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PhD defence by Paula Diaz Reigosa on Short-Circuit Instabilities in Silicon IGBTs and Silicon Carbide MOSFETs

Time

21.09.2017 kl. 13.30 - 16.00

Description

Paula Diaz Reigosa, Department of Energy Technology, will defend the thesis "Short-Circuit Instabilities in Silicon IGBTs and Silicon Carbide MOSFETs".

TITLE

Short-Circuit Instabilities in Silicon IGBTs and Silicon Carbide MOSFETs

PHD DEFENDANT

Paula Diaz Reigosa

SUPERVISOR

Professor Francesco Iannuzzo

MODERATOR

Associate Professor Huai Wang 

OPPONENTS

Professor Kjeld Pedersen, Dept. of Physics and Nanotechnology, Aalborg University (Chairman)
Professor Ichiro Omura, Kyushu Institute of Technology, Japan
Dr. Caroline Andersson, ABB Corporate Research, Switzerland

ABSTRACT

Power semiconductor devices are exposed to different type of stresses over their
lifetime, which they need to overcome in order to guarantee long-term reliable operation
up to 20 years or more. One of the most typical stresses that the device must withstand
is related to short-circuit events, which occur randomly during the component’s life.
Silicon-based IGBTs are good candidates for limiting the external current in case of
a short-circuit event, however their robustness is usually limited due to instabilities.
In this Ph.D. thesis, the short-circuit performance of silicon-based IGBTs has mainly
been evaluated, but since Wide-Band Gap (WBG) devices, such as SiC MOSFETs, are
rapidly growing as a potential substitute of silicon-based technologies, its robustness
with respect to short circuit is also addressed.

One of the most important experimental findings, and also the main motivation of
this thesis, is that IGBTs exposed under specific operational conditions suffer from high
frequency gate voltage oscillations (i.e., tens of MHz). Such oscillations are very critical
in case that they become unstable, which will cause the catastrophic damage of the
device. This failure type cannot be explained by any known short-circuit failure mechanism
reported in the literature. Therefore, the aim was to understand the destruction
mechanism that limits the short-circuit capability of the investigated IGBTs. This was
achieved by using a combination of finite-element device simulations and experimental
investigations of 1.2 kV, 1.7 kV, 3.3 kV, IGBTs with different technologies (i.e., planar,
trench and BIGT).

The experimental results demonstrate that the short-circuit ruggedness strongly depends
on the applied DC-link voltage, this means that at low DC-link voltages the
oscillations always occur, but at high DC-link voltages oscillations may not observed.
A sensitivity analysis on the oscillating behavior’s dependence revealed that there are
some factors which help to minimize the oscillations: low gate-emitter voltage, high
temperature and reduced stray inductance.

The root cause of the oscillation mechanism has been found to be as a consequence
of the input capacitance behaving as a time-varying element, leading to an amplification
mechanism involving the external circuit. As a major achievement of this work, it has
been possible to correlate the electric field distortions to gate capacitance variations, 
and thus, associate the capacitance variation with charge-storage effects occurring at
the surface of the IGBT. The analysis has demonstrated that the primary cause for
the excess electron density at the surface of the IGBT is the weak electric field in this
region, driven by the Kirk Effect. The carrier drift velocities have a strong impact on
the charge balance of the IGBT, especially at low DC-link voltages. Therefore, the low
drift velocities will cause electron accumulation effects at the surface of the IGBT due
to the weak electric field in this region. On the other hand, at high DC-link voltages,
the carrier drift velocities become saturated across the whole n-base, which means that
the charge-storage effect is no longer present and the input capacitance becomes fixed.

DOWNLOAD AS A PDF

PhD defence by Paula Diaz Reigosa on Short-Circuit Instabilities in Silicon IGBTs and Silicon Carbide MOSFETs

ALL ARE WELCOME. THE DEFENCE WILL BE IN ENGLISH.

AFTER THE DEFENCE THERE WILL BE AN INFORMAL RECEPTION AT PONTOPPIDANSTRAEDE 111 IN TNE COFFEE ROOM.

 

Host

Department of Energy Technology

Address

Pontoppidanstraede 111, auditorium

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