A Comprehensive Computational Modeling Approach for AlGaN/GaN HEMTs

Part I: Physical Insight Into Carbon-Doping-Induced Delayed Avalanche Action in GaN Buffer in AlGaN/GaN HEMTs

Abstract: Physics behind the improvement in breakdown voltage of AlGaN/GaN HEMTs with carbon-doping of GaN buffer is discussed. Modeling of carbon as acceptor traps and self-compensating acceptor/donor traps is discussed with respect to their impact on avalanche breakdown. Impact of carbon behaving as a donor as well as acceptor traps on electric field relaxation and avalanche generation is discussed in detail to establish the true nature of carbon in GaN that delays the avalanche action. This understanding of the behavior of carbon-doping in GaN buffer is then utilized to discuss design parameters related to carbon doped buffer. Design parameters such as undoped channel thickness and relative trap concentration induced by carbon-doping are discussed with respect to the performance metrics of breakdown voltage, leakage current, sheet charge density, and dynamic ON-resistance.

Part II: Proposals to Independently Engineer Donor and Acceptor Trap Concentrations in GaN Buffer for Ultrahigh Breakdown AlGaN/GaN HEMTs

Abstract: In part I of this paper, we developed physical insights into the role and impact of acceptor and donor traps—resulting from C-doping in GaN buffer—on avalanche breakdown in AlGaN/GaN HEMT devices. It was found that the donor traps are mandatory to explain the breakdown voltage improvement. In this paper, silicon doping is proposed and explored as an alternative to independently engineer donor trap concentration and profile. Keeping in mind the acceptor and donor trap relative concentration requirement for achieving higher breakdown buffer, as depicted in part I of this paper, silicon & carbon codoping of GaN buffer is proposed and explored in this paper. The proposed improvement in breakdown voltage is supported by physical insight into the avalanche phenomena and role of acceptor/donor traps. GaN buffer design parameters and their impact on breakdown voltage as well as leakage current are presented. Finally, a modified Si-doping profile in the GaN buffer is proposed to lower the C-doping concentration near GaN channel to mitigate the adverse effects of acceptor traps in GaN buffer.

Positive Threshold Voltage Shift in AlGaN/GaN HEMTs and E-Mode Operation By ${\mathrm{Al}}_{x}{\mathrm{Ti}}_{1-x}$ O Based Gate Stack Engineering

Abstract: In this paper, for the first time, we have experimentally demonstrated enhancement mode (e-mode) AlGaN/GaN high-electron-mobility transistor (HEMT) operation by integrating p-type high- $\kappa {\mathrm {Al}}_{x}{\mathrm {Ti}}_{1-x}$ O based gate stack. Concentration of Al in Al-Ti-O system was found to be a tuning parameter for the threshold voltage of GaN HEMTs. The high- $\kappa $ properties of ${\mathrm {Al}}_{x}{\mathrm {Ti}}_{1-x}$ O as a function of Al % are studied. Superiority of AlTiO over other p-oxides such as CuO and NiO x is proven statistically. Using the high- $\kappa $ and p-type AlTiO, in conjunction with a thinner AlGaN barrier under gate, 600-V e-mode GaN HEMTs are demonstrated with superior ON-state performance ( $\text{I}_{ \mathrm{\scriptscriptstyle ON}}~\sim ~400$ mA/mm and $\text{R}_{ \mathrm{\scriptscriptstyle ON}} ={8.9}\,\,\Omega $ -mm) and gate control over channel ( $\text{I}_{ \mathrm{\scriptscriptstyle ON}}/\text{I}_{ \mathrm{\scriptscriptstyle OFF}} = {10}^{{7}}$ , SS = 73 mV/dec, and gate leakage <200 nA/mm), beside improved safe operating area reliability.

