AlGaN/GaN High Electron Mobility Transistors (HEMTs) frequently show an UV-induced increase in ch... more AlGaN/GaN High Electron Mobility Transistors (HEMTs) frequently show an UV-induced increase in channel conductivity (persistent photoconductivity-PPC). PPC is a generic effect that exists for a wide range of HEMT configurations irrespective of the substrate material or the nominal buffer doping type and magnitude. Simulations demonstrate that the magnitude of the PPC effect can be explained by the UV-induced change in electronic band bending in the buffer below the 2D electron gas (2DEG). It only has a significant magnitude for p-type buffers that result from the presence of intentional or unintentional carbon doping and allows the net deep-level doping density (i.e., jN AN D j) below the 2DEG to be quantified, a parameter that is extremely difficult to evaluate by any other means. Transient measurements indicate a PPC recovery time extending to several days, with leakage along dislocations playing an important role in this recovery.
IEEE Transactions on Microwave Theory and Techniques, 2019
Envelope tracking (ET) is a promising power amplifier (PA) architecture for current and future co... more Envelope tracking (ET) is a promising power amplifier (PA) architecture for current and future communications systems, that uses dynamic modulation of the supply voltage to provide high efficiency and potentially very wide bandwidth over a large dynamic range of output power. The dynamic nature of the supply voltage can lead to a problematic variation in transistor gain however, particularly in GaN HEMTs. This paper describes and analyses this behaviour and the detrimental effect it can have on ET PAs. Contributing factors and origins of gain variation are described in detail along with how, for the first time, meaningful comparisons can be made between different devices. Using these guidelines, gain variation is shown to be a widespread issue effecting most GaN HEMTs presented in literature. To allow an analysis of the intrinsic device behaviour, an extended transistor model is developed that takes the effect of gate and source field plates into account. This model is refined using measurement data and used to demonstrate the fact that the parasitic gate-drain capacitance (C GD) is the main contributor to the small-signal gain variation; a significant part of the overall gain variation. Based on this knowledge, possible strategies to reduce gain variation at the transistor technology level are proposed, allowing the optimisation of GaN HEMTs specifically for ET PAs. One identified strategy involves reducing the length of the gate field plate, and is shown to be a viable approach to reduce the gain variation in GaN HEMTs, albeit at an increased RF/dc dispersion.
We demonstrate that the highly resistive Si substrate in GaN-on-Si RF HEMTs does not act as an in... more We demonstrate that the highly resistive Si substrate in GaN-on-Si RF HEMTs does not act as an insulator, but instead behaves as a conductive ground plane for static operation and can cause significant back-gateinduced current collapse. Substrate ramp characterization of the buffer shows good agreement with device simulations and indicates that the current collapse is caused by chargeredistribution within the GaN layer. Potential solutions, which alter charge storage and leakage in the epitaxy to counter this effect, are then presented.
This paper evaluates the applicability of pulsed I-V measurements as a tool for accurately extrac... more This paper evaluates the applicability of pulsed I-V measurements as a tool for accurately extracting nonlinear gallium nitride (GaN)-based heterojunction fieldeffect transistor (HFET) models. Two wafers with the identical layer structure but different growth conditions have been investigated. A series of I-V measurements was performed under dc and pulsed conditions demonstrating a dramatic difference in the kink effect and current collapse (knee walkout) suggesting different trapping behaviors. However, when radio frequency (RF) I-V waveform measurements, utilizing active harmonic load-pull, were used to study the impact of these traps on the RF performance, both wafers gave good overall RF performance with no significant difference observed. This absence of correlation between pulsed I-V measurement results and RF performance raises a question about the applicability of pulsed I-V measurements alone as a tool for extracting nonlinear device models in the case of GaN HFETs. Index Terms-Active harmonic load-pull, current collapse, gallium nitride (GaN), heterojunction field-effect transistor (HFET), high-electron mobility transistor, kink effect, knee walkout, pulsed I-V, trapping effect. I. INTRODUCTION D UE to the unique material properties of gallium nitride (GaN), particularly high-electron mobility, high break
The morphology and impact on leakage currents of two different ohmic metal stacks for GaN based t... more The morphology and impact on leakage currents of two different ohmic metal stacks for GaN based transistor devices is investigated in this work. The results have implications for the performance and reliability of a GaN transistor device. A low temperature Ta based and a higher temperature anneal Ti based metallization are compared. The low temperature process shows a smoother metal semiconductor interface together with several orders of magnitude lower vertical and lateral leakage compared to the conventional higher temperature process. In addition to the leakage tests, back bias ramping experiments are performed unveiling potential advantages of the conventional approach in mitigating current collapse. However the low leakage will enable higher voltage operation making the low temperature process the preferable choice for high power RF applications, if simultaneously current collapse can be controlled.
