A sparkling future for power electronics

Despite predictions that they could never be made, diamond power devices have bucked the doubters by reached the prototype stage. With an eye to the future, power semiconductors based on this diamond technology are expected to save more than 6,000MW of electrical power capacity in Japan alone with even greater savings envisioned in Europe. The first prototypes are based on very high purity, synthetic single-crystal diamond just 4mm across and 0.5mm thick. It is a well known and established fact that diamond holds great promise as a semiconductor substrate because of its very high thermal conductivity, five times that of copper; a breakdown field 30 times that of silicon; hole and electron mobilities several times better than silicon; and its intrinsic radiation hardness. However, nitrogen, the most common impurity in diamond, acts as an unusable deep (1.7eV) donor and, at an IEE seminar on 'Advances in Carbon Electronics', doubts were expressed that the technical barriers facing those involved in the formulation of device-grade synthetic diamond substrates would ever be breached. It must, therefore, have given Daniel Twitchen, of De Beers Industrial Diamonds, now renamed Element Six, exceptional pleasure to announce the successful demonstration of prototype devices at the same event. The team, based at Ascot, has for some time, been producing impressive synthetic diamond windows, up to 100mm across, for high-power microwave and laser sources. The current development involves growing a single crystal from a seed in a hydrogen-methane microwave-heated plasma, at more than 2,000°C. After synthesis, the epitaxial overlayers are separated from the substrate by a laser, and doping with boron from diborane creates the semiconductor properties. The end result being very efficient diodes able to block up to 2.5kV. The aim of the programme, being undertaken in conjunction with ABB, is to produce diodes capable of passing or blocking 100A at 20kV. Commercially available devices based on diamond substrates include ultra violet sensors manufactured by Richard Jackman and his colleagues in the Diamond Electronics Group based at University College, London (UCL). The large band gap that hinders diamond's use as a conventional semiconductor is put to use here as only photons of ultraviolet wavelength are sufficiently energetic to make the surface hydrogenated material photoconduct. The UCL sensors use lower cost polycrystalline material, but the interdigitated grid of gold electrodes are printed sufficiently close together that, in most cases, only single crystallites bridge the gaps, making the material effectively single crystalline. As commercial devices come to market, we can expect more and more technical 'tricks' being employed to make good use of this amazing electronic material. Element Six Daniel Twitchen at Element Six Dr Richard Jackman Pointers High power electronic devices, based on single-crystal synthetic diamond are expected to save large amounts of electrical energy because of their superior efficiency Prototype devices have already been fabricated and demonstrated Polycrystalline substrate diamond devices are already in commercial use as sensors, especially for ultraviolet use