J.C. De Jaeger

Punch-through in short-channel AlGaN/GaN HFETs

Short-channel punch-through effects are demonstrated in 0.17 /spl mu/m gate length AlGaN/GaN single heterojunction field-effect transistors. These take the form of a high output conductance and the strong dependence of pinch-off voltage on drain voltage. It is shown by simulation that they can be explained by poor confinement of charge at the AlGaN/GaN interface resulting in current flow within the bulk of the GaN layer. This is caused by there being a concentration of only /spl sim/1.5/spl times/10/sup 16/ cm/sup -3/ deep levels in the insulating GaN buffer layer. It is found that a net acceptor density of around 10/sup 17/ cm/sup -3/ is required to ensure suppression of short-channel effects.

Output power density of 5.1/mm at 18 GHz with an AlGaN/GaN HEMT on Si substrate

Microwave frequency capabilities of AlGaN/GaN high electron mobility transistors (HEMTs) on high resistive silicon (111) substrate for power applications are demonstrated in this letter. A maximum dc current density of 1 A/mm and an extrinsic current gain cutoff frequency (F/sub T/) of 50 GHz are achieved for a 0.25 /spl mu/m gate length device. Pulsed and large signal measurements show the good quality of the epilayer and the device processing. The trapping phenomena are minimized and consequently an output power density of 5.1 W/mm is reached at 18 GHz on a 2/spl times/50/spl times/0.25 /spl mu/m/sup 2/ HEMT with a power gain of 9.1dB.

193nm deep-ultraviolet solar-blind cubic boron nitride based photodetectors

Deep-ultraviolet (DUV) solar-blind photodetectors based on high-quality cubic boron nitride (cBN) films with a metal/semiconductor/metal configuration were fabricated. The design of interdigitated circular electrodes enables high homogeneity of electric field between pads. The DUV photodetectors present a peak responsivity at 180nm with a very sharp cutoff wavelength at 193nm and a visible rejection ratio (180 versus 250nm) of more than four orders of magnitude. The characteristics of the photodetectors present extremely low dark current, high breakdown voltage, and high responsivity, suggesting that cBN films are very promising for DUV sensing.

AlGaN-GaN HEMTs on Si with power density performance of 1.9 W/mm at 10 GHz

AlGaN-GaN high electron mobility transistors (HEMTs) on silicon substrate are fabricated. The device with a gate length of 0.3-/spl mu/m and a total gate periphery of 300 /spl mu/m, exhibits a maximum drain current density of 925 mA/mm at V/sub GS/=0 V and V/sub DS/=5 V with an extrinsic transconductance (g/sub m/) of about 250 mS/mm. At 10 GHz, an output power density of 1.9 W/mm associated to a power-added efficiency of 18% and a linear gain of 16 dB are achieved at a drain bias of 30 V. To our knowledge, these power results represent the highest output power density ever reported at this frequency on GaN HEMT grown on silicon substrates.

Effects of Self-Heating on Performance Degradation in AlGaN/GaN-Based Devices

A self-consistent electrothermal transport model that couples electrical and thermal transport equations is established and applied to AlGaN/GaN device structures grown on the following three different substrate materials: 1) SiC; 2) Si; and 3) sapphire. Both the resultant I-V characteristics and surface temperatures are compared to experimental I -V measurements and Raman spectroscopy temperature measurements. The very consistent agreement between measurements and simulations confirms the validity of the model and its numerical rendition. The results explain why the current saturation in measured I-V characteristics occurs at a much lower electric field than that for the saturation of electron drift velocity. The marked difference in saturated current levels for AlGaN/GaN structures on SiC, Si, and sapphire substrates is directly related to the different self-heating levels that resulted from the different biasing conditions and the distinctive substrate materials.

