Gallium nitride-based microwave high-power heterostructure field-effect transistors

The research described in this thesis has been carried out within a joint project between the Radboud Universiteit Nijmegen (RU) and the Technische Universiteit Eindhoven (TU/e) with the title: "Performance enhancement of GaN-based microwave power amplifiers: material, device and design issues". This project has been granted by the Dutch Technology Foundation STW under project number NAF 5040. The aims of this project have been to develop the technology required to grow state-of-the-art AlGaN/GaN epilayers on sapphire and semi-insulating (s.i.) SiC substrates using metal organic chemical vapor deposition (MOCVD) and to fabricate microwave (f > 1 GHz) high-power (Pout > 10 W) heterostructure field-effect transistors (HFETs) on these epitaxial films. MOCVD growth of AlGaN/GaN epilayers and material characterization has been done within the group Applied Materials Science (AMS) of RU. Research at the Opto-Electronic Devices group (OED) of TU/e has focused on both electrical characterization of AlGaN/GaN epilayers and design, process technology development, and characterization of GaN-based HFETs and CPW passive components. Although a considerable amount of work has been done during this research with respect to processing of CPW passive components on s.i. SiC substrates, this thesis focused on active AlGaN/GaN devices only. GaN is an excellent option for high-power/high-temperature microwave applications because of its high electric breakdown field (3 MV/cm) and high electron saturation velocity (1.5 x 107 cm/s). The former is a result of the wide bandgap (3.44 eV at RT) and enables the application of high supply voltages (> 50 V), which is one of the two requirements for highpower device performance. In addition, the wide bandgap allows the material to withstand much higher operating temperatures (300oC - 500oC) than can the conventional semiconductor materials such as Si, GaAs, and InP. A big advantage of GaN over SiC is the possibility to grow heterostructures, e.g. AlGaN/GaN. The resulting two-dimensional electron gas (2DEG) at the AlGaN/GaN heterojunction serves as the conductive channel. Large drain currents (> 1 A/mm), which are the second requirement for a power device, can be achieved because of the high electron sheet densities (> 1 x 1013 cm-2) and high electron saturation velocity. These material properties clearly indicate why GaN is a very suitable candidate for next-generation microwave high-power/high-temperature applications such as high-power amplifiers (HPAs) for GSM base stations, and microwave monolithic integrated circuits (MMICs) for radar systems. In this thesis we have presented the design, technology, and measurement results of n.i.d. AlGaN/GaN:Fe HFETs grown on s.i. 4H-SiC substrates by MOCVD. These devices have submicrometer T- or FP-gates with a gate length (Lg) of 0.7 µm and total gate widths (Wg) of 0.25 mm, 0.5 mm, and 1.0 mm, respectively. The 1.0 mm devices are capable of producing a maximum microwave output power (Pout) of 11.9 W at S-band (2 GHz - 4 GHz) using class AB bias conditions of VDS = 50 V and VGS = -4.65 V. It has to be noted that excellent scaling of Pout with Wg has been demonstrated. In addition, the associated power gain (Gp) ranges between 15 dB and 20 dB, and for the power added efficiency (PAE) values from 54 % up to 70 % have been obtained. These results clearly illustrate both the successful development of the MOCVD growth process, and the successful development and integration of process modules such as ohmic and Schottky contact technology, device isolation, electron beam lithography, surface passivation, and air bridge technology, into a process flow that enables the fabrication of state-of-the-art large periphery n.i.d. AlGaN/GaN:Fe HFETs on s.i. SiC substrates, which are perfectly suitable for application in e.g. HPAs at S-band

Application of waveform engineering to GaN HFET characterisation and class F design

In this work, the largely theoretical existing research on class F has been extended to include a measured waveform based analysis. The results demonstrate how optimum class F performance can be achieved using real devices and highlights a number of interesting issues that a designer of a class F amplifier should consider.EThOS - Electronic Theses Online ServiceGBUnited Kingdo

Application of waveform engineering to GaN HFET characterisation and class F design

In this work, the largely theoretical existing research on class F has been extended to include a measured waveform based analysis. The results demonstrate how optimum class F performance can be achieved using real devices and highlights a number of interesting issues that a designer of a class F amplifier should consider

Development of III-nitride bipolar devices: avalanche photodiodes, laser diodes, and double-heterojunction bipolar transistors

This dissertation describes the development of III-nitride (III-N) bipolar devices for optoelectronic and electronic applications. Research mainly involves device design, fabrication process development, and device characterization for Geiger-mode gallium nitride (GaN) deep-UV (DUV) p-i-n avalanche photodiodes (APDs), indium gallium nitride (InGaN)/GaN-based violet/blue laser diodes (LDs), and GaN/InGaN-based npn radio-frequency (RF) double-heterojunction bipolar transistors (DHBTs). All the epitaxial materials of these devices were grown in the Advanced Materials and Devices Group (AMDG) led by Prof. Russell D. Dupuis at the Georgia Institute of Technology using the metalorganic chemical vapor deposition (MOCVD) technique. Geiger-mode GaN p-i-n APDs have important applications in DUV and UV single-photon detections. In the fabrication of GaN p-i-n APDs, the major technical challenge is the sidewall leakage current. To address this issue, two surface leakage reduction schemes have been developed: a wet-etching surface treatment technique to recover the dry-etching-induced surface damage, and a ledged structure to form a surface depletion layer to partially passivate the sidewall. The first Geiger-mode DUV GaN p-i-n APD on a free-standing (FS) c-plane GaN substrate has been demonstrated. InGaN/GaN-based violet/blue/green LDs are the coherent light sources for high-density optical storage systems and the next-generation full-color LD display systems. The design of InGaN/GaN LDs has several challenges, such as the quantum-confined stark effect (QCSE), the efficiency droop issue, and the optical confinement design optimization. In this dissertation, a step-graded electron-blocking layer (EBL) is studied to address the efficiency droop issue. Enhanced internal quantum efficiency (ɳi) has been observed on 420-nm InGaN/GaN-based LDs. Moreover, an InGaN waveguide design is implemented, and the continuous-wave (CW)-mode operation on 460-nm InGaN/GaN-based LDs is achieved at room temperature (RT). III-N HBTs are promising devices for the next-generation RF and power electronics because of their advantages of high breakdown voltages, high power handling capability, and high-temperature and harsh-environment operation stability. One of the major technical challenges to fabricate high-performance RF III-N HBTs is to suppress the base surface recombination current on the extrinsic base region. The wet-etching surface treatment has also been employed to lower the surface recombination current. As a result, a record small-signal current gain (hfe) > 100 is achieved on GaN/InGaN-based npn DHBTs on sapphire substrates. A cut-off frequency (fT) > 5.3 GHz and a maximum oscillation frequency (fmax) > 1.3 GHz are also demonstrated for the first time. Furthermore, A FS c-plane GaN substrate with low epitaxial defect density and good thermal dissipation ability is used for reduced base bulk recombination current. The hfe > 115, collector current density (JC) > 141 kA/cm², and power density > 3.05 MW/cm² are achieved at RT, which are all the highest values reported ever on III-N HBTs.PhDCommittee Chair: Shen, Shyh-Chiang; Committee Member: Dupuis, Russell; Committee Member: Jiang, Zhigang; Committee Member: Mukhopadhyay, Saibal; Committee Member: Yoder, Dougla