Tunneling Through the Power Barrier
“There is a light at the end of the tunnel” is a phrase used by many. Now, Kevin Chen and his group at The Hong Kong University of Science and Technology say there is also “Power at the end of the Tunnel”. They have recently shown that a wide bandgap GaN-based power tunnel FET with normally-off operation can be realized on widely available baseline AlGaN/GaN heterostructures. The devices are claimed to offer record-low off-state leakage and record-high on/off current ratio at a high drain voltage. Advances in AlGaN/GaN HEMT technology have already shown that these devices are capable of beating silicon in terms of performance. Two of the most challenging but also highly desirable features of GaN power devices are normally-off operation with positive threshold voltage and low off-state leakage current at a high drain voltage. The GaN power tunnel FET, with its new current controlling scheme and a novel Schottky source configuration, delivers normally-off operation and low off-state leakage current simultaneously. According to Chen, the new power tunnel FET features a metal-2DEG (two-dimensional electron gas) tunnel junction that is controlled by an overlapping gate electrode. Since the current turn-on/off is mainly controlled by the tunnel junction instead of the 2DEG channel, positive threshold corresponding to normally-off operation is realized on the as-grown normally-on epi-wafers. This method of realizing normally-off operation is fundamentally different from the previous approaches that shift the threshold voltage of the 2DEG channel from negative to positive values. Furthermore, since the Schottky junction at the source electrode is naturally reverse biased in the off-state, excellent leakage blocking and high ION/IOFFratio can be obtained, on an epitaxial wafer without the need for a sophisticated buffer layer. At a source-drain voltage of 50 V, the drain leakage current is 10-11A/mm and the on/off current ratio is 1010. The off-state breakdown voltage of an AlGaN/GaN T-FET with a gate-drain spacing of 2 microns is 274 V, more than twice of that obtained in an AlGaN/GaN HEMT with the same gate-drain spacing. This new power tunnel FET technology could provide a low-cost approach to obtaining normally-off operation and low leakage since it does not require sophisticated techniques such as gate recess, fluorine implantation or an AlGaN buffer layer. However, the process must be optimized in order to improve the run-to-run reproducibility and uniformity, which is the next aim for the scientists.
The results of this research will be published in the paper “Normally-off AlGaN/GaN metal-2DEG tunnel-junction field effect transistors” by L. Yuan, H. Chen, and K. J. Chen in IEEE Electron Device Letters in March 2011.
Fig. 1: (a) Schematiccross-sectionof anAlGaN/GaN tunnel junction FET. (b) The conduction band energy diagramof the tunnel junction FET at the source junction. The labels A-C represent different bias conditions. A: VGS = -3 V, VDS = 10V; B: VGS = 0 V, VDS = 10V; C: VGS = 3 V, VDS = 10V.At zero gate bias, the tunnel barrier’s thickness is ~10nm and does not allow significant tunnel current, leading to the normally-off operation.
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Fig. 2: (a) low off-state current and high ON/OFF ratio; (b) Ids-Vds characteristics; (c) transfer characteristics.
