Diamond Transistors
Diamond Microwave Devices Limited has substantial legacy experience in the development of diamond as a semiconductor, and holds several patents related to diamond transistor structures.
Key findings from our diamond transistor R&D were published in the Journal of Applied Physics* detailing our work on boron delta-doped CVD diamond structures with surface delta layers <1nm thick. Hall-effect measurements indicated a substantial Hall mobility of up to 900 cm²/Vs, which would normally be indicative of material highly suitable for transistor action.
However, we also showed that the established analysis of Hall data using a single homogeneously doped layer can result in a dramatic over-estimate of mobility. The conductivity mobility is the important measure for most device applications and despite the high measured values of Hall mobility, the room temperature conductivity mobility was shown to be much lower in the range 1 – 4 cm²/Vs. Field-effect mobility extracted from our diamond FET structures was also in close agreement with the conductivity mobility.
This experimental work was consistent with our modeling which even for optimistic scenarios predicted a room temperature conductivity mobility of <40 cm²/Vs. Our work strongly indicates that to achieve high conductivity mobility in a boron doped surface delta structure on diamond and therefore achieve a practical diamond transistor, a high degree of carrier excitation is required from the lowest sub-band into 3D bulk states. The challenge is to achieve this at room temperature.
*Transport Behavior of Holes in Boron Delta-Doped Diamond Structures, R. S. Balmer, I. Friel, S. Hepplestone, J. Isberg, M. J. Uren, M. L. Markham, N. L. Palmer, J. Pilkington, P. Huggett, S. Majdi, and R. Lang, J. Appl. Phys., 113, (2013)
Technical Papers
Transport behaviour of holes in boron delta-doped diamond structures
R S Balmer, I Friel, S Hepplestone, J Isberg, M J Uren, M L Markham, N L Palmer, J Pilkington, P Huggett, S Majdi, and R Lang
J. Appl. Phys., 113, (2013)
Boron delta-doped diamond structures have been synthesized using microwave plasma chemical vapor deposition and fabricated into FET and gated Hall bar devices for assessment of the electrical characteristics.
A detailed study of variable temperature Hall, conductivity, and field-effect mobility measurements was completed. This was supported by Schrödinger-Poisson and relaxation time calculations based upon application of Fermi’s golden rule. A two carrier-type model was developed with an activation energy of ∼0.2 eV between the delta layer lowest subband with mobility ∼1 cm²/Vs and the bulk valence band with high mobility.
This new understanding of the transport of holes in such boron delta-doped structures has shown that although Hall mobility as high as 900 cm²/Vs was measured at room temperature, this dramatically overstates the actual useful performance of the device.
© 2013 American Institute of Physics
An impedance spectroscopic investigation of the electrical properties of delta-doped diamond structure
N Tumilty, J Welch, R Lang, C Wort, R Balmer, and R B Jackman
J. Appl. Phys. 106, (2009)
Impedance spectroscopy has been used to investigate the conductivity displayed by diamond doped with boron in an intrinsic-∂-layer-intrinsic multilayer system with differing ∂-layer thicknesses. Carrier transport within 5nm ∂-layer structures is complex, being dominated by conduction in the interfacial regions between the ∂-layer and the intrinsic regions, as well as conduction within the ∂-layer itself. In the case of 3.2 nm thick ∂-layers the situation appears improved with uncapped samples supporting only two conduction paths, one of which may be associated with transport outside the ∂-layer, the other low transport within the ∂-layer complex diamond structures. Introduction of the capping layer creates a third conduction within the ∂-layer associated with unwanted boron in the capping layer-∂-layer interface.
© 2009 American Institute of Physics
CVD Diamond MESFETs - technology and applications
R Lang, C Wort, R Balmer, I Friel, G Scarsbrook
2nd International Industrial Diamond Conference, 2007
Current semiconducting materials (Si and GaAs) are limited in the power (or power density) that they can offer for the next generation of high-power microwave sub-systems. Such systems are likely to require compact, low-mass, low-cost, reliable solutions for a range of applications that, perhaps, to date have been dominated by Travelling Wave Tubes (TWTs). Wide band gap materials such as GaN, SiC and diamond all offer substantial potential for such RF devices, with diamond having by far the best combination of material characteristics.