G. D. Via

Short-Channel Effect Limitations on High-Frequency Operation of AlGaN/GaN HEMTs for T-Gate Devices

AlGaN/GaN high-electron mobility transistors (HEMTs) were fabricated on SiC substrates with epitaxial layers grown by multiple suppliers and methods. Devices with gate lengths varying from 0.50 to 0.09 mum were fabricated on each sample. We demonstrate the impact of varying the gate lengths and show that the unity current gain frequency response (fT) is limited by short-channel effects for all samples measured. We present an empirically based physical model that can predict the expected extrinsic fT for many combinations of gate length and commonly used barrier layer thickness (tbar) on silicon nitride passivated T-gated AlGaN/GaN HEMTs. The result is that even typical high-aspect-ratio (gate length to barrier thickness) devices show device performance limitations due to short-channel effects. We present the design tradeoffs and show the parameter space required to achieve optimal frequency performance for GaN technology. These design rules differ from the traditional GaAs technology by requiring a significantly higher aspect ratio to mitigate the short-channel effects.

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

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.

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

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.

Effects of Sc 2 O 3 and MgO passivation layers on the output power of AlGaN/GaN HEMTs

The low temperature (100/spl deg/C) deposition of Sc/sub 2/O/sub 3/ or MgO layers is found to significantly increase the output power of AlGaN/GaN HEMTs. At 4 GHz, there was a better than 3 dB increase in output power of 0.5/spl times/100 /spl mu/m/sup 2/ HEMTs for both types of oxide passivation layers. Both Sc/sub 2/O/sub 3/ and MgO produced larger output power increases at 4 GHz than conventional plasma-enhanced chemical vapor deposited (PECVD) SiN/sub x/ passivation which typically showed /spl les/2 dB increase on the same types of devices. The HEMT gain also in general remained linear over a wider input power range with the Sc/sub 2/O/sub 3/ or MgO passivation. These films appear promising for reducing the effects of surface states on the DC and RF performance of AlGaN/GaN HEMTs.

Electric-Field-Driven Degradation in off-State Step-Stressed AlGaN/GaN High-Electron Mobility Transistors

The critical degradation voltage of AlGaN/GaN high-electron mobility transistors during off-state electrical stress was determined as a function of Ni/Au gate dimensions (0.1-0.17 μm), drain bias voltage, and source/drain-gate contact distance. Devices with different gate lengths and gate-drain distances were found to exhibit the onset of degradation at different source-drain biases but similar electric field strengths, showing that the degradation mechanism is primarily field driven. The degradation field was calculated to be ~ 1.8 MV/cm by Automatically Tuned Linear Algebra Software simulations. Transmission electron microscopy imaging showed creation of defects under the gate after dc stress.

RF Power Measurements of InAlN/GaN Unstrained HEMTs on SiC Substrates at 10 GHz

Unstrained high-electron mobility transistors (HEMTs) were fabricated from InAlN/GaN on semi-insulating SiC substrates. The devices had 0.24-mum T-gates with a total width of 2times150 mum. Final passivated performance values for these devices are I_max=1279 mA/mm, I_DSS=1182 mA/mm, R_c=0.43 Omegamiddotmm, rho_s=315 Omega/sq, f_T=45 GHz, f_max(MAG)=64 GHz, and g_m=268 mS/mm. Continuous-wave power measurements at 10 GHz produced P_sat=3.8 W/mm, G_t=8.6 dB, and PAE=30% at V_DS=20 V at 25% I_DSS. To our knowledge, these are the first power measurements reported at 10 GHz for this material

Proton irradiation of MgO- or Sc2O3 passivated AlGaN/GaN high electron mobility transistors

Abstract AlGaN/GaN high electron mobility transistors with either MgO or Sc 2 O 3 surface passivation were irradiated with 40 MeV protons at a dose of 5×10 9 cm −2 . While both forward and reverse bias current were decreased in the devices as a result of decreases in channel doping and introduction of generation–recombination centers, there was no significant change observed in gate lag measurements. By sharp contrast, unpassivated devices showed significant decreases in drain current under pulsed conditions for the same proton dose. These results show the effectiveness of the oxide passivation in mitigating the effects of surface states present in the as-grown structures and also of surface traps created by the proton irradiation.

Investigation of the effect of temperature during off-state degradation of AlGaN/GaN High Electron Mobility Transistors

Abstract AlGaN/GaN High Electron Mobility Transistors were found to exhibit a negative temperature dependence of the critical voltage ( V CRI ) for irreversible device degradation to occur during bias-stressing. At elevated temperatures, devices exhibited similar gate leakage currents before and after biasing to V CRI , independent of both stress temperature and critical voltage. Though no crack formation was observed after stress, cross-sectional TEM indicates a breakdown in the oxide interfacial layer due to high reverse gate bias.

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

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.

AlGaN/GaN HEMT on Diamond Technology Demonstration

This letter is a first report on the operation of AlGaN/GaN high-electron mobility transistors (HEMTs) atomically attached to a CVD diamond substrate. This technology demonstration shows the feasibility of producing GaN based devices on polycrystalline CVD diamond substrates to maximize heat extraction from devices operating at high power by situating the diamond substrates in the immediate proximity of the transistor channel. Such an approach offers tremendous opportunity for efficient and effective heat management of high power devices. We demonstrate the ability to preserve the electrical properties of AlGaN/GaN HEMTs throughout the GaN-on-diamond atomic attachment process and report on the fabricated DC and small-signal HEMT characteristics