Surface acoustic wave (SAW) filters using twodimensional electron gas (2DEG) as interdigital transducers (IDT) on AlGaN/GaN heterostructure has been demonstrated for the first time using a fluoride-based (CF4) plasma treatment technique. The CF4 plasma treatment is used to pattern 2DEG IDT on a planar surface without removing the top AlGaN layer. The RF characteristics of the SAW filters with planar 2DEG IDTs are compared with metal IDT SAW filters. It is shown that the massloading effects and the triple-transit-interference (TTI) are suppressed in the 2DEG IDT SAW devices owing to the removal of the metal IDTs. It is capable of reducing not only the passband ripple, but also the size of devices because 2DEG IDTs can be placed closer. In addition, the detection part of the SAW sensor can be performed on the top of the planar 2DEG IDTs rather than in the SAW propagation path. This novel SAW device can be integrated with high-electron mobility transistors (HEMTs) on AlGaN/GaN heterostructure to deliver a viable approach for single chip GaN wireless sensors.Index Terms -Two-dimensional electron gas, interdigital transducers, surface acoustic wave filters, AlGaN/GaN, fluoridebased plasma treatment, planar process.

We report a low-density drain HEMT (LDD-HEMT) that exhibits enhanced breakdown voltage and reduced current collapse. The LDD region is created by introducing negatively charged fluorine ions in the region between the gate and drain electrodes, effectively modifying the surface field distribution on the drain side of the HEMT without using field plate electrodes. Without changing the device physical dimensions, the breakdown voltage can be improved by 50% in LDD-HEMT and the current collapse can be reduced. No degradation of current cutoff frequency (f) and slight improvement in power gain cutoff frequency (f.a) are achieved in the LDD-HEMT, owing to the absence of any additional field plate electrode. Ie INTRODUCTIONNormally-off AlGaN/GaN HEMTs are desirable for microwave power amplifier and power electronics applications 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 [1-3]. To compensate the reduction in maximum current and achieve the same 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. The use of a field-plate, connected to the gate or source electrodes, can effectively enhance VBK by modifying the surface field distribution. The gate-terminated field plate [4], however, can introduce additional gate capacitances (CGS and CGD), which reduce the devices' gain and cutoff frequencies.The source-terminated field plate [5, 6], which requires a thick dielectric layer between the gate and field plate, proves effective in achieving enhanced VBK and mitigating the gain reduction.In this work, we present a simple approach of modifying the surface field distribution between the gate and drain without using a field plate electrode. The field modification is achieved by turning part or all of the region between gate and drain into a region with low density of 2DEG, effectively forming a Low-Density Drain (LDD). With the same device dimensions, the off-state breakdown voltage VBK improves from 60 V in an HEMT without LDD to over 90 V in a device with LDD. No degradation in ft and slight improvement of power gain andfma were observed in the LDD-HEMT owing to the absence of a field plate electrode and increased output resistance. II. DEVICE STRUCTURE AND FABRICATIONThe AlGaN/GaN HEMT structure used in this work grown on (0001) sapphire substrates in an Aixtron AIX 2000 HT Metal Organic Chemical Vapor Deposition (MOCVD) system. The HEMT structure consists of a low-temperature GaN nucleation layer, a 2.5-ptm-thick unintentionally doped GaN buffer layer and an AlGaN barrier layer with nominal 30% Al composition. The barrier layer consists of a 3-nm undoped spacer, a 15-nm c...

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