High microwave and noise performance of 0.17-/spl mu/m AlGaN-GaN HEMTs on high-resistivity silicon substrates

AlGaN-GaN high-electron mobility transistors (HEMTs) based on high-resistivity silicon substrate with a 0.17-/spl mu/m T-shape gate length are fabricated. The device exhibits a high drain current density of 550 mA/mm at V/sub GS/=1 V and V/sub DS/=10 V with an intrinsic transconductance (g/sub m/) of 215 mS/mm. A unity current gain cutoff frequency (f/sub t/) of 46 GHz and a maximum oscillation frequency (f/sub max/) of 92 GHz are measured at V/sub DS/=10 V and I/sub DS/=171 mA/mm. The radio-frequency microwave noise performance of the device is obtained at 10 GHz for different drain currents. At V/sub DS/=10 V and I/sub DS/=92 mA/mm, the device exhibits a minimum-noise figure (NF/sub min/) of 1.1 dB and an associated gain (G/sub ass/) of 12 dB. To our knowledge, these results are the best f/sub t/, f/sub max/ and microwave noise performance ever reported on GaN HEMT grown on Silicon substrate.

Investigation of the negative differential resistance reproducibility in AlN/GaN double-barrier resonant tunnelling diodes

AlN/GaN double-barrier resonant tunnelling diodes were grown by molecular beam epitaxy on GaN/sapphire template and processed into mesa diameters from 2 μm to 4 μm. The current-voltage characteristics were carried out in direct current operation and under-high vacuum. A sharp negative differential resistance (NDR) was detected in the forward bias at 120 K. The NDR was observed for the mesa size of 2 μm at 4 V with a peak-to-valley current ratio of 3.5. The measurement conditions were chosen to make NDR reproducible more than 50 times and apparent in both scan voltage directions after electrical treatment.

Proposal and Performance Analysis of Normally Off

Design considerations and performance of n++ GaN/InAlN/AlN/GaN normally off high-electron mobility transistors (HEMTs) are analyzed. Selective and damage-free dry etching of the gate recess through the GaN cap down to a 1-nm-thick InAlN barrier secures positive threshold voltage, while the thickness and the doping of the GaN cap influence the HEMT direct current and microwave performance. The cap doping density was suggested to be 2 × 1020 cm-3. To screen the channel from the surface traps, the needed cap thickness was estimated to be only 6 nm. Design is proved by an experiment showing a constant value of the HEMT dynamical access resistance, while a single-pulse experiment indicated almost collapse-free performance. On the other hand, it is found that the n++ GaN cap does not contribute to the HEMT drain current conduction, nor does it provide a path for the off-state breakdown. HEMTs with a gate length of 0.25 μm and a 4-μm source-to-drain distance show a drain-to-source current of 0.8 A/mm, a transconductance of 440 mS/mm, a threshold voltage of ~0.4 V, and a cutoff frequency of 50 GHz. A thin and highly doped GaN cap is also found to be suitable for the processing of normally on HEMTs by adopting the nonrecessed gate separated from the cap by insulation.

\hbox{n}^{++}

The absolute responsivity of a metal-semiconductor-metal (MSM) photodiode based on high quality AlN material has been tested from the vacuum ultraviolet (vuv) to the near UV wavelength range (44–360nm). The metal finger Schottky contacts have been processed to 2μm in width with spacing between the contacts of 4μm. In the vuv wavelength region, the measurement methodology is described in order to distinguish the contribution of the photoemission current from the internal diode signal. In the wavelength range of interest, AlN MSM is sensitive and stable under brief vuv irradiation. The MSM shows a 200∕360nm rejection ratio of more than four orders of magnitude and demonstrates the advantages of wide band gap material based detectors in terms of high rejection ratio and high output signal for vuv solar observation missions.

GaN/InAlN/AlN/GaN HEMTs With 1-nm-Thick InAlN Barrier

This letter reports the first millimeter-wave power demonstration of AlGaN/GaN high-electron-mobility transistors grown on a (110) silicon substrate. Owing to an AlN/AlGaN stress-mitigating stack and in spite of the twofold surface symmetry of Si (110), it is possible to obtain crack-free GaN layers for the fabrication of millimeter-wave power devices with high performance. The device exhibits a maximum dc drain current density of 1.55 A/mm at VGS = 0 V with an extrinsic transconductance of 476 mS/mm. An extrinsic current gain cutoff frequency of 81 GHz and a maximum oscillation frequency of 106 GHz are deduced from Sij parameters. At 40 GHz, an output power density of 3.3 W/mm associated with a power-added efficiency of 20.1% and a linear power gain of 10.6 dB is obtained.