Mattias Thorsell

An AlGaN/GaN HEMT-Based Microstrip MMIC Process for Advanced Transceiver Design

A MMIC process in AlGaN/GaN technology for advanced transceiver design has been developed. The process is based on microstrip technology with a complete model library of passive elements and AlGaN/GaN HEMTs. The transistor technology in this process is suitable for both power and low noise design, demonstrated with a power density of 5 W/mm, and an NFmin of 1.4 dB at X-band. Process stability of subcircuits, complementary to power amplifiers and LNAs, in a transceiver system have been investigated. The results indicate that an all AlGaN/GaN MMIC transceiver is realizable using this technology.

High-Efficiency LDMOS Power-Amplifier Design at 1 GHz Using an Optimized Transistor Model

A 10-W LDMOS harmonically tuned power amplifier at 1 GHz with state-of-the-art power-added efficiency of 80% is presented. The fundamental and second-harmonic load impedances are optimized for maximum efficiency while other harmonics are blocked by a low-pass load network. A simplified model of the transistor specialized for harmonically tuned and switched mode operations is proposed and used for the design. Good agreement between simulations and measurements is observed, indicating high accuracy of the model and design approach for these particular applications.

Thermal Study of the High-Frequency Noise in GaN HEMTs

The high-frequency noise performance of the GaN HEMT is studied for temperatures between 297-398 K. The access resistances RS and RD have a limiting effect on the noise performance, and in this paper, their temperature dependence is studied in detail for a 2 times 100 mum GaN HEMT. RS and RD show an increase of 0.71 and 0.86 %/K, respectively. The self-heating effect due to dissipated power is also studied to allow accurate intrinsic small-signal and noise parameter extraction. The thermal resistance is measured by infrared microscopy. Based on these results, a temperature dependent noise model including self-heating and temperature-dependent access resistances is derived and verified with measurements.

An X-Band AlGaN/GaN MMIC Receiver Front-End

This letter presents an integrated AlGaN/GaN X-band receiver front-end. This is to the authors knowledge the first published results of an integrated AlGaN/GaN MMIC receiver front-end. The receiver uses an integrated SPDT switch to reduce size, weight and cost compared to circulator based transceiver front-ends. The integrated front-end has more than 13 dB of gain and a noise figure of 3.5 dB at 11 GHz.

Dispersive Effects in Microwave AlGaN/AlN/GaN HEMTs With Carbon-Doped Buffer

Aluminium gallium nitride (AlGaN)/GaN high-electron mobility transistor performance is to a large extent affected by the buffer design, which, in this paper, is varied using different levels of carbon incorporation. Three epitaxial structures have been fabricated: 1) two with uniform carbon doping profile but different carbon concentration and 2) one with a stepped doping profile. The epitaxial structures have been grown on 4H-SiC using hot-wall metal-organic chemical vapor deposition with residual carbon doping. The leakage currents in OFF-state at 10 V drain voltage were in the same order of magnitude (10-4 A/mm) for the high-doped and stepped-doped buffer. The high-doped material had a current collapse (CC) of 78.8% compared with 16.1% for the stepped-doped material under dynamic I-V conditions. The low-doped material had low CC (5.2%) but poor buffer isolation. Trap characterization revealed that the high-doped material had two trap levels at 0.15 and 0.59 eV, and the low-doped material had one trap level at 0.59 eV.

Fast Multiharmonic Active Load–Pull System With Waveform Measurement Capabilities

A novel fast multiharmonic active load-pull system is presented, which allows for the acquisition of the complete voltage and current waveforms. The system is capable of measuring multiple different load impedance or input power states within a short time instance (typically 180 states within 20 ms). It also facilitates high-power capability (up to 100 W) and high-frequency operation (up to 50 GHz). Furthermore, the dynamic range at the higher order harmonics is significantly improved by introducing an equalizer in cascade with the wideband harmonic samplers. A new algorithm for presenting the wanted load impedance to the device-under-test is also proposed. The method allows for simultaneous optimization on multiple harmonics, converging within five iterations. This method is applicable to all open loop active load-pull systems and could significantly reduce the number of iterations, hence reducing the measurement time. This paper provides a detailed description of the measurement system, including on-wafer verification measurements.

Electrothermal Access Resistance Model for GaN-Based HEMTs

The high-power density in GaN-based high-electron-mobility transistors (HEMTs) increases demands on the accurate extraction and modeling of electrothermal effects such as self-heating. This paper presents a new electrothermal model of the access resistances in GaN HEMTs, taking into account both self heating and bias dependence. A coplanar ungated transfer length method (TLM) structure has been used to extract the resistance versus temperature, bias, and RF power. The temperature dependence is extracted from dc measurements at ambient temperatures between 293 and 443 K. Small-signal measurements are used to extract the time constants in the thermal impedance. The bias dependence of the current is characterized by isothermal large-signal RF measurements between 1 and 6 GHz. A new method for extracting the thermal resistance from the large-signal measurements together with temperature-dependent dc measurements is also presented.

SiC Varactors for Dynamic Load Modulation of High Power Amplifiers

SiC Schottky diode varactors with a high breakdown voltage, a high tuning ratio, and a low series resistance have been designed and fabricated. These characteristics are particularly necessary for the dynamic load modulation of high power amplifiers (PAs), which is an attractive alternative to other efficiency enhancement techniques. For a SiC Schottky diode varactor with a 50- radius fabricated by using a graded doping profile, a breakdown voltage of 40 V, a tuning range of 5.6, and a series resistance of 0.9 were achieved. The results show the great potential of this type of varactors for the use in the dynamic load modulation of high power amplifiers.

A Single-Ended Resistive

A broadband highly linear X-band mixer in AlGaN/GaN monolithic microwave integrated circuit technology has been designed, processed, and characterized. The design is based on a 4 times 100 mum AlGaN/GaN HEMT in a single-ended circuit topology. The mixer has an IF bandwidth of 2 GHz with a conversion loss (CL) of < 8 dB across the X-band with a minimum CL of 6.9 dB at 11 GHz. The large-signal performance is exemplified by IIP 3 levels of 22 and 30 dBm at local oscillator drive levels of 15 and 23 dBm, respectively. A minimum noise figure of 9 dB is achieved at 11.6 GHz.

X

Al2O3 films deposited by thermal and plasma-assisted atomic layer deposition (ALD) were evaluated as passivation layers for InAlN/AlN/GaN HEMTs. As a reference, a comparison was made with the more conventional plasma enhanced chemical vapor deposition deposited SiNx passivation. The difference in sheet charge density, threshold voltage, fT and fmax was moderate for the three samples. The gate leakage current differed by several orders of magnitude, in favor of Al2O3 passivation, regardless of the deposition method. Severe current slump was measured for the HEMT passivated by thermal ALD, whereas near-dispersion free operation was observed for the HEMT passivated by plasma-assisted ALD. This had a direct impact on the microwave output power. Large-signal measurements at 3 GHz revealed that HEMTs with Al2O3 passivation exhibited 77% higher output power using plasma-assisted ALD compared with thermal ALD.