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The compliant behavior of densely packed 10 × 10 µm2 square patterned InGaN layers on top of porous GaN is demonstrated. The elastic relaxation of the InGaN layers is enabled by the low stiffness of the porous GaN under layer. High resolution X-ray diffraction measurements show that upon InGaN re-growths on these InGaN-on-porous GaN pseudo-substrates, not only was the regrown layer partially relaxed, but the degree of relaxation of the InGaN pseudo-substrate layer on top of the porous GaN also showed an increase in the a-lattice constant. Furthermore, methods to improve the surface morphology of the InGaN layers grown by metal-organic chemical vapor deposition (MOCVD) were explored in order to fabricate InGaN pseudo-substrates for future optoelectronic and electronic devices. The largest a-lattice constant demonstrated in this study using this improved method was 3.209 Å, corresponding to a fully relaxed InGaN film with an indium composition of 0.056.

The authors report a novel structure to improve the high‐frequency performance of nitrogen‐polar GaN deep‐recess high electron mobility transistors (HEMTs) wherein a timed plasma etch was used to tailor the profile of the gate dielectric. A favourable trade‐off between gate fringing capacitance and transconductance was established, resulting in an improved current gain cutoff frequency times gate length (ft·LG) figure of merit compared to standard Schottky‐gate and metal‐insulator‐semiconductor (MIS) HEMTs. The etched gate dielectric MISHEMT demonstrated a maximum ft·LG of 15.0 GHz·µm for an LG of 100 nm. [ABSTRACT FROM AUTHOR]

We investigated the properties of indium-doped zinc oxide layers grown by metalorganic chemical vapor deposition on semi-insulating GaN(0001) templates. Specular and transparent films were grown with n-type carrier concentrations up to 1.82 × 1019 cm−3 as determined by Hall measurements, and all In-doped films had carrier concentrations significantly higher than that of a comparable undoped film. For low In flows, the carrier concentration increased accordingly with trimethyl-indium (TMIn) flow until a maximum carrier concentration of 1.82 × 1019 cm−3 was realized. For higher In flows, the carrier concentration decreased with increasing TMIn flow rate. Sheet resistance as low as 185 Ω/sq was achieved for the In-doped films, which is a significant decrease from that of a comparable undoped ZnO film. Our n-type doping studies show that In is an effective dopant for controlling the n-type conductivity of ZnO.

We present a systematic study of the impact of CF4 plasma treatment on GaN. It was found that CF4 plasma etches GaN at a slow rate and yields a smooth etched surface. The effect of CF4 plasma on electrical characteristics of GaN metal-semiconductor field-effect-transistor structures shows that the CF4 plasma introduces acceptors into the near surface region of the GaN, which depletes mobile electrons. It was further demonstrated that leakage current of AlGaN/GaN (or GaN) Schottky diodes can be significantly suppressed by proper CF4 plasma treatment. These unique properties of CF4 plasma can be utilized for the advanced processing of GaN transistors.

We describe the development of N-polar GaN-based high electron mobility transistors grown by N-2 plasma-assisted molecular beam epitaxy on C-face SiC substrates. High mobility AlGaN/GaN modulation-doped two-dimensional electron gas channels were grown, and transistors with excellent dc and small-signal performance were fabricated on these wafers. Large-signal dispersion was observed, and the trap states responsible for this were identified, and layer designs to remove the dispersive effects of these traps were demonstrated. Finally, an AlGaN-cap layer was used to reduce gate leakage in these devices, and a low-dispersion high breakdown voltage device was achieved. This detailed study of dispersion and leakage in N-polar GaN-based transistors establishes a technological base for further development of field effect devices based on N-polar III-nitrides. (C) 2007 American Institute of Physics.

The first AlGaN/GaN HEMT based differential oscillator is reported. The MMIC oscillates at a frequency of 4.16 GHz and provides 22.9 dBm of power from one side at a biasing of Vgs -1 V and Vds 20 V. The HEMTs each have a 0.7 um x 200 um gate. The second harmonic is 45 dB below the carrier and the third harmonic is more than 70 dB below the carrier. To our knowledge, this is the best reported harmonic performance for a GaN oscillator. The oscillator efficiency is between 4% and 9.4% depending on bias. The measured phase noise is -86.3 dBc and -115.7 dBc at offsets of 100 kHz and 1 MHz respectively. The phase noise at a 1 MHz offset is similar to the noise performance of FET based differential oscillators in other technologies.

The effect of noise figure of different AlGaN/GaN high electron-mobility transistor (HEMT) epitaxy structures is reported. The addition of a thin AlN layer between the barrier and channel gives better performance at biasings other than the best for minimum noise figure. However, varying Al composition in the HEMT barrier does not change the noise performance, contrary to a 2003 study by Lu et al. The measurements are checked with both the Pospieszalski and van der Ziel (Pucel) models. The models are used on six different samples, helping to reinforce the measurements and showing the strengths and weaknesses of each.

A novel mask design consisting of interdigitated multipixel arrays for the fabrication of high power light-emitting diodes (LED) with very low series resistance, reduced current crowding, and improving heat sinking is reported. Results suggest that the optical performance of high-power LEDs can be significantly improved by minimizing the series resistance and self-heating by adopting an interdigitated multipixel geometry.

A study to reduce the access resistance in AlGaN/GaN high electron mobility transistors using double-channel structures was conducted. From the results the carrier densities as high as 2.9 x 10(super 13) cm(super -2) and mobilities in the 1300 cm(super 2)/ V.s range were obtained in the access region.