Many applications that rely on organic electronic circuits still suffer from the limited switchin... more Many applications that rely on organic electronic circuits still suffer from the limited switching speed of their basic elements-the organic thin film transistor (OTFT). For a given set of materials the OTFT speed scales inversely with the square of the channel length, the parasitic gate overlap capacitance, and the contact resistance. For maximising speed we pattern transistor channels with lengths from 10 lm down to the sub-micrometre regime by industrially scalable UV-nanoimprint lithography. The reduction of the overlap capacitance is achieved by minimising the source-drain to gate overlap lengths to values as low as 0.2 lm by self-aligned electrode definition using substrate reverse side exposure. Pentacene based organic thin film transistors with an exceptionally low line edge roughness <20 nm of the channels, a mobility of 0.1 cm 2 /Vs, and an on-off ratio of 10 4 , are fabricated on 4 00 Â 4 00 flexible substrates in a carrier-free process scheme. The stability and spatial distribution of the transistor channel lengths are assessed in detail with standard deviations of L ranging from 185 to 28 nm. Such high-performing self-aligned organic thin film transistors enabled a ring-oscillator circuit with an average stage delay below 4 ls at an operation voltage of 7.5 V.
We examine the Moss-Burstein effect for InN and demonstrate an independent method for determing i... more We examine the Moss-Burstein effect for InN and demonstrate an independent method for determing its magnitude for high carrier concentration material. Consequently it is shown that the extent of the Moss-Burstein effect is less than 0.72 eV for a high carrier concentration sample with a 1.88 eV absorption edge. Early results are also provided for high band-gap low carrier concentration InN films that can be grown reprodcibly, vindicating the work of early groups in the field. The role of stoichiometry is examined in relation to point defects that appear to be common to many forms of InN.
The role of point defects related to the presence of excess nitrogen is elucidated for InN thin f... more The role of point defects related to the presence of excess nitrogen is elucidated for InN thin films grown by different techniques. Elastic recoil detection analysis has shown the presence of excess nitrogen in state-of-the-art InN films. Using x-ray photoelectron spectroscopy and x-ray diffraction it is shown that two distinct forms of point defects can be distinguished; one of these appears to be an interstitial form of nitrogen, common in some forms of polycrystalline InN. The other is associated with a combined biaxial and hydrostatic strain observed for molecular beam epitaxy ͑MBE͒ and chemical vapor deposition ͑CVD͒ grown films, and may be a mixture of the nitrogen-on-metal antisite defect and lower densities of indium vacancies and interstitial nitrogen. The high density of defects present in all the InN samples examined suggests that stoichiometry related point defects dominate the electrical and optical properties of the material. The difference in the type of point defect observed for polycrystalline ͑rf sputtered͒ and epitaxial ͑MBE and CVD͒ InN supports existing evidence that the Moss-Burstein effect is not an adequate description of the apparent band-gap difference between InN samples grown by different techniques.
AlGaN/GaN High Electron Mobility Transistors (HEMTs) frequently show an UV-induced increase in ch... more AlGaN/GaN High Electron Mobility Transistors (HEMTs) frequently show an UV-induced increase in channel conductivity (persistent photoconductivity-PPC). PPC is a generic effect that exists for a wide range of HEMT configurations irrespective of the substrate material or the nominal buffer doping type and magnitude. Simulations demonstrate that the magnitude of the PPC effect can be explained by the UV-induced change in electronic band bending in the buffer below the 2D electron gas (2DEG). It only has a significant magnitude for p-type buffers that result from the presence of intentional or unintentional carbon doping and allows the net deep-level doping density (i.e., jN AN D j) below the 2DEG to be quantified, a parameter that is extremely difficult to evaluate by any other means. Transient measurements indicate a PPC recovery time extending to several days, with leakage along dislocations playing an important role in this recovery.
IEEE Transactions on Microwave Theory and Techniques, 2019
Envelope tracking (ET) is a promising power amplifier (PA) architecture for current and future co... more Envelope tracking (ET) is a promising power amplifier (PA) architecture for current and future communications systems, that uses dynamic modulation of the supply voltage to provide high efficiency and potentially very wide bandwidth over a large dynamic range of output power. The dynamic nature of the supply voltage can lead to a problematic variation in transistor gain however, particularly in GaN HEMTs. This paper describes and analyses this behaviour and the detrimental effect it can have on ET PAs. Contributing factors and origins of gain variation are described in detail along with how, for the first time, meaningful comparisons can be made between different devices. Using these guidelines, gain variation is shown to be a widespread issue effecting most GaN HEMTs presented in literature. To allow an analysis of the intrinsic device behaviour, an extended transistor model is developed that takes the effect of gate and source field plates into account. This model is refined using measurement data and used to demonstrate the fact that the parasitic gate-drain capacitance (C GD) is the main contributor to the small-signal gain variation; a significant part of the overall gain variation. Based on this knowledge, possible strategies to reduce gain variation at the transistor technology level are proposed, allowing the optimisation of GaN HEMTs specifically for ET PAs. One identified strategy involves reducing the length of the gate field plate, and is shown to be a viable approach to reduce the gain variation in GaN HEMTs, albeit at an increased RF/dc dispersion.
We demonstrate that the highly resistive Si substrate in GaN-on-Si RF HEMTs does not act as an in... more We demonstrate that the highly resistive Si substrate in GaN-on-Si RF HEMTs does not act as an insulator, but instead behaves as a conductive ground plane for static operation and can cause significant back-gateinduced current collapse. Substrate ramp characterization of the buffer shows good agreement with device simulations and indicates that the current collapse is caused by chargeredistribution within the GaN layer. Potential solutions, which alter charge storage and leakage in the epitaxy to counter this effect, are then presented.
