Showing papers by "Eric R. Heller published in 2016"

3.8-MV/cm Breakdown Strength of MOVPE-Grown Sn-Doped $\beta $ -Ga 2 O 3 MOSFETs

Abstract: A Sn-doped (100) $\beta $ -Ga2O3 epitaxial layer was grown via metal–organic vapor phase epitaxy onto a single-crystal, Mg-doped semi-insulating (100) $\beta $ -Ga2O3 substrate. Ga2O3-based metal–oxide–semiconductor field-effect transistors with a 2- $\mu \text{m}$ gate length ( $L_{G})$ , 3.4- $\mu \text{m}$ source–drain spacing ( $L_{\textrm {SD}})$ , and 0.6- $\mu \text{m}$ gate–drain spacing ( $L_{\textrm {GD}})$ were fabricated and characterized. Devices were observed to hold a gate-to-drain voltage of 230 V in the OFF-state. The gate-to-drain electric field corresponds to 3.8 MV/cm, which is the highest reported for any transistor and surpassing bulk GaN and SiC theoretical limits. Further performance projections are made based on layout, process, and material optimizations to be considered in future iterations.

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

Abstract: 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.

Benefits of Considering More than Temperature Acceleration for GaN HEMT Life Testing

Abstract: The purpose of this work was to investigate the validity of Arrhenius accelerated-life testing when applied to gallium nitride (GaN) high electron mobility transistors (HEMT) lifetime assessments, where the standard assumption is that only critical stressor is temperature, which is derived from operating power, device channel-case, thermal resistance, and baseplate temperature. We found that power or temperature alone could not explain difference in observed degradation, and that accelerated life tests employed by industry can benefit by considering the impact of accelerating factors besides temperature. Specifically, we found that the voltage used to reach a desired power dissipation is important, and also that temperature acceleration alone or voltage alone (without much power dissipation) is insufficient to assess lifetime at operating conditions.

Electroluminescence Microscopy of Cross-Sectioned AlGaN/GaN High-Electron Mobility Transistors

Abstract: We report an electroluminescence (EL) microscopy study of operating cross-sectioned AlGaN/GaN high-electron mobility transistors. By examining devices in a cross-sectional view, the distribution and intensity of photons emitted from underneath the optically opaque metal of the gate and drain structures can be studied. The location and the shape of EL bright spots were quantitatively compared with simulated device behavior, revealing a strong correlation between the measured EL intensity and the expected distribution of hot electrons in the channel. Under constant low-power conditions, the bulk of the EL signal migrates from the drain edge of the gate field plate to the drain edge of the source-connected field plate (SCFP) as the drain bias is increased. Hot electrons have been cited as a dominant contributor to device degradation for some devices, so quantifying their location and bias dependence is critical to understanding how this degradation rate might scale with bias and device design. In addition, devices both with and without an SCFP were imaged to quantitatively investigate the influence of the field plate on the EL signal. Finally, a measurement of the spectra of EL signals is used to estimate the temperature of hot electrons in the device.

Nanometer-Scale Strain Measurements in AlGaN/GaN High-Electron Mobility Transistors During Pulsed Operation

Abstract: Electric, thermal, and mechanical strain fields drive the degradation of AlGaN/GaN high-electron mobility transistors (HEMTs). The resulting mechanical strains within the devices are particularly important. However, a lack of high-resolution measurements of device deformation has limited progress in understanding the related phenomena. This paper presents the atomic force microscope measurements of thermomechanical deformation of AlGaN/GaN HEMT devices during pulsed operation. We investigate the devices with various operating conditions: drain–source voltage, $V_{\mathrm {\mathrm {DS}}}$ , of 0–50 V; drain–source power of 0–6 W/mm; and operating frequency of 55–400 kHz. As $V_{\mathrm {\mathrm {DS}}}$ increases, thermomechanical deformation decreases, especially in the region above the gate. An electrothermomechanical model closely matches with and helps to explain the measurements. According to the model, the maximum periodic tensile thermal stress, which occurs at the drain-side edge of the gate footprint, is 55% larger for $V_{\mathrm {\mathrm {DS}}} = 10$ V than for $V_{\mathrm {\mathrm {DS}}} = 48$ V for the same device power. The maximum tensile thermal stress in the device depends on the gate temperature and not the maximum device temperature. As $V_{\mathrm {\mathrm {DS}}}$ increases, the hotspot moves away from the gate, leading to lower gate temperature rise and lower tensile thermal stress.

Direct measurement of defect and dopant abruptness at high electron mobility ZnO homojunctions

Abstract: Due to a strong Fermi-level mismatch, about 10% of the electrons in a 5-nm-thick highly Ga-doped ZnO (GZO) layer grown by molecular beam epitaxy at 250 °C on an undoped ZnO buffer layer transfer to the ZnO (Debye leakage), causing the measured Hall-effect mobility (μH) of the GZO/ZnO combination to remarkably increase from 34 cm2/V s, in thick GZO, to 64 cm2/V s. From previous characterization of the GZO, it is known that ND = [Ga] = 1.04 × 1021 and NA = [VZn] = 1.03 × 1020 cm−3, where ND, NA, and [VZn] are the donor, acceptor, and Zn-vacancy concentrations, respectively. In the ZnO, ND = 3.04 × 1019 and NA = 8.10 × 1018 cm−3. Assuming the interface is abrupt, theory predicts μH = 61 cm2/V s, with no adjustable parameters. The assumption of abruptness in [Ga] and [VZn] profiles is confirmed directly with a differential form of depth-resolved cathodoluminescence spectroscopy coupled with X-ray photoelectron spectroscopy. An anneal in Ar at 500 °C for 10 min somewhat broadens the profiles but causes no appreciable degradation in μH and other electrical properties.

Debye tail mobility enhancement in ZnO:Ga/ZnO structures

Abstract: A highly-Ga-doped ZnO (GZO) layer of thickness d grown by molecular-beam epitaxy on an undoped ZnO buffer layer exhibits enhanced mobility μ due to electron diffusion (about 2 nm) from the low-mobility GZO into the high-mobility ZnO. For d = 300 nm, the combined GZO/ZnO structure has Hall mobility μ = 34.2 cm2/V-s, due almost entirely to electrons in the GZO, whereas for d = 50, 25, or 5 nm, μ = 37.0, 43.4, and 64.1 cm2/V-s, respectively, due to the influence of electrons in the ZnO. This observation of an increase of μ with decrease in d is very unusual for thin films of GZO on various substrates. However, Poisson analysis and degenerate scattering theory accurately predict the measured values of μ vs d with no adjustable parameters. For the case d = 5 nm, only 9.7% of the electrons from the GZO diffuse into the ZnO, but those closest to the interface can have μ > 200 cm2/V-s, raising the overall mobility from 34 to 64 cm2/V-s. More complicated structures can produce higher percentages of electrons in the ZnO and thus even higher mobilities. For example, simulation shows that six repeated units of a 1-nm-GZO/2-nm-ZnO structure will have 43% of the electrons in the ZnO and an average mobility of 152 cm2/V-s. This structure has roughly the same conductance as that of a GZO-only layer having the same total thickness (18 nm), but a much lower free-carrier concentration and thus a much higher transmittance in the near IR. This “Debye-tail” technology allows optimization of the conductance/transmittance tradeoff for different applications of transparent conductive oxides.