Sebastian Gustafsson

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.

Application Relevant Evaluation of Trapping Effects in AlGaN/GaN HEMTs With Fe-Doped Buffer

This paper investigates the impact of different iron (Fe) buffer doping profiles on trapping effects in microwave AlGaN/gallium nitride (GaN) high electron mobility transistors (HEMTs). We characterize not only the current collapse due to trapping in the buffer, but also the recovery process, which is important in the analysis of suitable linearization schemes for amplitude modulated signals. It is shown that the simple pulsed dc measurements of current transients can be used to investigate transient effects in the RF power. Specifically, it is revealed that the design of the Fe-doping profile in the buffer greatly influences the recovery time, with the samples with lower Fe concentration showing slower recovery. In contrast, traditional indicators, such as S-parameters and dc as well as pulsed I-V characteristics, show very small differences. An analysis of the recovery shows that this effect is due to the presence of two different detrapping processes with the same activation energy (0.6 eV) but different time constants. For highly doped buffers, the faster process dominates, whereas the slower process is enhanced for less doped buffers.

A novel active load-pull system with multi-band capabilities

In this paper a novel active open-loop multi-band load-pull measurement system is presented. The system uses few components where the main parts are an Oscilloscope and an Arbitrary Waveform Generator (AWG). The used AWG is a sophisticated signal generator with two outputs, letting the user take full control over the injected signals in the frequency band 0-4 GHz. This enables new kinds of measurements to be performed, such as multi-band load-pull and in particular inter-modulation product load-pull, which this paper focuses on. The paper describes the measurement setup briefly and the calibration and load-pull algorithms used. As an example to demonstrate the capabilities of the system, a multi-band load-pull is carried out on a GaN HEMT transistor. The frequency bands are 2.14 GHz and 2.65 GHz which create IM3 products at 1.63 GHz and 3.16 GHz. It is shown that the efficiency is affected by the choice of load termination presented at the IM3 frequencies.

Prediction of Nonlinear Distortion in Wideband Active Antenna Arrays

In this paper, we propose a technique for comprehensive analysis of nonlinear and dynamic characteristics of multi-antenna transmitters (TXs). The analysis technique is enabled by the development of a Volterra series-based dual-input model for power amplifiers (PAs), which is capable of considering the joint effects of PA nonlinearity, antenna crosstalk, and mismatch for wideband modulated signals. By combining multiple instances of the PA model with linear dynamic antenna simulations, we develop the analysis technique. The proposed method allows the prediction of the output signal of every antenna in an arbitrarily sized TX array, as well as the total far-field radiated wave of the TX for any input signal with low computational effort. A 2.12-GHz four-element TX demonstrator based on GaAs PAs is implemented to verify simulation results with measurements. The proposed technique is a powerful tool to study hardware characteristics as, for example, the effects of antenna design and element spacing. It can be used in cases where experiments are not feasible, and thus aid the development of next generation wireless systems.

Wideband RF characterization setup with high dynamic range low frequency measurement capabilities

In this paper, a wideband RF characterization setup with low-frequency (LF) measurement capabilities is presented. Simultaneous LF and RF measurements enable studies of trapping phenomena, thermal effects and other LF-related nonlinear distortion in microwave devices using realistic wideband RF stimuli. The setup is capable of measuring up to 4 GHz through the RF path, and from DC to 125 MHz through the LF path. A study is made on how device linearity is affected by the choice of power supply unit (PSU). LF measurements reveal significant differences in baseband termination in the PSUs, consequently affecting the linearity of the measured RF output spectrum.

Low-Pressure-Chemical-Vapor-Deposition SiNx passivated AlGaN/GaN HEMTs for power amplifier application

A Low-Pressure-Chemical-Vapor-Deposition (LPCVD) bilayer SiNx passivation scheme has been investigated and developed, which effectively suppress the current collapse in AlGaN/GaN HEMTs. Low current slump is very helpful for microwave power amplifier application. Compared to in-situ SiNx passivations by metal-organic-chemical-vapor-deposition (MOCVD) and ex-situ SiNx passivations by plasma-enhanced-chemical-vapor-deposition (PECVD), the LPCVD SiNx exhibits the quickest recovery time from double-sweep IV curves. From pulsed IV and load-pull measurements, LPCVD SiNx is also confirmed to deliver superior large signal performance. The bilayer LPCVD SiNx passivated device shows negligible current slump (<;6%). These characteristics are directly reflected in the large signal operation, where HEMTs with bilayer LPCVD SiNx have highest output power (2.9 W/mm at 3 GHz).

Vector-corrected nonlinear multi-port IQ-mixer characterization using modulated signals

In this paper, large-signal operation of IQ-mixers is studied using a vector-corrected four-port measurement setup with modulated signals as stimuli. The measurement setup presents unique characterization possibilities since it has two ports at low/baseband frequencies and two ports at RF, making it ideal for characterization of frequency-translating devices such as mixers. A commercial upconverting IQ-mixer is studied, with the I and Q input signals residing at incommensurate frequency grids, enabling separation of the nonlinear distortion generated in the I and Q branches. Frequency-domain and time-domain measurements reveal imbalances between the I and Q branches in terms of conversion gain and nonlinear distortion. It is also shown for the same mixer that operating the I and Q branches concurrently has limited influence on both conversion gain and nonlinear distortion, compared to non-concurrent operation.