The decrease of transconductance g(sub m) and current gain cutoff frequency f(sub T) at high drain current levels in AlGaN/GaN high-electron mobility transistors (HEMTs) severely limit the linearity and power performance of the devices at high frequencies. The increase of differential source access resistance r(sub s), with drain current is shown to play an important role in the fall of g(sub m) and f(sub T).

The investigation of drain current dispersion effects in AlGaN-GaN HEMTs by means of pulsed, transient, and small signal measurements is done. The simulations show that surface traps behave as hole traps during transients, interacting with holes attracted at the AlGaN surface by the negative polarization charge.

A 36-dBm high-linearity single-ended common-source class-B monolithic-microwave integrated-circuit power amplifier is reported in GaN high electron-mobility transistor technology. We also describe the design and simulation of highly linear and highly efficient common-source and common-drain class-B power amplifiers. Single-ended class-B amplifiers with bandpass filtering have equivalent efficiency and linearity to push-pull configurations. The common-source class-B circuit demonstrates high linearity, greater than 35 dBc of third-order intermodulation (IM3) suppression and high power-added efficiency (PAE) of 34%. Simulations of common-drain class-B designs predict a PAE of 54% with a superior IM3 suppression of more than 45 dBc over a wider range of bias due to the strong series-series negative feedback offered by the load resistance. Index Terms--GaN high electron-mobility transistor (HEMT), high linearity, intermodulation suppression, monolithic-microwave integrated-circuit (MMIC) power amplifiers.

DC current-switching and power-switching transients of various GaN-based FET structures are investigated. Two different characteristics are compared, namely, thermal and electronic transients. While the thermal transients are mainly reflected in changes in channel carrier mobility, the electronic transients are dominated by charge instabilities caused by the polar nature of the material. The discussion of the electronic transients focuses, therefore, on instabilities caused by polarization-induced image charges. Three structures are discussed, which are: 1) a conventional AlGaN/GaN heterostructure FET; 2) an InGaN-channel FET; and 3) an AlGaN/GaN double-barrier structure. In structures 2) and 3), field-induced image charges are substituted by doping impurities, eliminating this source of related instability. This is indeed observed. Index Terms--GaN, heterostructure field-effect transistors (HFETs), switching experiments, transient analysis.

Breakdown enhancement potential of the field plate (FP) technique is investigated, in a context of AlGaN/GaN power high electron mobility transistors (HEMT). Higher breakdown enhancement is possible using a higher insulator dialectric constant. The methodology used here can be extended to other FP structures in other lateral field-effect transistors (FET).

Research into the power density of AlGaN/GaN HEMTs is presented.

A review of the development of GaN electronics at the University of California, Santa Barbara is presented.

A new study investigates the impact of surface states on GaN-HEMT devices when the GaN and AlGaN layers are grown in conditions for optimum electrical performance.

In this paper, recent progress of AlGaN/GaN-based power high-electron-mobility transistors (HEMT's) is reviewed. Remarkable improvement in performances was obtained through adoption of high Al contents in the AlGaN layer. The mobility in these modulation-doped structures is about 1200 [cm.sup.2] [multiplied by] [V.sup.-1] [multiplied by] [s.sup.-1] at 300 K with sheet densities of over 1 x [10.sup.13] [cm.sup.-2]. The current density is over 1 A/mm with gate-drain breakdown voltages up to 280 V. [F.sub.t] values up to 52 GHz have been demonstrated. Continuous wave (CW) power densities greater than 3 W/mm at 18 GHz have been achieved. Index Terms - FET, GaN, HEMT, microwave, transistor.

Previous investigations of noise in mutually synchronized coupled-oscillator systems are extended to include the effects of phase noise introduced by externally injected signals. The analysis is developed for arbitrarily coupled arrays and an arbitrary collection of coherent injected signals, and is illustrated with the specific case of linear chains of nearest neighbor coupled oscillators either globally locked (locking signal applied to each array element) or with the locking signal applied to a single-array element. It is shown that the general behavior is qualitatively similar to a single injection-locked oscillator, with the output noise tracking the injected noise near the carrier, and returning to the free-running array noise far from the carrier, with intermediate behavior significantly influenced by the number of array elements and injection strength. The theory is validated using a five-element GaAs MESFET oscillator array operating at X-band. Index Terms: Array noise, globally locked, injection locked, injection strength, noise admittance, noise offset frequency, phase noise, power spectral density, single element locked, synchronization.

Phase noise in mutually synchronized oscillator systems is analyzed for arbitrary coupling and injection-locking topologies, neglecting amplitude noise, and amplitude modulation (AM) to phase modulation (PM) conversion. When the coupling phase is chosen properly (depending on the oscillator model), the near-carrier phase noise is reduced to 1/N that of a single oscillator, provided the coupling network is reciprocal. This is proved in general, and illustrated with specific cases of globally coupled and nearest-neighbor coupled oscillator chains. A slight noise degradation is found for unilaterally coupled (nonreciprocal) chains. The 1/N reduction for reciprocal coupling applies over nearly the entire range of free-running frequency distributions required for beam-scanning, and is verified experimentally using a linear chain of coupled GaAs MESFET voltage-controlled oscillators (VCO's) operating at X-band. The effect of a nonoptimum coupling phase on the phase noise of the system is also studied. As the coupling phase deviates from the optimum value, the phase noise increases significantly near the locking range edge for noise offset frequency near the carrier. Index Terms - AM noise, coupling topology, noise admittance, oscillators, phase noise, power spectral density.