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High performance AlGaN/GaN HEM


MOCVD-grown 0.25 um gate-length AlGaN/GaN HEMTs have been fabricated on 6H-SiC substrates. These 0.25 um gate-length devices exhibited maximum drain current density as high as 1.28 A/mm, peak extrinsic transconductance of 310 mS/mm, unity gain cut-off frequency (fT) of 51 GHz, and maximum frequency oscillation (fmax) of 115 GHz. At 18 GHz, a CW output power density of 6.7 W/mm with power-added-efficiency (PAE) of 26 % was obtained, yielding the highest reported power performance of AlGaN/GaN HEMTs at 18 GHz. In accordance with the impressive power results, excellent microwave noise performance was also obtained. High-performance passivated AlGaN/GaN high electron-mobility transistors (HEMTs) with 0.25 um gate-length for low noise applications. The devices exhibited a minimum noise figure (NFmin) of 0.98 dB and an associated gain (Ga) of 8.97 dB at 18 GHz at VDS = 8 V and IDS = 235 mA/mm.


Large Signal Analytic Model for Gallium Nitride HEMTs


A temperature-dependent large signal model for continuous wave (CW) and pulsed mode operation is presented and applied to gallium nitride HEMTs on silicon carbide (SiC) substrates. The model includes thermal, RF dispersion, and bias-dependent capacitance model elements, and is suitable for application with a harmonic balance simulator. Temperature- and bias-dependent on-wafer pulsed I?V and S-parameter measurements from 27 C to 175 C were performed to examine trapping and thermal effects, and for use in determining temperature- and bias- dependent parameterized model coefficients for the nonlinear model. Large signal measurement and model results were presented for gallium nitride HEMTs fabricated on SiC. The nonlinear model showed good agreement with measured continuous wave power sweep data at an elevated temperature of 150 C, and with measured pulsed load-pull data



Broadband Gallium Nitride Cascode HEMT Nonuniform Distributed Amplifier


High power density (the output power per input capacitance ratio) gallium nitride HEMTs allow multioctave high-power amplifiers. Gallium nitride HEMTs combine a frequency response similar to GaAs-based devices with significantly higher current densities and breakdown voltages. To demonstrate the high power and broadband amplifier concept, a monolithic gallium nitride cascode-connected HEMT nonuniform distributed amplifier (NDA) has been designed. Distributed amplifiers (DAs) offer broadband operation by incorporating gain elements in a synthetic, lumped-element approximate transmission lines, realized by the transistor capacitances and intervening inductances. The measured small-signal S-parameters of the NDA resulted in 9 dB gain at 8 GHz, with a 3 dB bandwidth of DC to 9.5 GHz. The measured performance yielded a saturated output power of 3-6W over a dc-8 GHz bandwidth with an associated PAE of 13-31%.



High Efficiency Broadband Gallium Nitride Push-Pull Power Amplifier


.A highly efficient, linear, broadband gallium nitride HEMTs push-pull microwave power amplifier has been achieved using discrete devices. This is the first demonstration of GaN-based Class B amplifiers and the efficiency achieved is the highest for a linear gallium nitride power amplifier to date. Instrumental was a low-loss, planar three-coupled-line balun with integrated biasing. The challenge of realizing low-loss microwave baluns and the complex biasing necessary for a push-pull amplifier was addressed by using a symmetric three-coupled-line balun with an integrated biasing scheme. Using two 1.5 mm Gallium Nitride HEMTs, a push-pull amplifier yielded 42 % power-added efficiency (PAE) with 28.5 dBm input power at 5.2 GHz, and a 3 dB bandwidth of 4.0?8.5 GHz was achieved at Class B bias condition. The output power at 3 dB gain compression was 36 dBm under CW operation. Along with the high efficiency, good linearity was obtained compared to single-ended operation


Monolithic Gallium Nitride Low-Noise Amplifiers


In addition to potential of gallium nitride HEMTs for power applications, investigations on microwave noise performance of GaN-based devices have been conducted for robust low-noise receiver system under high temperature and harsh environment. This is possible because the high breakdown voltage and low intrinsic carrier generation characteristics of wide-bandgap gallium nitride HEMTs allow elimination of additional protection circuits, and can operate at higher temperatures where conventional devices based on GaAs and Si can not be used. High-frequency noise performance of the devices was measured using an ATN NP5B noise parameter test system. The noise model for intrinsic devices based on a resistor temperature model of Pospiezalski was studied in conjunction with noise de-embeding technique. Based on the noise model, LNA circuit was designed using device width and source inductance as design parameters to achieve simultaneous noise and input matching. The simulated result on the designed two-stage K-band LNA predicted less than 2 dB noise figure and 15 dB gain over 18-24 GHz. To achieve compact layout based on coplanar waveguide (CPW) transmission line, CPW discontinuities such as bend, tee, and cross were examined using an electromagnetic simulator up to 60 GHz and the elements were successfully modeled using lumped element models.


Design and modeling of RF passive components


Development of high Q inductor model and fabrication, Design and analysis of low loss broadband microwave transformer (balun): The balun is a key component in balanced (push-pull) amplifiers. Proposed an analytic design method and demonstrated 0.5 dB insertion loss over 4-12 GHz.


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