This paper evaluates the applicability of pulsed I-V measurements as a tool for accurately extrac... more This paper evaluates the applicability of pulsed I-V measurements as a tool for accurately extracting nonlinear gallium nitride (GaN)-based heterojunction fieldeffect transistor (HFET) models. Two wafers with the identical layer structure but different growth conditions have been investigated. A series of I-V measurements was performed under dc and pulsed conditions demonstrating a dramatic difference in the kink effect and current collapse (knee walkout) suggesting different trapping behaviors. However, when radio frequency (RF) I-V waveform measurements, utilizing active harmonic load-pull, were used to study the impact of these traps on the RF performance, both wafers gave good overall RF performance with no significant difference observed. This absence of correlation between pulsed I-V measurement results and RF performance raises a question about the applicability of pulsed I-V measurements alone as a tool for extracting nonlinear device models in the case of GaN HFETs. Index Terms-Active harmonic load-pull, current collapse, gallium nitride (GaN), heterojunction field-effect transistor (HFET), high-electron mobility transistor, kink effect, knee walkout, pulsed I-V, trapping effect. I. INTRODUCTION D UE to the unique material properties of gallium nitride (GaN), particularly high-electron mobility, high break
The morphology and impact on leakage currents of two different ohmic metal stacks for GaN based t... more The morphology and impact on leakage currents of two different ohmic metal stacks for GaN based transistor devices is investigated in this work. The results have implications for the performance and reliability of a GaN transistor device. A low temperature Ta based and a higher temperature anneal Ti based metallization are compared. The low temperature process shows a smoother metal semiconductor interface together with several orders of magnitude lower vertical and lateral leakage compared to the conventional higher temperature process. In addition to the leakage tests, back bias ramping experiments are performed unveiling potential advantages of the conventional approach in mitigating current collapse. However the low leakage will enable higher voltage operation making the low temperature process the preferable choice for high power RF applications, if simultaneously current collapse can be controlled.
Many applications that rely on organic electronic circuits still suffer from the limited switchin... more Many applications that rely on organic electronic circuits still suffer from the limited switching speed of their basic elements-the organic thin film transistor (OTFT). For a given set of materials the OTFT speed scales inversely with the square of the channel length, the parasitic gate overlap capacitance, and the contact resistance. For maximising speed we pattern transistor channels with lengths from 10 lm down to the sub-micrometre regime by industrially scalable UV-nanoimprint lithography. The reduction of the overlap capacitance is achieved by minimising the source-drain to gate overlap lengths to values as low as 0.2 lm by self-aligned electrode definition using substrate reverse side exposure. Pentacene based organic thin film transistors with an exceptionally low line edge roughness <20 nm of the channels, a mobility of 0.1 cm 2 /Vs, and an on-off ratio of 10 4 , are fabricated on 4 00 Â 4 00 flexible substrates in a carrier-free process scheme. The stability and spatial distribution of the transistor channel lengths are assessed in detail with standard deviations of L ranging from 185 to 28 nm. Such high-performing self-aligned organic thin film transistors enabled a ring-oscillator circuit with an average stage delay below 4 ls at an operation voltage of 7.5 V.
We examine the Moss-Burstein effect for InN and demonstrate an independent method for determing i... more We examine the Moss-Burstein effect for InN and demonstrate an independent method for determing its magnitude for high carrier concentration material. Consequently it is shown that the extent of the Moss-Burstein effect is less than 0.72 eV for a high carrier concentration sample with a 1.88 eV absorption edge. Early results are also provided for high band-gap low carrier concentration InN films that can be grown reprodcibly, vindicating the work of early groups in the field. The role of stoichiometry is examined in relation to point defects that appear to be common to many forms of InN.
The role of point defects related to the presence of excess nitrogen is elucidated for InN thin f... more The role of point defects related to the presence of excess nitrogen is elucidated for InN thin films grown by different techniques. Elastic recoil detection analysis has shown the presence of excess nitrogen in state-of-the-art InN films. Using x-ray photoelectron spectroscopy and x-ray diffraction it is shown that two distinct forms of point defects can be distinguished; one of these appears to be an interstitial form of nitrogen, common in some forms of polycrystalline InN. The other is associated with a combined biaxial and hydrostatic strain observed for molecular beam epitaxy ͑MBE͒ and chemical vapor deposition ͑CVD͒ grown films, and may be a mixture of the nitrogen-on-metal antisite defect and lower densities of indium vacancies and interstitial nitrogen. The high density of defects present in all the InN samples examined suggests that stoichiometry related point defects dominate the electrical and optical properties of the material. The difference in the type of point defect observed for polycrystalline ͑rf sputtered͒ and epitaxial ͑MBE and CVD͒ InN supports existing evidence that the Moss-Burstein effect is not an adequate description of the apparent band-gap difference between InN samples grown by different techniques.
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Papers by Hassan Hirshy