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Conference report  IEDM


no silicon substrate under the gate region [where the heat is generated]. With the STAD approach, the silicon substrate is still under the gate electrode, which has improved thermal performance.”


These novel FETs were formed on silicon (111) substrates, because this orientation has a smaller lattice mismatch with AlN than silicon (100). After the epitaxial stack is formed, which features a 3 nm-thick Al0.45


Ga0.55 a 150 nm-thick GaN channel and a 2 µm-thick Al0.18


Figure 7. Engineers at Panasonic have developed a variant of the natural


N barrier, Ga0.82


N


buffer, the substrate is thinned to 125 µm and trenches are formed around the drain contact with reactive ion etching.


To assess the performance of these transistors, the team also produced control devices without a STAD. The breakdown voltage of these devices saturated at 650 V, while the breakdown voltage of the STAD FETs increased with gate-drain distance, exceeding 2 kV for a separation of 20 µm. The transfer characteristics (IDS


-VGS


revealed no change in threshold voltage with the introduction of a trench around the drain contact, indicating no deterioration to the two-dimensional electron gas (2DEG) channel. High-temperature performance of the STAD FETs was assessed by measuring the buffer leakage at 100 °C. The team found that this leakage is several orders of magnitude lower than that of the control at 500 V (see Figure 6).


Barriers supress leakage


A novel GaN architecture has also been developed at Panasonic to deliver low reverse-leakage current, fast recovery times, and a breakdown voltage of 600 V. This diode could be used in power supply circuits, including those fitted to hybrid electric vehicles.


In these types of applications, silicon incumbents will compete with SiC and GaN variants in the power switching market. One of the key differences between the two wide bandgap diodes is that GaN has a lateral configuration, while the configuration in the SiC device is vertical. A lateral configuration is superior, according to the Panasonic team, because it has an inherently lower capacitance.


“We believe the area of the top electrodes is the dominant origin of capacitance,” says Panasonic’s Tetsuzo Ueda. “By minimizing the area of the top two electrodes, we can reduce the total capacitance with the lateral configuration. We are attaching the electrodes directly to the 2DEG, so we believe we can reduce the area sufficiently.”


Panasonic’s novel diode features triple junctions of AlGaN and GaN on a silicon substrate (see Figure 7). The undoped multi-junctions behave as an insulator when the device is under reverse bias, thanks to balancing of the fixed polarization-induced charges at the top and bottom surfaces. This structure, which Panasonic refers to as the ‘Natural Super Junction’, does not require precise control of the doping, and a low operating voltage and contact resistance is possible


superjunction diode that includes a p- GaN barrier controlling layer,which supresses leakage current


)


by applying Ni/Au anode and Ti/Al cathode contacts to the sidewalls of the junctions.


This particular structure suffers from a high leakage current, which prevents high operating voltages. But the engineers at Panasonic have recently developed a way to combat that by adding a p-type GaN blocking layer. Simulations show that with this addition the depletion resulting from the p-GaN layer increases the tunnelling distance, thereby supressing the tunnelling currents and the reverse leakage current.


The paper presented by Panasonic researchers at the IEDM meeting in Washington detailed devices with this architecture, which were formed using MOCVD. These devices were compared with commercial SiC diodes. This effort determined that the GaN multi-junction diode has a significantly lower capacitance than its SiC rival, produces a blocking voltage up to 600 V and delivers 18 A at 1.5 V. Boost converter circuits have been built with Panasonic’s diode and a GaN-based, normally off GIT, which has an on-state resistance of 100 mΩ and a breakdown voltage of 600 V. Operating at 100 kHz, the efficiency of this convertor exceeds 98 percent, outperforming the combination of SiC Schottky barrier diode and silicon free-wheeling diode that can drive the GaN-based GIT.


Panasonic is starting to try and exploit the commercial promise of this diode, which outperforms its SiC rival in terms of circuit-level efficiency and enables a reduction in component count.


This novel GaN device detailed at IEDM by Panasonic, plus that described by imec and the advances in III-V transistors reported by many groups, highlights the promise of the compounds to exceed what is possible with silicon. And at IEDM 2012, it’s a sure bet that they’ll be many more announcements echoing this theme.


© 2012 Angel Business Communications. Permission required.


January/February 2012 www.compoundsemiconductor.net 19


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