Annual Program Plan—Year One—for
Physics-Based Modeling and Characterization of Wide
Bandgap Semiconductor Devices
Background
The development of high-temperature, high-power, high-voltage devices using SiC in
place of Si is very promising. However, before such devices can be successfully fielded,
a number of important issues must be addressed. For example, in SiC MOSFETs, less
than optimal channel mobility results from high levels of interface traps and nearinterfacial fixed oxide traps, as well as surface roughness. The threshold voltage is not
optimized due to charge trapping in these states. Transient operation of these solid state
switches is also affected by these traps. Time-dependent wear-out mechanisms of the
gate oxide have not been determined. Physics based device modeling using numerical
simulators can confirm physical models of device operation and provide insight into
effects unknowable from the terminal characteristics. This information can in turn be fed
to device designers and manufacturers to produce a better device.
Scope of Effort
This effort focuses on the physics-based modeling of wide band-gap (WBG)
semiconductor electronics devices, in particular those made with silicon carbide (SiC), as
well as their characterization. In general, the modeling work will be done at the
University of Maryland (UMD) and the device characterization will be performed at
ARL; but some modeling may be done at ARL under the guidance of Professor
Goldsman, and some UMD students may perform device characterization at ARL under
the guidance of ARL scientists. All WBG devices of joint interest may be characterized
and modeled under this agreement. Specifically, this includes the characterization and
modeling of 4H-SiC MOSFETs, both lateral test structures and power devices rated for
1,200 V and 5 A.
Timeline / Deliverables
The following schedule gives an estimate of the work undertaken by UMD and ARL
during the course of this agreement:
Year One (August 16, 2004 to August 15, 2005)
1. ARL will characterize, as a function of temperature and applied voltage, 4H-SiC
lateral MOSFETs provided by Cree from SiC MOSFET fabrication runs under a
separate cooperative agreement. This device characterization will include charge
pumping, I-V, C-V, and conductance measurements. ARL and Cree will compare
results, specifically mobility and interface trap density values, obtained from device
characterization of these 4H SiC MOSFETs. In addition, ARL will perform
preliminary reliability measurements on these devices, including threshold voltage
stability measurements as a function of applied bias stress, time, and temperature.
ARL will attempt to separate the effects due to interface trapped charge and to fixed
charge in the SiC MOSFET oxide layer. ARL will provide device characterization
results to UMD, including ID-VGS and ID-VDS data, extracted threshold voltage and
channel mobility, and interface trap density as a function of band-gap energy. ARL
will also provide device design information, including device geometry and material
parameters such as doping density.
2. UMD will perform a study of low-field transport in the inversion layer channel of a
4H-SiC n-channel MOSFET. This work will emphasize the effects of quantum
confinement at the SiC/SiO2 interface, whereby the two low-lying conduction bands
are split into a number of subbands. Quantum effects will also be included in
calculating the carrier concentrations, the scattering rates, and screening effects. The
rates of scattering, due to charge trapped at interface traps and near-interfacial oxide
traps, surface roughness, and acoustic phonons, will be developed for quasi-2D and
will account for the screening of mobile charge carriers in the inversion layer and
will be solved in a self-consistent manner. The dependence of inversion-layer carrier
transport properties, such as mobility, will then be investigated as a function of both
temperature and trapped charge (both at the interface and in the near-interfacial
oxide region). The modeling results will be compared with data provided by ARL in
Item # 1, and will then be incorporated into the 4H-SiC MOSFET device simulator at
UMD.
3. UMD will begin a temperature-dependent study of high-field transport in the inversion
layer channel of a 4H-SiC n-channel MOSFET using UMD’s full-band Monte-Carlo
simulator. Scattering rates due to the following will be considered, including
acoustic phonons, polar optical phonons, interface-trap charge, near-interfacial oxide
trapped charge, and surface roughness. ARL will provide trapped charge
information obtained in Item # 1, along with surface roughness information obtained
from a separate ARL cooperative agreement with Cree. The output of this work,
providing important information regarding the physical mechanisms affecting
inversion-channel current flow and saturation will be incorporated into the 4H-SiC
MOSFET device simulator at UMD.
4. UMD will begin the modeling of the 4H-SiC power DMOSFET, using the UMD 4HSiC MOSFET device simulator as a starting point. This work will require
transforming the 2D MOSFET code, which is based largely on surface transport, to
surface and bulk codes reflecting DMOSFET geometries. ARL will provide UMD
with device design information, including device geometries, dimensions, and
doping profiles, where available.
5. UMD will design Si test-structures that can be used as benchmarks for SiC chargepumping and C-V measurements performed at ARL under Item #1. The teststructures will consist mainly of various-sized MOSFETs and capacitors, and laid out
using the Cadence tools set. The designs will be submitted to MOSIS for fabrication.