Manuel Trejo

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.

Thermal Properties of AlGaN/GaN HFETs on Bulk GaN Substrates

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

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.

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

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.

Non-Arrhenius Degradation of AlGaN/GaN HEMTs Grown on Bulk GaN Substrates

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.

Comparative Study of AlGaN/GaN HEMTs on Free-Standing Diamond and Silicon Substrates for Thermal Effects

In this work, we compare for the first time the performance results of AlGaN/GaN HEMTs processed on a free-standing chemical vapor deposition (CVD) polycrystalline diamond substrate and a silicon substrate with nominally the same epitaxial AlGaN/GaN layers both grown by metal-organic chemical vapor deposition (MOCVD). The objective of this work is to compare the small signal and DC trends of the transistors fabricated on the different substrates as a function of temperature. Wafer scale results were obtained from both wafers for 2 x 150 µm devices with gate lengths of 0.18µm and 0.20µm for the silicon and CVD diamond wafers respectively.

SMALL SIGNAL AND DC CHARACTERISTICS OF ULTRA-THIN GaN/AlN/GaN HFETs

This paper presents high performance device results using an ultra-thin AlN/GaN structure on sapphire substrate with a 100-nm T-gate. Excellent dc and RF characteristics are reported, including an extrinsic transconductance of 500 mS/mm and an extrinsic ft/fmax(U) ratio of 78/111-GHz which is among the highest reported for AlN/GaN HFETs. Low gate leakage results are also presented despite the small barrier thickness and absence of a gate dielectric. Modeling of the small signal parameters is also discussed to gain an understanding of the limiting and contributing performance factors.