THESIS
2008
xi, 88 leaves : ill. ; 30 cm
Abstract
Wide band-gap AlGaN/GaN high electron mobility transistors (HEMTs) are emerging as outstanding candidates for a wide range of applications, from radio-frequency and microwave power amplifiers, to high-efficiency power converters. Conventional AlGaN/GaN HEMTs show depletion-mode (D-mode) or normally-on operation with negative threshold voltage. Many circuit applications, however, prefer enhancement-mode (E-mode) or normally-off devices with a positive threshold voltage. For example, E-mode HEMT allows the elimination of the negative voltage supplier in power amplifiers (PAs) and low noise amplifiers (LNAs) circuits; thus reducing the circuit complexity and system cost. In power switches, the normally off operation is favored for operation safety. Recently, a novel fluorine plasma treatme...[
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Wide band-gap AlGaN/GaN high electron mobility transistors (HEMTs) are emerging as outstanding candidates for a wide range of applications, from radio-frequency and microwave power amplifiers, to high-efficiency power converters. Conventional AlGaN/GaN HEMTs show depletion-mode (D-mode) or normally-on operation with negative threshold voltage. Many circuit applications, however, prefer enhancement-mode (E-mode) or normally-off devices with a positive threshold voltage. For example, E-mode HEMT allows the elimination of the negative voltage supplier in power amplifiers (PAs) and low noise amplifiers (LNAs) circuits; thus reducing the circuit complexity and system cost. In power switches, the normally off operation is favored for operation safety. Recently, a novel fluorine plasma treatment technology was demonstrated to provide a self-aligned approach of converting D-mode HEMT to E-mode HEMT. For practical implementation of this technology, it is of great importance to study the stability of fluorine ions and the reliability of E-mode HEMTs.
In this work, the reliability of E-mode HEMT fabricated by fluorine plasma treatment technology is investigated using both high temperature thermal stress and high-field electrical stress. No detectable changes were observed in the devices that went through thermal stress at 350°C for 153 hours, indicating excellent thermal stability of fluorine ions. A moderate but persistent negative shift (-0.25 V) was observed in the threshold voltage after 288 hours of electrical stress at room temperature in an unpassivated device. Two solutions, namely dual-gate structure and surface passivation, to improve the electrical reliability are proposed and proved to be effective by experimental results. The dual-gate structure can effectively shield the high electric-field from influencing the fluorine plasma treated region. The passivation approach is capable of effectively stabilizing the fluorine ions on the surface.
Device reliability is also often affected by the gate leakage. Since the fluorine plasma treatment technique is applied in the gate region, it is particularly important to characterize the gate leakage in E-mode AlGaN/GaN HEMTs. Using the dual-gate structure, the surface and bulk components of the gate leakage current in both E-mode and D-mode gates are independently characterized, from room temperature up to 250°C. No obvious differences are observed between the E-mode and D-mode devices, while E-mode gate exhibits higher surface leakage at higher temperatures.
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