GaN Schottky barrier diodes threaten to overturn SiC

Soaring energy prices and an increased awareness of environmental issues are spurring the development of products that consume less energy. For computers and other consumer and industrial electronic products, one area with the potential for improvement is the switch-mode power supplies (SMPSs) that convert AC mains into various DC formats. Ideally, these supplies would now be delivering higher efficiencies through the employment of SiC high-voltage Schottky barrier diodes, but the high price of these devices is preventing them from displacing the less efficient silicon equivalents that still dominate the high-power semiconductor market.

However, it will soon be possible to enjoy the benefits of a wide-bandgap semiconductor – high breakdown fields, good thermal conductivities, and high electron mobilities and saturation drift velocities – by turning to GaN instead. On sapphire substrates, GaN promises to be a much cheaper alternative to SiC. This combination of materials may raise a few eyebrows, because it is widely believed to suffer from sapphire s low conductivity – something that ultimately leads to poor thermal resistances and hot, unreliable devices. But at Velox Semiconductor, located in Somerset, NJ, we have demonstrated the folly of this argument with GaN-on-sapphire diodes that are incorporated in an insulating frame.

The compatibility with an insulating frame is a big advantage over SiC, because it reduces the cooling demands of the heat sinks employed in the SMPSs. In these modules we have found that our devices deliver efficiencies comparable to SiC and significantly better than those of silicon.

The manufacturing cost-savings over SiC equivalents stem from cheaper, larger substrates and a lower epiwafer growth temperature. We are building our devices on 100 mm sapphire, which has primarily been developed for the multibillion-dollar GaN LED industry as it transitions away from 2 and 3 inch wafers. Multiple vendors are now supplying the larger-sized material, and prices are falling thanks to healthy competition and continuous manufacturing improvements. There might also be an option of scaling to even larger substrates in the near-term, as 150 mm research and development material is already available from leading manufacturers.

In contrast, 100 mm SiC substrates are only available in limited quantities at outrageous prices, so manufacturing and development is undertaken on the 75 mm platform. This smaller size is still pricey and we have found that its cost per unit area is typically four times that of sapphire (table 1).

Working with larger wafers brings an obvious advantage – more chips per wafer. However, there is a secondary benefit, as the majority of semiconductor equipment available is developed for substrates with diameters of 100 mm or more. This means that it is easier to equip a fab with production equipment and there is more choice for outsourcing processes.

Further gains are realized from GaN s lower growth temperature (1000–1100 °C versus 1500–1600 °C for SiC). Reactor parts for SiC growth are also very expensive, not particularly reliable and suffer from a much smaller supply base.

Producing marketable nitride power devices requires the growth of high-quality, uniform thick epitaxial layers. GaN does not suffer from "micropipe" defects that plague SiC, but dislocations can result from lattice mismatches between the substrate and the epilayers. This problem is also encountered by LED manufacturers, and like them we have developed proprietary nucleation and buffer growth techniques to reduce dislocation densities.

Conductive atomic force microscopy images reveal the high quality of our nitride film, which has a typical conductive dislocation density of 103 cm–2 (figure 1). These dislocations are probably responsible for reverse leakage currents, but their low density allows us to manufacture reliable low-leakage devices.

We produce our Schottky diode epiwafers in-house by growing thick GaN and AlGaN layers on insulating sapphire substrates. A patented interdigitated geometry is used to add contacts that maximize the conducting current for the smallest possible chip size (figure 2). This design also minimizes the lateral spreading resistance – the resistance due to current flowing from the bond pads to the device s corners – and this in turn reduces the diode s forward voltage and improves its efficiency when deployed in a SMPS.

Our Schottky diode leakage current is reduced through proprietary surface preparation and passivation techniques. At room temperature these devices have a leakage current of less than 200 µA at 600 V reverse bias, but this rises to 600 µA at 125 °C (figure 3a). Forward voltages of 1.7 and 2.0 V are required to deliver 6 A at room temperature and at 125 °C, respectively (figure 3b). I-V curves show that the behavior of these diodes is close to that of an ideal diode.

Our GaN devices, like their SiC cousins, have a very short, temperature-independent reverse recovery time that is vastly superior to silicon diodes (figure 4). The device s behavior is essentially the same at room temperature and at 125 °C.

Better packages
A major additional advantage of GaN devices over their SiC equivalents is compatibility with an insulating frame, something that stems from the excellent insulation that is provided by sapphire. When these devices are housed in the very common lead-framed plastic TO-220 packages they can be screwed directly to the heat sink and produce a thermal resistance that is typically just 1.8 °C/W.

SiC devices, in comparison, have a much higher thermal resistance. If non-isolated lead frames are used, then an isolating pad must be inserted between the package and the heat sink, and this increases thermal resistance by 1.0–1.5 °C/W. This additional pad can be avoided by switching to an isolated frame, but any benefit is marginal because this design has a 3 °C/W thermal resistance.

The significant reduction in thermal resistance provided by our diodes is a major plus point for SMPS designers. That s because it can allow heat sinks to operate at higher temperatures, or devices to run cooler for an equivalent heat-sink temperature.

The benefit is significant for diodes operating at 8 A and 2.5 V, which will dissipate 20 W. If the heat sink is operating at 100 °C, typical silicon or SiC devices with a thermal resistance of 3 °C/W will have a junction temperature of 160 °C. This compares to 136 °C for a GaN diode under the same conditions, thanks to the lower thermal resistance. The low operating temperature improves GaN diode reliability.

We evaluated our devices by inserting them into two SMPSs that normally incorporate SiC diodes. These 1000 W, 48 V power supplies originally featured two of Infineon s SDT06S60 diodes (6 A, 600 V) in the power factor correction circuits. We replaced them with our equivalent, the GaN VSD06060 diodes.

Our GaN diodes perform as well as their SiC equivalents at room temperature and at 72 °C, according to measurements comparing the performance of the SMPSs employing both device types (figure 5). The tests used thermocouples to measure the temperatures of the diodes and heat sinks at various input voltages and loads. We also recorded the total power supply efficiency at these operating conditions. No difference in system performance was measured between the GaN and SiC devices.

We have also conducted standard RF emission tests for our power supplies, which have been measured in modified and original forms and compared against standards for class B equipment. No difference was observed between supplies featuring GaN and SiC diodes, and in both cases the equipment easily passed the class B requirements.

This series of tests demonstrates that our GaN diodes deliver the performance of SiC, but at a fraction of the cost. This product is at the final stages of qualification and should make the transition to production at the end of this year. STMicroelectronics is helping us with the qualification and marketing of these diodes. Future versions of the device operating at high currents and voltages could be used in hybrid electric vehicles and we are developing diodes for this application. However, our next product will be a GaN-on-silicon transistor that promises to deliver very low on-resistance and superior switching characteristics compared to silicon devices.

Further reading
I Cohen et al. 2005 12th Annual IEEE Applied Power Electronics Conference and Exposition 1 311.
www.veloxsemi.com
B Shelton US Patent No. 7084475.