure design gives the confined eigenfunctions and eigenenergies. The intrasubband and the intersubband scattering rates are calculated based on the Fermi's golden rule considering LO phonon-plasmon screening. The high field characteristics are extracted from the full-band Monte Carlo simulation of heterostructures at 300 K. The motion of electrons in the 2DEG and the bulk is treated through an integrated Monte Carlo program which outputs the steady state zone population, transient dynamics and the velocity-field curves for a few heterostructure designs. The critical field for saturation does not change significantly from bulk values, however an improved peak velocity is calculated at a higher 2DEG density. The velocity at low 2DEG densities is impacted by the antiscreening of LO phonons which plays an important role in shaping the zone population. A comparison with the experimental measurements is also carried out and possible origins of the discrepancies with experiments is discussed.

Field-plated (FP) depletion-mode MOVPE-grown $\beta$-Ga$_2$O$_3$ lateral MESFETs are realized with superior reverse breakdown voltages and ON currents. A sandwiched SiN$_x$ dielectric field plate design was utilized that prevents etching-related damage in the active region and a deep mesa-etching was used to reduce reverse leakage. The device with L$_{GD}$ = 34.5 $\mu$m exhibits an ON current (I$_{DMAX}$) of 56 mA/mm, a high I$_{ON}$/I$_{OFF}$ ratio $>$ 10$^8$ and a very low reverse leakage until catastrophic breakdown at $\sim$ 4.4kV. The highest measurable V$_{BR}$ recorded was 4.57 kV (L$_{GD}$ = 44.5 $\mu$m). An LFOM of 132 MW/cm$^2$ was calculated for a V$_{BR}$ of $\sim$ 4.4 kV. The reported results are the first $>$ 4kV-class Ga$_2$O$_3$ transistors to surpass the theoretical FOM of Silicon. These are also the highest I$_{DMAX}$ and lowest R$_{ON}$ values achieved simultaneously for any $\beta$-Ga$_2$O$_3$ device with V$_{BR}$ $>$ 4kV to date. This work highlights that high breakdown voltages (V$_{BR}$), high lateral figure of merit (LFOM) and high ON currents can be achieved simultaneously in $\beta$-Ga$_2$O$_3$ lateral transistors., Comment: 4 pages, 5 figures

A comprehensive study of drain current dispersion effects in $\beta$-Ga$_2$O$_3$ FETs has been done using DC, pulsed and RF measurements. Both virtual gate effect in the gate-drain access region and interface traps under the gate are most plausible explanations for the experimentally observed pulsed current dispersion and high temperature threshold voltage shift respectively. Unpassivated devices show significant current dispersion between DC and pulsed IV response due to gate lag effect characterized by time constants in the range of 400~$\mu s$ to 600~$\mu s$. An activation energy of 99~$meV$ is estimated from temperature dependent Arrhenius plots. A variable range hopping based slow transport in conjunction with the observed shallow trap level is attributed to the observed slow transient response of drain current with respect to time. Reactive ion etching step during the device fabrication is most likely responsible for introducing the traps. Effect of traps can be minimized by using surface passivation layers, in this case, Silicon Nitride which shows significant improvement in the current dispersion and RF cutoff frequency. This work demonstrates the detrimental effect the traps can have on the current dispersion which significantly limits the high frequency operation of the device.

The $\alpha$ phase of $Ga_{2}O_{3}$ is an ultra-wideband semiconductor with potential power electronics applications. In this work, we calculate the low field electron mobility in $\alpha-Ga_{2}O_{3}$ from first principles. The 10 atom unit cell contributes to 30 phonon modes and the effect of each mode is taken into account for the transport calculation. The phonon dispersion and the Raman spectrum are calculated under the density functional perturbation theory formalism and compared with experiments. The IR strength is calculated from the dipole moment at the $\Gamma$ point of the Brillouin zone. The electron-phonon interaction elements (EPI) on a dense reciprocal space grid is obtained using the Wannier interpolation technique. The polar nature of the material is accounted for by interpolating the non-polar and polar EPI elements independently as the localized nature of the Wannier functions are not suitable for interpolating the long-range polar interaction elements. For polar interaction the full phonon dispersion is taken into account. The electron mobility is then calculated including the polar, non-polar and ionized impurity scattering.

