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Simulation and modeling of silicon carbide devices

Simulation und Modellierung von Siliziumkarbid-Bauelementen
: Uhnevionak, Viktoryia
: Pichler, Peter; Weigel, Robert

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Erlangen, 2015, XI, 146 pp.
Erlangen-Nürnberg, Univ., Diss., 2015
URN: urn:nbn:de:bvb:29-opus4-61975
Bundesministerium für Bildung und Forschung BMBF
PICF 10; 01SF0804; MobiSiC
Dissertation, Electronic Publication
Fraunhofer IISB ()
wide-bandgap-semiconductor; silicon carbide; simulation; hall mobility; n-channel-FET; Traps

In recent years, silicon carbide (SiC) became an attractive material and opened new perspectives in power electronics due to its superior material properties. The wide bandgap, high thermal conductivity and high breakdown electric field make SiC a material of choice for power MOSFETs. The incorporation of SiC MOSFETs, for example, in power converters allows to decrease their weight and size. This can be a big advantage for many applications including electric cars. The ability of SiC devices to withstand high temperatures simplifies the thermal management of the electrical systems. However, the commercial use of MOSFETs is currently limited by technological problems which result in low channel mobility and high turn-on voltage. The aim of this thesis was to understand and explain the mechanisms which control the channel mobility in SiC MOSFETs using numerical simulation and to develop a self-consistent simulation methodology for a description of their electrical behavior. For technological progress, development and optimization of semiconductor devices, TCAD simulation became an increasingly important tool of investigation. However, for SiC devices TCAD simulation currently is a big challenge. Most of the simulation models were developed for silicon, and, thus, can not adequately describe the transport properties of SiC devices. Moreover, because of a high density of interface traps at the SiC/SiO2 interface, which strongly degrade the channel mobility of SiC MOSFETs, an accurate interface trap model is of primary importance for the simulation. In the framework of the MobiSiC (Mobility Engineering for SiC Devices) project lateral n-channel 4H-SiC MOSFETs have been fabricated and electrically characterized by current-voltage and Hall-effect measurements. The effects of temperature and bulk potential engineering upon the transport properties in the channel of SiC MOSFETs have been studied. The interpretation of the electrical measurements, i. e. current-voltage characteristics (ID(VG)) as well as sheet carrier density and channel mobility obtained from the Hall-effect measurements (ninv(VG), 𝜇 (VG)), has been performed within this work using numerical simulation with Sentaurus Device of Synopsys. For an accurate evaluation of the Hall-effect measurements, a new method for the calculation of Hall factors was developed. It is based on the fact that both Hall factor and mobility depend on the same mechanisms by which the charge carriers are scattered. The method of calculation accounts for all electron scattering mechanisms in the active area of the device. Thus, for the first time, an accurate Hall factor has been calculated for the channel of MOSFETs and applied for the correction of the Hall-effect measurements. Experimental data, for example from Hall-effect measurements, is often used to characterize the density of interface traps. In this work, a new method, which allows a more accurate characterization, is suggested. In the first step, the densities of the interface traps versus trap energy (DIT (ET)) are extracted from the Hall-effect and capacitance-voltage measurements using a conventional method. Afterwards, the extracted DIT (ET) distributions are introduced into Sentaurus Device and optimized numerically to minimize the deviations between the numerically simulated characteristics (ninv(VG), 𝜇 (VG), and ID (VG)) and the experimentally measured data. The numerical simulation allows to take into account, for instance, the effects of potential drop along the channel between source and drain as well as the Fermi-Dirac distribution of the electrons. These effects are neglected when the interface trap density is extracted conventionally from the experimental data. Thus, it is expected that the new method produces physically more reasonable results on the DIT (ET) distributions. Based on the experimental and simulated results, origin and nature of the interface defects are discussed. The simulation methodology, in which the method of the Hall factor calculation and the method of the DIT (ET) extraction are accounted for, could consistently describe the temperature dependence as well as the doping dependence of the transport properties of SiC MOSFETs studied in this thesis. On the basis of a good agreement between simulations and measurements, a comprehensive interpretation of the scattering mechanisms in the channel of SiC MOSFETs with different doping concentrations and at different temperatures has been performed. One of the main findings from this work is that a decrease of the interface trap density is not the only factor which can improve the performance of SiC MOSFETs. For example, their performance can be improved significantly by decreasing the doping concentration of the channel. It was also found that the doping concentration of the channel affects the temperature dependence of the channel mobility: At elevated temperatures for highly doped MOSFETs it increases with increasing temperature while for lowly doped ones it decreases.