Characterization of Microwave Transistors for Robust Receivers and High Efficiency Transmitters

The next generation of integrated transceiver front-ends needs both robust low noise amplifiers and high power amplifiers on a single-chip. The Aluminium Gallium Nitride / Gallium Nitride (AlGaN/GaN) High Electron Mobility Transistors (HEMT) is a suitable semiconductor technology for this purpose due to its high breakdown voltage and high electron mobility. In this thesis the AlGaN/GaN HEMT’s thermal properties, noise and survivability have been characterized for the intended use in robust high power transceivers. Furthermore, a new characterization setup for load modulated high efficiency power amplifiers have been developed. The thermal properties of AlGaN/GaN HEMTs have been carefully investigated considering self-heating and its effect on small-signal parameters and high frequency noise. Self-heating is a severe problem for a high power transistor on any semiconductor material, including GaN. In addition to reliability problems, the performance of the operating HEMT degrades with temperature. The access resistances showed a large temperature dependence, which was also verified with TLM measurements. Due to the large self-heating, the temperature dependence of the access resistances has to be taken into account in the modeling of the AlGaN/GaN HEMT. A temperature dependent small-signal noise model was derived and verified through fabricated amplifiers. Design strategies for robust low noise amplifiers are discussed and implemented using the derived model. The new characterization setup gives new possibilities to characterize the performance of load modulated amplifiers. Recent results on load modulated amplifiers show promising efficiency improvements in back-off operation. Therefore a new measurement setup was developed that performs dynamic load modulation at the transistor terminals. This method should be useful to further improve the performance of load modulated amplifiers for high efficiency operation. The measurement setup is based on an active load-pull setup, where a modulated input signal is used to synthesize a time varying output power. The load impedance is dynamically controlled with the envelop of the input signal, following an optimum efficiency load trajectory. This gives better insight into device operation and possible improvements

Investigation of Isolation Approaches and the Stoichiometry of SiNx Passivation Layers in “Buffer-Free” AlGaN/GaN Metal–Insulator–Semiconductor High-Electron-Mobility Transistors

Critical process modules for the fabrication of metal–insulator–semiconductor high-electron-mobility transistors (MISHEMTs) based on a novel ‘buffer-free’ AlGaN/GaN heterostructure grown with metal–organic chemical vapor deposition (MOCVD) are presented. The methods of isolation and passivation for this type of heterostructure are investigated. Utilizing nitrogen implantation, it is possible to achieve off-state destructive breakdown voltages (BVs) of 2496 V for gate–drain distances up to 25 μm, whereas mesa isolation techniques limit the BV below 1284 V. The stoichiometry of the SiNx passivation layer displays a small impact on the static and dynamic on-resistance. However, MISHEMTs with Si-rich passivation show off-state gate currents in the range of 1–100 μA mm−1 at voltages above 1000 V, which is reduced below 10 nA mm−1 using a stoichiometric SiNx passivation layer. Destructive BVs of 1532 and 1742 V can be achieved using gate-integrated and source-connected field plates for MIHEMTs with stoichiometric and Si–rich passivation layers, respectively. By decreasing the field plate lengths, it is possible to achieve BVs of 2200 V. This demonstrates the implementation of MISHEMTs with high-voltage operation and low leakage currents on a novel “buffer-free” heterostructure by optimizing the SiNx stoichiometry

Vector-corrected Nonlinear Multi-port IQ-mixer Characterization using Modulated Signals

In this paper, large-signal operation of IQ-mixers isstudied 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 frequencygrids, 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

Semi-physical nonlinear circuit model with device/physical parameters for HEMTs

A nonlinear circuit model (NCM) with physical parameters is proposed for direct simulation of the RF characteristics of GaN high-electron-mobility transistors (GaN HEMTs) on the basis of device structure. The physical equations are used for the construction of the model in order to connect strongly the model parameters with the device/physical parameters. Hyperbolic tangent functions are used as the model equations to ensure good model convergence and rapid simulation (short simulation time). The usefulness of these equations is confirmed by technology computer aided design (TCAD) simulation. The number of model parameters for the nonlinear components (Ids, Cgs, Cgd) is reduced to 17 by using common physical parameters for modeling the drain current and capacitance. The accuracy of this model is verified by applying to GaN HEMTs. The modeled I–V and capacitance characteristics agree well with the measurement data over a wide voltage range. Furthermore, this model can be used for the accurate evaluation of S-parameters and large-signal RF characteristics

