Trade-off for optimizing drift region of diamond power devices
Résumé
In power devices such as Schottky Barrier Diode or Field Effect Transistor, the breakdown voltage is linked to the design of the drift layer but also to the physical properties of the material used. Diamond with its high critical electric field due to its large band gap, opens the way to unipolar power components able to withstand very high voltage with outstanding figures of merit [1]. Nevertheless, a particular attention has to be paid to the design of the drift layer to take benefit of these superlative properties. Indeed, the drift region thickness, doping level and consequently the punch through or non-punch through designs must be well designed to optimize the ON state resistance for any breakdown voltage (OFF state). Here, a focus on the optimization of the Ron.S(BV) figure of merit has been carried out, while optimizing the drift layer. Based on the ionization integral calculation with impact ionization coefficients adapted to diamond [2], we performed an accurate analysis of the reciprocal punch through factor as function of the breakdown voltage. This theoretical study has been first applied in a one dimensional semi-analytical approach of the breakdown voltage [3] and point out that optimized drift layer showed a Ron.S improvement up to 55% for a specified breakdown voltage. An additional two dimensional cylindrical coordinate analysis was performed to quantify the radius effect on the breakdown voltage value for different drift region designs. These simulation results were applied to experimental devices from literature to show how some trade off can improve their performances.
This work offer preliminary design rules to fabricate more efficient unipolar diamond power devices. This analysis also points out that thicknesses and doping levels required to achieve such structures are quite challenging for crystal growth in the context of high voltage power devices.