Diamond, an Ultra Wide Bandgap Semiconductor—Challenges for Power and RF Electronic Applications

Great strides in diamond wafer technology and diamond epitaxy have inspired new concepts for diamond electronics particularly for power conversion and RF applications. For diamond, the ultra wide bandgap supports high fields, the high electron and hole mobilities support low resistance and bipolar current transport, and the highest bulk thermal conductivity enables high power applications. A specific example includes PIN diodes that demonstrate current density greater than 100 kA/cm2 and high frequency operation in receiver protect circuits.<br/>However, the relatively high activation energy of substitutional p- and n-type dopants in diamond has limited the development of diodes and field effect transistors. This presentation highlights two approaches to mitigate this limitation. At the center of this research is growth of high purity, epitaxial diamond layers by plasma enhanced CVD.<br/>The first approach considers diodes that operate much like a vacuum tube where injected carriers drift at the saturation velocity in the applied field. The current transport, which is described by the Mott Gurney expression, is considered as space charge limited current. High current PIN diodes prepared with epitaxial intrinsic and n-type (phosphorus doped) layers are described. An updated figure of merit for diamond power diodes considering this effect is discussed.<br/>Another alternative to substitutional impurity doping is interface charge transfer at a diamond-dielectric interface. Optimized configurations result in the formation of a hole accumulation layer, which is not limited by thermal activation. However, the hole transport shows a mobility that is much lower than predicted. It is widely accepted that the low mobility is due to scattering from the near interface negative charges transferred into the dielectric layer.<br/>Following the concept of modulation doping at heterostructure interfaces, we have proposed and demonstrated a dielectric layer configuration that results in a nearly ten-fold mobility increase for the accumulated holes at the diamond interface. In this approach MoO₃ is used as the charge transfer dielectric, and Al₂O₃ is employed as the modulation doping spacer layer. The charge transfer is driven by the energy difference between the diamond valence band and the charge transfer states in the MoO₃. The relative distribution of the charge near the interface is deduced from photoemission spectroscopy. The thickness of the spacer layer is shown to affect the hole accumulation layer.<br/>The advantages and limitations of space charge limited current and interface charge transfer doping will be presented. New approaches including photo-enhanced doping will also be described.<br/>Research supported by the U.S. Department of Energy (DOE) Office of Science, under Award No. DE-SC0021230 and the NSF through grant DMR-2003567.