Date of Award

Fall 2017

Document Type

Open Access Dissertation

Department

Electrical Engineering

Sub-Department

College of Engineering and Computing

First Advisor

Asif Khan

Abstract

High Aluminum content AlxGa1-xN (x > 30%) has attracted intense research interest nowadays as “Ultra Wide Band Gap (UWBG)” material due to large band gap (> 3.4 eV). The critical electric field (EC) of UWBG semiconductor AlGaN is significantly higher than GaN. Moreover, electron saturation velocity is also comparable to GaN. These attractive material properties are the needs for getting large breakdown voltage and high thermal stability which are the requirements for next generation power semiconductor devices. Theoretical possibilities show UWBG AlGaN based devices are an emerging class and can outperform conventional GaN based devices even at elevated temperatures. This dissertation focuses on design, fabrication, and characterization of UWBG semiconductor AlGaN based Field Effect Transistors. Along with detailed electrical and thermal characterization, a brief study on optical characterization is also performed to understand the potential of UWBG semiconductor AlGaN based devices as UV detector which has many important commercial and military applications.

The work started with brief explanation of material growth and characterization. This includes discussion on structural and surface morphological analysis of the active epilayers grown on AlN (3μm)/Sapphire templates conventionally used for Deep Ultra Violet Light Emitting Diodes (DUV LEDs). The off-axes (102) X-ray peak line width of this AlN buffers was measured to be 350 arc-sec which translates the overall defect density close to (1-3) × 108 cm-2. The device epilayers were grown on these templates pseudo morphically. Electrical characterizations of these epilayers are performed by eddy current method and mercury probe Capacitance-Voltage (C-V) method. The transmission spectra measurement is also performed to get the optical absorption edge.

A series of experiments have been made to realize the Field Effect Transistors (FETs). As a first step of making UWBG based Field Effect Transistors an n-Al0.5Ga0.5N channel Metal Semiconductor Field Effect Transistor (MESFET) was fabricated by Selective Area Growth (SAG) technique for the first time which shows 60 mA/mm current with good gate control. For studying the effect of gate insulator another set of similar type of devices were fabricated with SiO2 as gate insulator which reduces gate current by a factor of 1000. These devices show very high optical responsivity as 1.2×106 A/W which drops after 290 nm of wavelength and shows greater promise as solar blind UV detector. After that Aluminum mole fraction was increased to 65% in the MESFET channel layer. Nonlinearity in Ohmic contacts motivated to develop a new approach of Selective Area Graded n-AlGaN based Ohmic contact. Extensive thermal characterizations have been performed. It is found that up to 200 oC the change in drain saturation current is only < 10%. These devices also showed 254 nm UV detection capabilities at 200 oC.

For getting better switching performance i-Al0.65Ga0.35N channel based HEMT was designed and fabricated. In this case i-Al0.85Ga0.15N and n-Al0.85Ga0.15N barriers were employed separately. Large bandgap of the barrier layer results extremely low gate leakage current in the order of 10-9 A/mm. The peak current obtained was 250 mA/mm which is the highest till to date for UWBG semiconductor AlGaN channel based HEMT. The on current to off current ratio (ION/IOFF) was > 107 which shows the good switching quality of this power semiconductor device. Even at elevated temperature of 250 oC, the ION/IOFF ~ 105. The change in drain saturation current is < 10% at 250 oC which is similar for devices on expensive bulk AlN substrate. The breakdown voltage was close to 800V for gate to drain separation of 9 µm for these un-passivated devices with no filed plate and edge termination.

All these results indicate that UWBG AlGaN channel based FETs have greater promise for next generation high power and high temperature power electronic applications in low cost platform. Also, the optical characterization reveals the possibility to open new device applications scope where photonic and electronic devices can be used on the same chip for high temperature operation.

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