Silicon carbide and gallium nitride challenge the status of silicon circuits

Silicon, which has long been considered as the most ideal IC material, is to be replaced by another new-developed material. Novel new compounds, such as silicon carbide and gallium nitride, can enable electronics power switches will not break down easily.

If the history of technology has taught us anything in the field of microelectronics, silicon carbide must be a winner. It has defeated most other materials, and only been kept out of applications such as photonics by physical properties that make it innately unsuitable. Where silicon can compete directly with materials that offer better nominal properties, it almost always wins. However, when it comes to power electronics, silicon may well have met its match.

As a material for power electronics, silicon gives an excellent performance, but it has a weakness: when hit with a high voltage, it breaks down and starts to conduct out of control. If power is not removed from the device it will simply burn itself to death, with a strong likelihood of roasting the other electronics that it was meant to protect. It is possible to boost the breakdown voltage – the point at which the transistor stops being able to control power – but this leads to higher resistance when switched on, which means more heat and power wasted.

To deal with the breakdown issue, while working at General Electric in the early 1980s Indian electrical engineer Dr B Jayant Baliga invented the insulated-gate bipolar transistor (IGBT) – a hybrid of the two most commonly-encountered types of transistor used now. For the most part, it behaves like the bipolar transistor that was first invented by William Shockley in the 1950s; but it has a gate, sitting behind a fairly thick layer of insulation, much like the metal-oxide semiconductor field-effect transistor (MOSFET) used in every computer.

This blended design has made it the power device of choice where breakdown is a problem. A hybrid electric car may have tens of IGBTs that are used to convert battery power into drive power – and back again.

In general, an IGBT with a given breakdown voltage will beat a power MOSFET in terms of on-state resistance; but MOSFETs are more flexible and much faster. This has become increasingly important in power designs, as circuits that switch faster tend to be smaller. For one, they can get away with smaller passive components, such as inductors and capacitors that form a necessary part of most power circuitry.

Two key materials that stand out for use in power devices are silicon carbide (SiC), and gallium nitride (GaN). One key advantage that GaN and SiC have over silicon is that, for a given on-resistance they do not break down as readily as silicon due to a much larger band gap – the energy it takes to move electrons out of bonds with atoms in the crystal lattice and turn them into unbound, conduction electrons.

The strength of electric field that a device can resist rises roughly with the square of the band-gap energy, so that both SiC and GaN with a band gap more than three times higher than silicon can withstand a field ten times stronger. This allows devices to be made smaller for a given breakdown voltage requirement.

The Figure of Merit pulls together a number of factors that govern the effectiveness of power semiconductor. There are several in common use now, although designers commonly employ the Figure of Merit proposed by Dr Baliga in 1989. This focuses on electron mobility and band gap, and so helps predict how much energy a device will lose when conducting. Because GaN has much higher electron mobility than either silicon or SiC, but its band gap is not very different to that of SiC, it tends to win on this figure of merit.

The availability of silicon processing for GaN is one reason why it may encroach on SiC's core market. SiC has a structure very similar to that of diamond – and a hardness to go with it. Manufacturers have had to develop novel ways to cut through the material, using high-powered lasers for example, to build power semiconductors the way they have with silicon.

University of Warwick spin-out Anvil Semiconductor says that it has developed a way of depositing SiC layers on silicon wafers, that the company claims will lead to a step reduction in the cost of manufacturing power switches. According to Anvil, the process will make it possible to produce SiC devices at costs close to those of conventional silicon.

For once, silicon looks as though it is going to give way to other materials in the quest to improve power efficiency – although it seems destined to continue in the game by providing the literal foundation for these new classes of device.