AlGaN-GaN High Electron Mobility Transistors Research Articles

Power and Linearity Characteristics of Field-Plated Recessed-Gate AlGaN–GaN HEMTs

Record power density and high-efficiency operation with AlGaN-GaN high-electron mobility transistor (HEMT) devices have been achieved by adopting a field-plated gate-recessed structure. Devices grown on SiC substrate yielded very high power density (18.8 W/mm with 43% power-added efficiency (PAE) as well as high efficiency (74% with 6 W/mm) under single-tone continuous-wave testing at 4 GHz. Devices also showed excellent linearity characteristics when measured under two-tone continuous-wave signals at 4 GHz. When biased in deep-class AB (33 mA/mm, 3% I/sub max/) device maintained a carrier to third-order intermodulation ratio of 30 dBc up to a power level of 2.4 W/mm with 53% PAE; increasing bias current to 66 mA/mm (6% I/sub max/) allowed high linear operation (45 dBc) up to a power level of 1.4 W/mm with 38% PAE.

High Microwave and Noise Performance of 0.17-<tex>$muhbox m$</tex>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.

A gallium~nitride push~pull microwave power amplifier

A highly efficient linear, broad-band AlGaN-GaN high electron-mobility transistor (HEMT) push-pull microwave power amplifier has been achieved using discrete devices. Instrumental was a low-loss planar three-coupled-line balun with integrated biasing. Using two 1.5-mm GaN HEMTs, a push-pull amplifier yielded 42% power-added efficiency with 28.5-dBm input power at 5.2 GHz, and a 3-dB bandwidth of 4-8.5 GHz was achieved with class-B bias. The output power at 3-dB gain compression was 36 dBm under continuous-wave operation. Along with the high efficiency, good linearity was obtained compared to single-ended operation. The second harmonic content of the amplifier was more than 30 dB down over the 4-8.5-GHz band, and a two-tone excitation measurement gave an input third-order intercept point of 31.5 dBm at 8 GHz. These experimental results and an analysis of the periodic load presented by the output balun suggest the plausibility of broad-band push-pull operation for microwave systems with frequency diversity.

AlGaN-GaN HEMTs on SiC with CW power performance of >4 W/mm and 23% PAE at 35 GHz

In this work, continuous wave Ka-band power performance of AlGaN-GaN high electron-mobility transistors grown on semi-insulating SiC substrates are reported. The devices, with gate lengths of 0.25 /spl mu/m, exhibited maximum drain current density of 1.1 A/mm and peak extrinsic transconductance of 285 mS/mm. At 35 GHz, an output power density of 4.13 W/mm with 23% of power-added efficiency (PAE) and 7.54 dB of linear gain were achieved at a drain bias of 30 V. These power results represent the best power density, PAE, and gain combination reported at this frequency. The drain bias dependence of the Ka-band power performance of these devices is also presented.

Unpassivated AlGaN-GaN HEMTs with minimal RF dispersion grown by plasma-assisted MBE on semi-insulating 6H-SiC substrates

High electron mobility transistors (HEMTs) are fabricated from AlGaN-GaN heterostructures grown by plasma-assisted molecular beam epitaxy (MBE) on semi-insulating 6H-SiC substrates. At a sheet charge density of 1.3 × 10/sup 13/ cm/sup -2/, we have repeatedly obtained electron mobilities in excess of 1350 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> /Vs. HEMT devices with a gate length of 1μm, a gate width of 200 μm, and a source-drain spacing of 5 μm show a maximum drain current of 1.1 A/mm and a peak transconductance of 125 mS/mm. For unpassivated HEMTs, we measured a saturated power output of 8.2-W/mm continuous wave (cw) at 2 GHz with an associated gain of 11.2 dB and a power-added efficiency of 41%. The achievement of high-power operation without a surface passivation layer suggests that free surface may not be the dominant source of radio-frequency (RF) dispersion in these MBE-grown structures. This data may help discriminate between possible physical mechanisms of RF dispersion in AlGaN-GaN HEMTs grown by different techniques.

Engineering of an insulating buffer and use of AlN interlayers: two optimisations for AlGaN–GaN HEMT-like structures

The semi-insulating character of GaN epitaxial layers can be achieved by the control of the early stages of growth on the substrate. Adding two low temperature (LT) AlN interlayers is a technique enough powerful to reduce threading dislocation densities by up to one order of magnitude. A compressive strain as high as 2.8 x 10 -3 is induced in the uppermost GaN epilayer. The global structure is kept semi-insulating so that it is a perfect template for undoped AlGaN-GaN HEMTs (High Electron Mobility Transistors). HEMTs with interlayers present better two dimensional electron gas (2DEG) properties: up to 20% higher carrier density (n s ) and 40% higher mobility. Typically n s is as high as 1.7 x 10 13 cm -2 for a record mobility of 1200 cm 2 /Vs. The improvement of the mobility can be correlated to the reduction of nano-scale V-shaped defects in the AlGaN (less morphological-relaxation). The improvement of n s could be explained by the higher piezo-doping resulting from GaN extra-compression and AlGaN weaker relaxation. As a consequence, the DC transistors characteristics are improved: in 2 μm gate transistors, the maximum current and transconductance are increased by up to 80% and 20%, respectively, and could be extrapolated to values as high as 1500 mA/mm and 250 mS/mm for 0.2 μm gate devices.

Submicron AlGaN/GaN HEMTs with very high drain current density grown by plasma-assisted MBE on 6H-SiC

High electron mobility transistors (HEMTs) were fabricated from AlGaN/-GaN layers grown by plasma-assisted molecular beam epitaxy on semi-insulating 6H-SiC substrates. Room-temperature Hall effect measurements yielded a polarization-induced 2DEG sheet charge of 1.3/spl middot/10/sup 13/ cm/sup -2/ and a low-field mobility of 1300 cm/sup 2//V/spl middot/s. Submicron gates were defined with electron beam lithography using an optimized two-layer resist scheme. HEMT devices repeatedly yielded drain current densities up to 1798 mA/mm, and a maximum transconductance of 193 mS/mm. This is the highest drain current density in any AlGaN-GaN HEMT structure delivering significant microwave power reported thus far. Small-signal testing of 50-/spl mu/m wide devices revealed a current gain cutoff frequency f/sub T/ of 52 GHz, and a maximum frequency of oscillation f/sub max/ of 109 GHz. Output power densities of 5 W/mm at 2 GHz, and 4.9 W/mm at 7 GHz were recorded from 200-/spl mu/m wide unpassivated HEMTs with a load-pull setup under optimum matching conditions in class A device operation.

1 - 8-GHz GaN-based power amplifier using flip-chip bonding

We report the first gallium nitride (GaN)-based broad-band power amplifier. The circuit was fabricated on an AlN substrate using AlGaN-GaN power high-electron mobility transistors (HEMTs), grown on sapphire substrates, which were flip-chip bonded for thermal management. The amplifier employed a modified traveling-wave power amplifier (TWPA) topology that eliminated the backward wave of conventional TWPAs. Using four HEMTs each with 0.75-/spl mu/m gate length and 0.75-mm gate periphery, a small-signal gain of /spl sim/7 dB was obtained with a bandwidth of 1-8 GHz. At mid-band, an output power of 3.6 W was obtained when biased at V/sub ds/=18 V and 4.5 W when biased at V/sub ds/=22 V.