Evaluation of Thermal Versus Plasma-Assisted ALD Al2O3 as Passivation for InAlN/AlN/GaN HEMTs

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, f(T) and f(max) 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

Tuning composition in graded AlGaN channel HEMTs toward improved linearity for low-noise radio-frequency amplifiers

Compositionally graded channel AlGaN/GaN high electron mobility transistors (HEMTs) offer a promising route to improve device linearity, which is necessary for low-noise radio-frequency amplifiers. In this work, we demonstrate different grading profiles of a 10-nm-thick AlxGa1-xN channel from x = 0 to x = 0.1 using hot-wall metal-organic chemical vapor deposition (MOCVD). The growth process is developed by optimizing the channel grading and the channel-to-barrier transition. For this purpose, the Al-profiles and the interface sharpness, as determined from scanning transmission electron microscopy combined with energy-dispersive x-ray spectroscopy, are correlated with specific MOCVD process parameters. The results are linked to the channel properties (electron density, electron mobility, and sheet resistance) obtained by contactless Hall and terahertz optical Hall effect measurements coupled with simulations from solving self-consistently Poisson and Schr\uf6dinger equations. The impact of incorporating a thin AlN interlayer between the graded channel and the barrier layer on the HEMT properties is investigated and discussed. The optimized graded channel HEMT structure is found to have similarly high electron density (∼9 7 10 12 cm-2) as the non-graded conventional structure, though the mobility drops from ∼ 2360 cm2/V s in the conventional to ∼ 960 cm2/V s in the graded structure. The transconductance gm of the linearly graded channel HEMTs is shown to be flatter with smaller g m ′ and g m ″ as compared to the conventional non-graded channel HEMT implying improved device linearity

Nonlinear Characterisation and Modelling of Microwave Semiconductor Devices

There is an increasing need for more accurate models taking into account the nonlinearities and memory effects of microwave transistors. The memory effects are especially important for transistor technologies suffering from relatively large low frequency dispersion, such as GaN baed HEMTs. Nonlinear measurement systems are today available off-the-shelf, but the use of them is still limited. It is therefore important to demonstrate the possibilities these new systems brings to the device characterisation and modelling community. This thesis deals with electrothermal characterisation and modelling of GaN based HEMTs, and also development and utilisation of new nonlinear measurement systems.The electrothermal properties of the AlGaN/GaN heterostructure were characterised, and it was shown that a thermal response is present up to 100 MHz. Moreover, a new characterisation method, making use of nonlinear measurements, allowed for isothermal measurements of the current transport through the access resistances of a GaN based HEMT. A new current transport model was proposed to correctly reproduce the isothermal IV characteristics. Furthermore, the temperature dependence of the high frequency noise was characterised, showing that the major limiting factors for the low noise performance were the access resistances. The combination of high power and low noise makes the GaN based HEMT suitable for monolithically integrated GaN based transceiver front-ends. The first steps toward a transceiver were taken by designing and manufacturing a GaN based receiver front-end consisting of an SPDT switch and an LNA.A new fast multi harmonic active load-pull system was developed, with waveform acquisition capabilities. The speed of the load-pull system was increased by the use of an improved optimisation routine for presenting the wanted load impedances. The load-pull system was capable of presenting dynamically varying load impedances to a transistor, enabling faster device characterisation without the need to build complete amplifiers. The system was also used to characterise the nonlinear distortion in SiC varactors. It was shown that the nonlinear distortion increases the losses, and hence a new general Q-factor description was proposed. Furthermore, a new characterisation method was proposed which enabled the study of memory effects in transistors driven by modulated signals

