Prospects of bipolar diamond devices

Abstract

The prospects of diamond bipolar devices are analysed theoretically and experimentally in respect to the problem of deep doping, especially the deep donor in diamond. For this purpose a set of p– n– p bipolar junction transistors (BJTs) is fabricated on p-type diamond substrates by epitaxial growth using boron ( E A=0.4 eV) and nitrogen ( E D=1.7 eV) as the p- and n-type dopants respectively. It is shown that at the boron/nitrogen junction a p– n junction is formed. The built-in potential of the junction is determined by the ionised boron/nitrogen impurities. The specific features of the fabricated devices are the high resistivity of the nitrogen doped base (10 GΩ·cm at 20°C) and a significant leakage current of the reverse biased p– n junctions. These factors limit the transistor action to d.c.-operation in the nA-current range and to temperatures below 200°C where leakage starts to dominate. The values of the static current gain I C/ I B are measured in the common base mode 200 and in the common emitter mode 1.1. The theoretical section of the paper deals with the calculation of the static current gain of diamond pnp transistor structures in dependence of the donor energy level, temperature and frequency. Both the theoretical and the experimental results indicate that diamond bipolar transistors with a nitrogen doped n-type base can exhibit a current gain β of up to 30,000 in the d.c.-regime provided the leakage of the p– n junctions is sufficiently low. High-gain diamond transistors operating in GHz-frequency can be expected as soon as n-doping by shallow donor like phosphorous ( E D<0.5 eV) becomes available.