Kelson D. Chabak

3.8-MV/cm Breakdown Strength of MOVPE-Grown Sn-Doped

A Sn-doped (100) β-Ga₂O₃ epitaxial layer was grown via metal-organic vapor phase epitaxy onto a single-crystal, Mg-doped semi-insulating (100) β-Ga₂O₃ substrate. Ga₂O₃-based metal-oxide-semiconductor field-effect transistors with a 2-μm gate length (L_G), 3.4-μm source-drain spacing (L_SD), and 0.6-μm gate-drain spacing (L_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.

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We report the first CW Ka-band radio-frequency (RF) power measurements at 35 GHz from a passivated Al_0.82In_0.18N/GaN high-electron mobility transistor on SiC with 9.8-nm-thin barrier. This device delivered a maximum of 5.8 W/mm with a power-added efficiency of 43.6% biased at V_DS = 20 V and 10% I_DSS when matched for power at CW. The device was grown by metal-organic chemical vapor deposition with 2.8-¿m source-drain spacing and a gate length of 160 nm. An excellent ohmic contact was obtained with an R_c of 0.62 ¿·mm. The maximum extrinsic transconductance was 354 mS/mm with an I_DSS of 1197 mA/mm at a V_GS of 0 V, an ft of 79 GHz, and an f_max of 113.8 GHz.

-Ga 2 O 3 MOSFETs

We report on electrical characterization and uniformity measurements of the first conventionally processed AlGaN/GaN high electron mobility transistors (HEMTs) on free-standing chemical-vapor-deposited (CVD) diamond substrate wafers. DC and RF device performance is reported on HEMTs fabricated on ~ 130-¿m-thick and 30-mm round CVD diamond substrates without mechanical carrying wafers. A measured fT ·LG product of 12.5 GHz ·¿m is the best reported data for all GaN-on-diamond technology. X-band power performance of AlGaN/GaN HEMTs on diamond is reported to be 2.08 W/mm and 44.1% power added efficiency. This letter demonstrates the potential for GaN HEMTs to be fabricated on CVD diamond substrates utilizing contact lithography process techniques. Further optimization of the epitaxy and diamond substrate attachment process could provide for improvements in thermal spreading while preserving the electrical properties.

High-Power Ka-Band Performance of AlInN/GaN HEMT With 9.8-nm-Thin Barrier

The effects of proton irradiation energy on dc, small signal, and large signal rf characteristics of AlGaN/GaN high electron mobility transistors (HEMTs) were investigated. AlGaN/GaN HEMTs were irradiated with protons at fixed fluence of 5 × 1015/cm2 and energies of 5, 10, and 15 MeV. Both dc and rf characteristics revealed more degradation at lower irradiation energy, with reductions of maximum transconductance of 11%, 22%, and 38%, and decreases in drain saturation current of 10%, 24%, and 46% for HEMTs exposed to 15, 10, and 5 MeV protons, respectively. The increase in device degradation with decreasing proton energy is due to the increase in linear energy transfer and corresponding increase in nonionizing energy loss with decreasing proton energy in the active region of the HEMTs. After irradiation, both subthreshold drain leakage current and reverse gate current decreased more than 1 order of magnitude for all samples. The carrier removal rate was in the range 121–336 cm−1 over the range of proton energies employed in this study.

Full-Wafer Characterization of AlGaN/GaN HEMTs on Free-Standing CVD Diamond Substrates

We report on Sn-doped β-Ga2O3 MOSFETs grown by molecular beam epitaxy with as-grown carrier concentrations from 0.7 × 1018 to 1.6 × 1018 cm−3 and a fixed channel thickness of 200 nm. A pulsed current density of >450 mA/mm was achieved on the sample with the lowest sheet resistance and a gate length of 2  μm. Our results are explained using a simple analytical model with a measured gate voltage correction factor based on interface charges that accurately predict the electrical performance for all doping variations.