Nonlinear Characterisation and Modelling of Microwave Semiconductor Devices

There is an increasing need for more accurate models taking into account the nonlinearities and memory effects of microwave transistors. The memory effects are especially important for transistor technologies suffering from relatively large low frequency dispersion, such as GaN baed HEMTs. Nonlinear measurement systems are today available off-the-shelf, but the use of them is still limited. It is therefore important to demonstrate the possibilities these new systems brings to the device characterisation and modelling community. This thesis deals with electrothermal characterisation and modelling of GaN based HEMTs, and also development and utilisation of new nonlinear measurement systems.The electrothermal properties of the AlGaN/GaN heterostructure were characterised, and it was shown that a thermal response is present up to 100 MHz. Moreover, a new characterisation method, making use of nonlinear measurements, allowed for isothermal measurements of the current transport through the access resistances of a GaN based HEMT. A new current transport model was proposed to correctly reproduce the isothermal IV characteristics. Furthermore, the temperature dependence of the high frequency noise was characterised, showing that the major limiting factors for the low noise performance were the access resistances. The combination of high power and low noise makes the GaN based HEMT suitable for monolithically integrated GaN based transceiver front-ends. The first steps toward a transceiver were taken by designing and manufacturing a GaN based receiver front-end consisting of an SPDT switch and an LNA.A new fast multi harmonic active load-pull system was developed, with waveform acquisition capabilities. The speed of the load-pull system was increased by the use of an improved optimisation routine for presenting the wanted load impedances. The load-pull system was capable of presenting dynamically varying load impedances to a transistor, enabling faster device characterisation without the need to build complete amplifiers. The system was also used to characterise the nonlinear distortion in SiC varactors. It was shown that the nonlinear distortion increases the losses, and hence a new general Q-factor description was proposed. Furthermore, a new characterisation method was proposed which enabled the study of memory effects in transistors driven by modulated signals

Characterization of Microwave Transistors for Robust Receivers and High Efficiency Transmitters

The next generation of integrated transceiver front-ends needs both robustlow noise amplifiers and high power amplifiers on a single-chip. The AluminiumGallium Nitride / Gallium Nitride (AlGaN/GaN) High Electron MobilityTransistors (HEMT) is a suitable semiconductor technology for this purposedue to its high breakdown voltage and high electron mobility. In this thesisthe AlGaN/GaN HEMT’s thermal properties, noise and survivability havebeen characterized for the intended use in robust high power transceivers.Furthermore, a new characterization setup for load modulated high efficiencypower amplifiers have been developed.The thermal properties of AlGaN/GaN HEMTs have been carefully investigatedconsidering self-heating and its effect on small-signal parameters andhigh frequency noise. Self-heating is a severe problem for a high power transistoron any semiconductor material, including GaN. In addition to reliabilityproblems, the performance of the operating HEMT degrades with temperature.The access resistances showed a large temperature dependence, which was alsoverified with TLM measurements. Due to the large self-heating, the temperaturedependence of the access resistances has to be taken into account in themodeling of the AlGaN/GaN HEMT. A temperature dependent small-signalnoise model was derived and verified through fabricated amplifiers. Designstrategies for robust low noise amplifiers are discussed and implemented usingthe derived model.The new characterization setup gives new possibilities to characterize theperformance of load modulated amplifiers. Recent results on load modulatedamplifiers show promising efficiency improvements in back-off operation.Therefore a new measurement setup was developed that performs dynamicload modulation at the transistor terminals. This method should be usefulto further improve the performance of load modulated amplifiers for high efficiencyoperation. The measurement setup is based on an active load-pullsetup, where a modulated input signal is used to synthesize a time varyingoutput power. The load impedance is dynamically controlled with the envelopof the input signal, following an optimum efficiency load trajectory. This givesbetter insight into device operation and possible improvements

Fast Multi Harmonic Active Load-Pull

A fast multi harmonic active load-pull setup is presented in this paper. The heart of the system is a large signalnetwork analyzer (LSNA, Maury/NMDG MT4463). The LSNA measures the voltage and current time domainwaveforms at the device terminals. The load reflection coefficient (ΓL(f)) is realized by injecting an amplitude andphase controlled signal at the wanted harmonic on the output of the transistor, according to:ΓL(f) = a2(f0)b2(f0)=A(VI, VQ)ej(ω0+θ(VI,VQ))b2(f0)(1)The speed of the system is significantly improved by using the LSNA in modulation mode. This allows formultiple impedance states to be presented during the measurement time of the LSNA. More than 180 impedancestates can be measured within 21 ms. The sample clock of the LSNA and the vector modulators, controlling theload impedance, needs to be synchronized. A digital pattern generator (DPG) is therefore used in the setup toenable the synchronization, and also to trigger the start of the measurement. Furthermore, a new optimizationalgorithm is implemented to reduce the number of iterations needed to realize the desired load impedance. Theoptimization is based on the Newton-Raphson algorithm, and convergence is obtained within 3-5 iterations for oneharmonic. The optimization is carried out simultaneously for all wanted harmonics, and multiple impedance states.This significantly reduce the total measurement time for multi harmonic load-pull measurements