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III-nitride normally-off low-density-drain high electron mobility transistors (LDD-HEMTs)

Authors Song, Di
Issue Date 2007
Summary Since the first demonstration of the AlGaN/GaN high electron mobility transistor (HEMT) over a decade ago, there has been rapid development in wide bandgap GaN-based materials and devices. Wide bandgap AlGaN/GaN HEMTs are promising candidates for next-generation microwave power amplifiers and power electronics components (e.g. switches and converters) owing to their large power handling capabilities. Tremendous progress has been made in the performance of the conventional AlGaN/GaN depletion-mode HEMTs by improvement in material growth, epi-structure design and device processing techniques. Meanwhile, more advanced device structures are being explored for further performance improvement. In circuit applications, normally-off AlGaN/GaN HEMTs are desirable because they offer simplified circuit configurations and favorable operating conditions for device safety. However, the normally-off AlGaN/GaN HEMTs usually exhibit lower maximum drain current compared to their normally-on counterparts, especially when the threshold voltage is increased to ~+1 V to assure the complete turn-off of the 2DEG channel at zero gate bias. To compensate the reduction in maximum current and maintain similar power handling capability, the breakdown voltage (VBK) needs to be further improved, preferably not at the cost of increased gate-to-drain distance which inevitably increases the device size. In this work, a low-cost approach of modifying the surface field distribution between the gate and drain developed. The field modification is achieved by turning the entire or part of the region between the gate and drain into a region with low density of 2DEG, effectively forming Low-Density Drain (LDD). With the same device dimensions, the off-state breakdown voltage VBK improves from 60 V in a device without LDD to over 90 V in a device with LDD. No degradation in ƒT and slight improvement of power gain and ƒmax were observed in the LDD-HEMT owing to the absence of a field plate electrode and increased output resistance (RDS). In addition, reduction in current collapse is observed in LDD-HEMTs that are not passivated by SiN layer, indicating a passivation effect of the fluorine plasma treatment. A small-signal equivalent circuit model is also developed that enables the extractions of all the model parameters of the LDD-HEMT. The LDD-HEMT also exhibits improved linearity and reduced noise figure compared to the standard HEMTs.
Note Thesis (M.Phil.)--Hong Kong University of Science and Technology, 2007
Language English
Format Thesis
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