Dependence on proton energy of degradation of AlGaN/GaN high electron mobility transistors

Micro-Raman thermography, microphotoluminescence spectroscopy, and thermal simulation were used to study the thermal properties of AlGaN/GaN heterostructure field-effect transistors grown on semi-insulating bulk GaN substrates. A bulk GaN thermal conductivity of 260 was determined from temperature measurements on operating devices in combination with finite-difference thermal simulations. This is significantly higher than typical thin GaN epilayer thermal conductivities, due to a lower dislocation density in bulk GaN. Despite the thermal conductivity of bulk GaN being lower than that of SiC, transistors on bulk GaN exhibited a similar thermal resistance as GaN-on-SiC devices, attributed to the absence of a thermal boundary resistance between the device epilayers and substrate for GaN-on-GaN devices.

High pulsed current density β-Ga2O3 MOSFETs verified by an analytical model corrected for interface charge

This letter presents transistor device results on ultrathin AIN/GaN high-electron mobility transistors grown on a sapphire substrate with high dc/RF performance, including low gate leakage and high transconductance. Devices with 80and 180-nm T-gates are compared, which demonstrate drain-induced OFF-state gate leakage currents below 10-6 A/mm and extrinsic transconductance gm ~ 500 mS/mm by utilizing a ~2-3 nm amorphous oxide layer formed under the T-gate during processing. In addition, excellent dc results such as RC <; 0.50 Ω · mm and pulsed IDS,max ~1.75 A/mm are reported. Small-signal RF performance using an 80-nm T-gate achieved ft >; 100 GHz operation, which is among the best so far reported for AIN/GaN technology.

Thermal Properties of AlGaN/GaN HFETs on Bulk GaN Substrates

We report on a dc/RF performance of lattice-strained AlInN/GaN high-electron mobility transistors (HEMTs) on SiC substrate. HEMT devices were fabricated with gate periphery of 2 × 150 μm with an 80-nm T-gate and ~2.5-μm source-drain spacing. Fabricated devices simultaneously demonstrated up to 2.11 A/mm with f_t-ext = 104 GHz and f_t-int = 113 GHz. The high performance is attributed to the combination of low R_sh ~ 230 Ω/sq (μ ~ 1079 cm²/V · s, n_s ~ 2.39 × 10¹³ cm^-2) and thin ~110-Å total barrier thickness with a short gate length. Other device parameters include R_c = 0.29 Ω · mm, I_g,leak = 27.9 μA/mm, g_m,peak = 432 mS/mm, and V_th = -5.8 V. To our knowledge, this is among the highest current densities reported for any HEMT operating with a unity current gain frequency exceeding 100 GHz.

High-Performance AlN/GaN HEMTs on Sapphire Substrate With an Oxidized Gate Insulator

A GaN high electron mobility transistor monolithic microwave integrated circuit (MMIC) designer typically has to choose a device design either for high-gain millimeter-wave operation with a short gate length, or for high-power-density X-band operation with a much larger gate/field-plate structure. We provide the designer the option of incorporating two different devices by implementing a 0.14-μm gate length GaN MMIC process capable of high-efficiency Ka-band operation while simultaneously achieving high power density in the same process flow. The key process enabler simply uses the capacitor top plate in the MMIC process as a field plate on the passivation layer. On two separate devices on the same chip using the same MMIC process flow, we demonstrate 7.7 W/mm at 35 GHz and VDS = 30 V on a standard 4 × 65-μm T-gated FET and then 12.5 W/mm at 10 GHz and VDS = 60 V on a 4 × 75-μm T-gated FET by adding a field plate. These are the highest reported power densities achieved simultaneously at X-band and Ka-band in a single wideband GaN MMIC process.

Strained AlInN/GaN HEMTs on SiC With 2.1-A/mm Output Current and 104-GHz Cutoff Frequency

The reliability of AlGaN/GaN HEMTs processed on bulk GaN substrates was studied using electrical and optical methods, showing a decreasing degradation with increasing baseplate temperature (Tb). Generation of traps spatially located in both intrinsic and extrinsic HEMT regions was found to be most pronounced for OFF-state bias stress performed at room Tb, while increasing Tb up to 150°C decreased trap generation underneath the gate perimeter. This was attributed to degradation driven by hot electrons as it should dominate over defect-related degradation mechanisms in GaN-on-GaN devices.