$\mathrm{\beta}$-Gallium oxide ($\mathrm{\beta\mbox{-}Ga_{2}O_{3}}$) is an emerging widebandgap semiconductor for potential application in power and RF electronics applications. Initial theoretical calculation on a 2-dimensional electron gas (2DEG) in $\mathrm{\beta\mbox{-}(Al_{x}Ga_{1-x})_{2}O_{3}/Ga_{2}O_{3}}$ heterostructures show the promise for high speed transistors. However, the experimental results do not get close to the predicted mobility values. In this work, We perform more comprehensive calculations to study the low field 2DEG transport properties in the $\mathrm{\beta\mbox{-}(Al_{x}Ga_{1-x})_{2}O_{3}/Ga_{2}O_{3}}$ heterostructure. A self-consistent Poisson-Schrodinger simulation of heterostructure is used to obtain the subband energies and wavefunctions. The electronic structure, assuming confinement in a particular direction, and the phonon dispersion is calculated based on first principle methods under DFT and DFPT framework. Phonon confinement is not considered for the sake of simplicity. The different scattering mechanisms that are included in the calculation are phonon (polar and non-polar), remote impurity, alloy and interface-roughness. We include the full dynamic screening polar optical phonon screening. We report the temperature dependent low-field electron mobility.

A comprehensive current-voltage (I-V) characterization is performed for three different Schottky contacts; Pt, Ni and Ti, to unintentionally doped (UID) \{beta}-(Al0.19Ga0.81)2O3 grown by molecular beam epitaxy (MBE) on \{beta}-Ga2O3 for temperatures ranging between 25C -300C. Reciprocal space mapping shows the (Al0.19Ga0.81)2O3 films are strained and lattice matched to the substrate. Schottky Barrier Height (SBH), ideality factor (n), and series resistance (Rs) are extracted from the I-V characteristics for the three types of metals and temperatures. Room temperature capacitance-voltage (C-V) measurements revealed fully depleted \{beta}-(Al0.19Ga0.81)2O3 layer. Extracted room temperature SBHs after zero field correction for Pt, Ni and Ti were 2.39 eV, 2.21 eV, and 1.22 eV respectively. Variation of SBHs with metal clearly indicates the dependence on work function.

This work reports an investigation of electron transport in monoclinic \beta-Ga2O3 based on a combination of density functional perturbation theory based lattice dynamical computations, coupling calculation of lattice modes with collective plasmon oscillations and Boltzmann theory based transport calculations. The strong entanglement of the plasmon with the different longitudinal optical (LO) modes make the role LO-plasmon coupling crucial for transport. The electron density dependence of the electron mobility in \beta-Ga2O3 is studied in bulk material form and also in the form of two-dimensional electron gas. Under high electron density a bulk mobility of 182 cm2/ V.s is predicted while in 2DEG form the corresponding mobility is about 418 cm2/V.s when remote impurities are present at the interface and improves further as the remote impurity center moves away from the interface. The trend of the electron mobility shows promise for realizing high electron mobility in dopant isolated electron channels. The experimentally observed small anisotropy in mobility is traced through a transient Monte Carlo simulation. It is found that the anisotropy of the IR active phonon modes is responsible for giving rise to the anisotropy in low-field electron mobility., Comment: to be appeared in Journal of Materials Research

A theoretical investigation of extremely high field transport in an emerging wide-bandgap material $\beta-Ga_2O_3$ is reported from first principles. The signature high-field effect explored here is impact ionization. Interaction between a valence-band electron and an excited electron is computed from the matrix elements of a screened Coulomb operator. Maximally localized Wannier functions (MLWF) are utilized in computing the impact ionization rates. A full-band Monte Carlo (FBMC) simulation is carried out incorporating the impact ionization rates, and electron-phonon scattering rates. This work brings out valuable insights on the impact ionization coefficient (IIC) of electrons in $\beta-Ga_2O_3$. The isolation of the $\Gamma$ point conduction band minimum by a significantly high energy from other satellite band pockets play a vital role in determining ionization co-efficients. IICs are calculated for electric fields ranging up to 8 MV/cm for different crystal directions. A Chynoweth fitting of the computed IICs is done to calibrate ionization models in device simulators., Comment: 13 pages, 4 figures, 1 table, in press J. Appl. Phys

