The Design, Fabrication, and Characterization of Waffle-substrate-based n-channel IGBTs in 4H-SiC
thesisposted on 27.07.2021, 00:20 by Md monzurul AlamMd monzurul Alam
Power semiconductor devices play an important role in many areas, including household
appliances, electric vehicles, high speed trains, electric power stations, and renewable energy
conversion. In the modern era, silicon based devices have dominated the semiconductor
market, including power electronics, because of their low cost and high performance. The
applications of devices rated 600 V - 6.5 kV are still dominated by silicon devices, but they
are nearly reaching fundamental material limits. New wide band gap materials such as silicon
carbide (SiC) offer significant performance improvements due to superior material properties
for such applications in and beyond this voltage range. 4H-SiC is a strong candidate
among other wide band gap materials because of its high critical electric field, high thermal
conductivity, compatibility with silicon processing techniques, and the availability of high
quality conductive substrates.
Vertical DMOSFETs and insulated gate bipolar transistors (IGBT) are key devices for
high voltage applications. High blocking voltages require thick drift regions with very light
doping, leading to specific on-resistance (RON,SP ) that increases with the square of blocking
voltage (VBR). In theory, superjunction drift regions could provide a solution because of a
linear dependence of RON,SP on VBR when charge balance between the pillars is achieved
through extremely tight process control. In this thesis, we have concluded that superjunction
devices inevitably have at least some level of charge imbalance which leads to a quadratic
relationship between VBR and RON,SP . We then proposed an optimization methodology to
achieve improved performance in the presence of this inevitable imbalance.
On the other hand, an IGBT combines the benefits of a conductivity modulated drift
region for significantly reduced specific on-resistance with the voltage controlled input of a
MOSFET. Silicon carbide n-channel IGBTs would have lower conduction losses than equivalent
DMOSFETs beyond 6.5 kV, but traditionally have not been feasible below 15 kV. This
is due to the fact that the n+ substrate must be removed to access the p+ collector of the
IGBT, and devices below 15 kV have drift layers too thin to be mechanically self-supporting.
In this thesis, we have demonstrated the world’s first functional 10 kV class n-IGBT with
a waffle substrate through simulation, process development, fabrication and characterization.
The waffle substrate would provide the required mechanical support for this class of devices.
The fabricated IGBT has exhibited a differential RON,SP of 160 mohm
.cm2, less than half of
what would be expected without conductivity modulation. An extensive fabrication process
development for integrating a waffle substrate into an active IGBT structure is described
in this thesis. This process enables an entirely new class of moderate voltage SiC IGBTs,
opening up new applications for SiC power devices.