On the Role of Interface States in AlGaN/GaN Schottky Recessed Diodes: Physical Insights, Performance Tradeoff, and Engineering Guidelines

Abstract: In this paper, the impact of donor and acceptor states at the Schottky interface of fully recessed AlGaN/GaN Schottky diode is physically modeled using device TCAD and detailed experiments. This allowed us to develop physical insights into recessed AlGaN/GaN diode’s reverse breakdown, reverse leakage, and ON-state performance as a function of interface states and provided design guidelines to engineer fully recessed AlGaN/GaN Schottky diode for the maximum reverse breakdown and least reverse leakage without compromising its ON-state performance. It has been observed that donor states are responsible for high reverse leakage and reduced breakdown performance in Schottky diodes. On the other hand, the presence of acceptor states at the interface improves the diode leakage and breakdown voltage. Experiments involve a number of dry and wet surface treatments to: 1) validate computational findings and 2) find ways to cure or passivate donor states affected Schottky interface/recessed region. The introduction of acceptor traps at the Schottky interface has been proposed and experimentally verified using the Fluorine implant to cure donor state-affected Schottky interface, which improves the breakdown and reverse leakage characteristics significantly.

Two-dimensional electron gases induced by spontaneous and piezoelectric polarization charges in N- and Ga-face AlGaN/GaN heterostructures

Abstract: Carrier concentration profiles of two-dimensional electron gases are investigated in wurtzite, Ga-face AlxGa1−xN/GaN/AlxGa1−xN and N-face GaN/AlxGa1−xN/GaN heterostructures used for the fabrication of field effect transistors. Analysis of the measured electron distributions in heterostructures with AlGaN barrier layers of different Al concentrations (0.15

Two dimensional electron gases induced by spontaneous and piezoelectric polarization in undoped and doped AlGaN/GaN heterostructures

Abstract: Two dimensional electron gases in Al x Ga 12x N/GaN based heterostructures, suitable for high electron mobility transistors, are induced by strong polarization effects. The sheet carrier concentration and the confinement of the two dimensional electron gases located close to the AlGaN/GaN interface are sensitive to a large number of different physical properties such as polarity, alloy composition, strain, thickness, and doping of the AlGaN barrier. We have investigated these physical properties for undoped and silicon doped transistor structures by a combination of high resolution x-ray diffraction, atomic force microscopy, Hall effect, and capacitance‐voltage profiling measurements. The polarization induced sheet charge bound at the AlGaN/GaN interfaces was calculated from different sets of piezoelectric constants available in the literature. The sheet carrier concentration induced by polarization charges was determined

Polarization effects, surface states, and the source of electrons in AlGaN/GaN heterostructure field effect transistors

Abstract: The origin of the two-dimensional electron gas (2DEG) in AlGaN/GaN heterostructure field effect transistors is examined theoretically and experimentally. Based on an analysis of the electrostatics, surface states are identified as an important source of electrons. The role of the polarization-induced dipole is also clarified. Experimental Hall data for nominally undoped Al0.34Ga0.66N/GaN structures indicate that ∼1.65 eV surface donors are the actual source of the electrons in the 2DEG, which forms only when the barrier thickness exceeds 35 A.

Electron and hole mobilities in silicon as a function of concentration and temperature

Abstract: An analytical expression has been derived for the electron and hole mobility in silicon based on both experimental data and modified Brooks-Herring theory of mobility. The resulting expression allows one to obtain electron and hole mobility as a function of concentration up to \sim 10^{20} cm-3in an extended and continuous temperature range (250-500 K) within ± 13 percent of the reported experimental values.

The effect of surface passivation on the microwave characteristics of undoped AlGaN/GaN HEMTs

Abstract: Surface passivation of undoped AlGaN/CaN HEMT's reduces or eliminates the surface effects responsible for limiting both the RF current and breakdown voltages of the devices. Power measurements on a 2/spl times/125/spl times/0.5 /spl mu/m AlGaN/GaN sapphire based HEMT demonstrate an increase in 4 GHz saturated output power from 1.0 W/mm [36% peak power-added efficiency (PAE)] to 2.0 W/mm (46% peak PAE) with 15 V applied to the drain in each case. Breakdown measurement data show a 25% average increase in breakdown voltage for 0.5 /spl mu/m gate length HEMT's on the same wafer. Finally, 4 GHz power sweep data for a 2/spl times/75/spl times/0.4 /spl mu/m AlGaN/GaN HEMT on sapphire processed using the Si/sub 3/N/sub 4/ passivation layer produced 4.0 W/mm saturated output power at 41% PAE (25 V drain bias). This result represents the highest reported microwave power density for undoped sapphire substrated AlGaN/GaN HEMT's.