We investigate the high-field transport in monoclinic \(\beta\)-Ga\textsubscript{2}O\textsubscript{3}} using a combination of ab initio calculations and full band Monte Carlo (FBMC) simulation. Scattering rate calculation and the final state selection in the FBMC simulation use complete wave-vector (both electron and phonon) and crystal direction dependent electron phonon interaction (EPI) elements. We propose and implement a semi-coarse version of the Wannier-Fourier interpolation method [F. Giustino, M. L. Cohen, and S. G. Louie, Physical Review B, vol. 76, no. 16, 2007] for short-range non-polar optical phonon (EPI) elements in order to ease the computational requirement in FBMC simulation. During the interpolation of the EPI, the inverse Fourier sum over the real-space electronic grids is done on a coarse mesh while the unitary rotations are done on a fine mesh. This paper reports the high field transport in monoclinic \(\beta\)-Ga\textsubscript{2}O\textsubscript{3}} with deep insight on the contribution of electron-phonon interactions, and velocity-field characteristics for electric fields ranging up to 450 kV/cm in different crystal directions. A peak velocity of $2\times10^{7}& cm/s is estimated at an electric field of 200 kV/cm.

The thermoelectric transport coefficients of electrons in two recently emerged transition metal dichalcogenides(TMD), MoS2 and WSe2, are calculated by solving Boltzmann Transport equation and coupled electrical and thermal current equations using Rode iterative technique. Scattering from localized donor impurities, acoustic deformation potential, longitudinal optical (LO) phonons, and substrate induced remote phonon modes are taken into account. Hybridization of TMD plasmon with remote phonon modes is investigated. Dynamic screening under linear polarization response is explored in TMDs sitting on a dielectric environment and the screened electron-phonon coupling matrix elements are calculated. The effect of screening and substrate induced remote phonon mediated scattering on the transport coefficients of the mentioned materials is explained. The transport coefficients are obtained for a varying range of temperature and doping density for three different types of substrates SiO2, Al2O3, and HfO2. The thermoelectric properties of interest including Seebeck coefficient, Peltier coefficient, and electronic thermal conductivity are calculated.

Control of faceting during epitaxy is critical for nanoscale devices. This work identifies the origins of gaps and different facets during regrowth of InGaAs adjacent to patterned features. Molecular beam epitaxy (MBE) near SiO2 or SiNx led to gaps, roughness, or polycrystalline growth, but metal modulated epitaxy (MME) produced smooth and gap-free "rising tide" (001) growth filling up to the mask. The resulting self-aligned FETs were dominated by FET channel resistance rather than source-drain access resistance. Higher As fluxes led first to conformal growth, then pronounced {111} facets sloping up away from the mask., Comment: 18 pages, 7 figures

This work reports the design and analysis of an n-type tunneling field effect transistor based on InN. The tunneling current is evaluated from the fundamental principles of quantum mechanical tunneling and semiclassical carrier transport. We investigate the RF performance of the device. High transconductance of 2 mS/um and current gain cut-off frequency of around 460 GHz makes the device suitable for THz applications. A significant reduction in gate to drain capacitance is observed under relatively higher drain bias. In this regard, the avalanche breakdown phenomenon in highly doped InN junctions is analyzed quantitatively for the first time and is compared to that of Si and InAs.

N-polar GaN channel mobility is important for high frequency device applications. In this Letter, we report the theoretical calculations on the surface optical (SO) phonon scattering rate of two-dimensional electron gas (2-DEG) in N-polar GaN quantum well channels with high-k dielectrics. The effect of SO phonons on 2-DEG mobility was found to be small at >5 nm channel thickness. However, the SO mobility in 3 nm N-polar GaN channels with high-k dielectrics is low and limits the total mobility. The SO scattering for SiNx dielectric GaN was found to be negligible due to its high SO phonon energy., Comment: 4 figures