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Title: Development of 4H-SiC power MOSFETs for high voltage applications
Author: Rong, Hua
ISNI:       0000 0004 5922 1019
Awarding Body: University of Warwick
Current Institution: University of Warwick
Date of Award: 2015
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Silicon carbide is a promising wide bandgap semiconductor for high-power, high-temperature and high frequency devices, owing to its high breakdown electric field strength, high thermal conductivity and ability to grow high quality SiO2 layers by thermal oxidation. Although the SiC power MOSFET (metal-oxide-semiconductor field effect transistor) is preferred as a power switch, it has suffered from low channel mobility with only single digit field effect mobility achieved using standard oxidation process (1200◦C thermal oxidation). As such, this thesis is focussed on the development of 4H-SiC MOSFETs (both lateral and vertical MOSFETs) to improve the channel mobility and breakdown characteristics of these devices. In this work, high temperature nitridation using N2O has been investigated on MOS capacitors and MOSFETs, both with gate oxides grown directly in N2O environment or in a O2 ambient followed by a N2O post-oxidation annealing process. Results have demonstrated that at high temperature (>1200◦C) there is a significant improvement in the interface trap density to as low as (1.5x10^11cm-2eV-1) and field effect channel mobility (19cm2/V.s) of 4H-SiC MOSFET compare with a lower temperature (between 800 and 1200◦C) oxidation (1x10^12cm-2eV-1 and 4cm2/V.s). Nitridation temperatures of 1300◦C was found to be the most effective method for increasing the field effect channel mobility and reducing threshold voltage. The number of working devices per sample also increased after N2O nitridation at 1300◦C as observed for both lateral and vertical MOSFETs. Other post oxidation techniques have also been investigated such as phosphorous passivation using solid SiP2O7 planar diffusion source (PDS). The peak value of the field effect mobility for 4H-SiC MOSFET after phosphorus passivation is approximately 80cm2/V.s, which is four times more than the valued obtained using high temperature N2O annealing. Different JTE structures have been designed and simulated including single-zone JTE, space modulated JTE (SMJTE) and the novel two-step mesa JTE structures. It was found that for the same doping concentration the SM two-zone JTE and SMJTE have higher breakdown voltage than the single zone JTE. With SMJTE, the device could achieve more than 90% of the ideal parallel plane voltage from simulations and 86% from the breakdown test of the fabricated devices.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: TK Electrical engineering. Electronics Nuclear engineering