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SiC MOSFETs have very large interface trap densities which degrade device
performance. The effect of traps on inversion layer mobility and inversion charge concentration has
been studied, and mobility models suitable for inclusion in Drift!Diffusion simulators have been
developed for steady state operation of SiC MOSFET devices. Here, we attempt to model the
transient behavior of SiC MOSFETs, and at the same time, extract the time constants for the filling
and emptying of interface traps. As compared to the inversion layer, interface traps in SiC
MOSFETs are slow in reacting to change in gate bias. So, at the positive edge of a gate pulse, we
see a large current in the MOSFET, which then decays slowly to the steady state value as the
interface traps fill up. We have developed a generation/recombination model for minority carriers in
a SiC MOSFET based on the Shockley!Read!Hall recombination model for electrons and holes. In
our model, the generation/recombination takes place between minority carriers in the inversion
layer, and the traps at the SiC!SiO
2
interface. Comparing our simulated current vs. time curves to
experiment, we have been able to extract time constants for the filling and emptying of traps at the
SiC!SiO
2
interface.
In steady state operation of SiC MOSFETs, occupied interface traps cause mobility degradation and
reduction in mobile charge in the inversion layer, thereby lowering current and degrading device
performance [1!4]. In MOSFET switching applications, the rate at which the interface traps can be
filled and emptied will affect the transient response of the device and also pose stability concerns.
Here, we describe how we characterize the transient behavior of SiC MOSFETs, and discuss how to
extract time constants for the filling/emptying of interface traps. Our method combines
measurements with detailed numerical device modeling. We introduce a generation!recombination
model for carriers in the semiconductor that can occupy the interface traps, and thereby obtain a
time dependent trap occupation model. These are incorporated in our drift diffusion simulator for
SiC MOSFETs [5]. Simulated transient characteristics are compared with experiment to extract trap
physics.
The occupation of traps at the interface of a SiC MOSFET can be thought of as recombination of a
mobile carrier in the semiconductor with an empty trap state at the interface. Similarly, emission of
an electron from a trap will cause a generation event in the semiconductor. Thus, this generation!
recombination process is a single carrier process. In the case of filling/emptying of the interface
traps, there is a net shift in charge from the semiconductor to the interface, or vice!versa. An initial
net non!zero generation!recombination rate takes a finite time to become zero and reach steady
state, giving a constant occupied trap density and mobile charge concentration.
Materials Science Forum Vols. 556-557 (2007) pp 847-850
online at http://www.scientific.net
© (2007) Trans Tech Publications, Switzerland
Online available since 2007/Sep/15
All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of the
publisher: Trans Tech Publications Ltd, Switzerland, www.ttp.net. (ID: 72.66.90.198-15/06/09,21:50:07)