Advanced GaN HEMTs for high performance microwave power amplifiers

The ever increasing demand for high power levels at higher frequencies from the industry has stimulated extensive research in gallium nitride (GaN) transistor technology over the past two decades. This has led to significant advances of the technology, but the degradation in the device performance due to device self-heating and trap generation in the device epilayers during device operation is still a major challenge with the current GaN high electron mobility transistor (HEMT) technology. This thesis focuses on minimising device self-heating effects by means of efficient heat distribution within the device. Two approaches are analysed in this work. Firstly, the impact on the device DC performance of improved wafer growth conditions by using method called hot-wall MOCVD (metal organic chemical vapour deposition) are investigated. It was found that 2 µm × 100 µm devices on this wafer exhibit only 4% degradation in the saturated output current density at 20 V compared with 13% for devices fabricated on a wafer grown by standard MOCVD growth. This improved performance was attributed to lower thermal boundary resistance achieved by improved growth quality of the epitaxial material layers. In the second approach, the impact on self-heating was investigated through the use of a distributed device channel, i.e. introducing inactive regions along the device channel to distribute the hot spots in the device. Here a planar isolation method was used to achieve planar distributed gate devices that led to low leakage currents below 200 nA/mm at gate voltage of -20 V. A decrease in the peak channel temperature of 30°C was found through thermal simulations over a single 100 µm wide gate finger. Moreover, these distributed channel devices with gate periphery of 10 ×100 µm showed 13 % higher saturated current density than standard devices with the same active device area. The other major issue addressed in this thesis is the so-called current collapse which is a degradation in the output current caused by electron trapping in the device structure. An alternative solution to the conventionally used dielectric passivation is proposed and it entails the use of a thick undoped GaN cap layer to reduce the surface effects by moving the surface further away from the device channel. Drain lag measurements show 15% and 35% decrease in the current at quiescent bias decrease points of [-7 V; 10 V] and [-7 V; 20 V] respectively for the proposed structure compared with 80% decrease and complete current collapse at these quiescent bias points in the same geometry devices on a standard wafer with 2 nm GaN cap layer and a thin 10 nm thin SiNx passivation, respectively. The 10 nm thin passivation layer does not minimise the surface effects, but it protects the devices from oxidation. Finally, a single stage class A amplifier was demonstrated using the developed technology exhibiting peak output power of 30 dBm at 10 GHz and associated power added efficiency of 44% and gain of 10 dB. Also, gain of at least 9.4 dB was shown over 8-13 GHz bandwidth

A planar distributed channel AlGaN/GaN HEMT technology

This brief presents AlGaN/GaN high electron mobility transistor (HEMT) devices with improved thermal and dc current-voltage (I-V) performance using a novel method of obtaining a distributed channel device, i.e., the total semiconductor area between the ohmic contacts comprise conducting and nonconducting regions. A novel oxygen (O2) plasma treatment technique is used to realize the inactive or nonconducting regions. Multifinger devices with 1-mm gate periphery exhibit extremely low gate leakage currents below 0.2 μA/mm at a gate voltage of -20 V and an increase in the saturated output current by 14% at 20-V drain voltage. Moreover, performed dc I-V measurements at various ambient temperatures show that the proposed method not only increases the saturated output currents by over 10% for 1 × 100 μm 2 gate devices but also significantly reduces their knee walkout voltage from 6 to 3 V at 300 K. These results show that this device design approach can exploit further the potential of the GaN material system for transistor applications

AlGaN/GaN HEMT with Distributed Gate for Improved Thermal Performance

This paper reports a novel type of distributed gate (DG) HEMT fabricated using isolation by oxygen plasma. The technique results in planar devices with low gate leakage currents of only 1.3 μA/mm at -20 V gate voltage for devices with gate periphery of 1 mm. The DG-HEMT improves the thermal performance by reducing the current drop at higher drain voltages leading to higher output powers

Speed limitations of resonant tunneling diode-based photodetectors

In this work, we study multiple epitaxial layer structures incorporating a resonant tunneling diode photodetector utilizing the In0.53Ga0.47As/InP material system for operation at the near-infrared region of 1.55 and 1.31 micrometers. We study the photodetection speed of response for these devices and the physical limitations affecting their bandwidth. We show that resonant tunneling diode-based photodetectors have bandwidth limitations due to the charge accumulation near the barriers and report on an operating bandwidth reaching up to 1.75 GHz in particular structures, which is the highest number reported for such detectors to the authors’ best knowledge

Systemic Functional Evaluative Complements in Academic Book Reviews

Bakalaura darbā aprakstīti vērtējošo papildinājumu lietojuma zinātnisko grāmatu recenzijās pētījuma rezultāti, vadoties pēc Martina (2000) izstrādātās vērtēšanas teorijas. Darba teorētiskā daļa ir bastīta uz Martina un Vaita (2005), Martina (2000), Halideja (1994) kā arī Blora un Bloras (2004) darbiem. Darba mērķis bija uzskaitīt un norādīt visbiežāk leksiskos valodas līdzekļus, kas funkcionē kā vērtējošie papildinājumi, pētot zinātnisko grāmatu recenzijas dažādās lingvistikas nozarēs. Pielietotā pētniecības metode ir korpusa lingvistikas pētniecības metode. Iegūtie dati liecina, ka pozitīvi vērtējošo papildinājumu skaits ir trīs reizes augstāks nekā negatīvo. Turklāt dažādās lingvistikas nozarēs tiek izmantoti līdzīgi leksikas līdzekļi, lai paustu vērtējumu. Atslēgas vārdi: vērtējošā teorija; vērtējošie papildinājumi; priekšmeta papildinājumi; intensīvie papildinājumi; grāmatu recenzijasThe present BA thesis describes the results of the research into the use of evaluative Complements in academic book reviews which were examined following Martin’s (2000) theory of appraisal. The background of the research has been based mainly on the works of Martin and White (2005), Martin (2000), Hyland (2000), Halliday (1994) and Bloor and Bloor (2004). The goal of the research was to enumerate and point out the most common lexical means functioning as evaluative Complements by analysing academic book reviews in different branches of linguistics. The research method used was corpus-linguistic research method. The obtained data indicate that the use of positive evaluative Complements in academic book reviews is three times higher than that of the negative ones. Moreover, different linguistic branches share similar lexical means of expressing evaluation. Key words: appraisal theory; evaluative Complements; object Complements; intensive Complements, book review

AlGaN/GaN HEMTs With Reduced Self‐Heating

No abstract available

AlGaN/GaN HEMTs With Reduced Self‐Heating

No abstract available

Comparative Study of AlGaN/GaN HEMTs with and without the Buffer on SiC Substrates

No abstract available

Thick GaN Capped AlGaN/GaN HEMTs for Reduced Surface Effects

No abstract available