| dc.description.abstract | Exploring the attractive electrical properties of the Silicon Carbide (SiC) for power devices, the
characterization and the analysis of 4H-SiC pin diodes is the main topic of this Ph.D. document. In
particular, the thesis concerns the development of an auto consistent, analytical, physics based
model, created for accurately replicating the power diodes behavior, including both on-state and
transient conditions.
At the present, the fabrication of SiC devices with the given performances is not completely
obvious because of the lack of knowledge still existing in the physical properties of the material,
especially of those related to carrier transport and of their dependences on process parameters.
Among these, one can cite the degree of doping activation, the carrier lifetime into epitaxial layers
that will be employed and the sensitivity of some physical parameters to temperature changes.
Therefore, a set of investigative tools, designed especially for SiC devices, cannot be regarded as
secondary objective. It will be useful both for process monitoring, becoming essential to the tuning
of technological processes used for the implementation of the final devices, and for a proper
diagnostics of the realized devices. Following this need, in our research activity firstly a predictive,
static analytical model, including temperature dependence, is developed. It is able to explain the
carrier transport in diffused regions as function of the injection level and turns also useful for better
understanding the influence of physical parameters, which depend in a significant way from the
processed material, on device performances. The model solves the continuity equation in double
carrier conditions, taking into account the effects due to varying doping profile of the junction, the
spatial dependence of physical parameters on both doping and injection level and the modification
of the electric field of the region with the injection regime. The model includes also the device
characterization at high temperatures to analyze the influence of thermal issues on the overall
behavior up to temperature of 250°C. The accuracy of the static model has been extensively
demonstrated by numerous comparisons with numerical results obtained by the SILVACO
commercial simulator.
Secondly, with the aim to properly account for the dynamic electrical behavior of a diode with
generic structure, the static model has been incorporated in a more general, self-consistent model,
allowing the analysis of the device behavior when it is switched from an arbitrary forward-bias
condition. In particular, the attention is focused on an abrupt variation of diode voltage due to an
instantaneous interruption of the conduction current: although this situation is notably interesting
for the study of the switching behavior of diodes, the voltage transitory is also traditionally used in
different techniques of investigation to extract more information about the mean carrier lifetime.
This occurs, for example, in the conventional Open Circuit Voltage Decay (OCVD) technique,
where the voltage decay due to the current interruption is useful for an indirect measure of minority
carrier lifetime in the epitaxial layer.
Because of its heavy dependence on processes, the carrier lifetime is an important parameter to
be monitored, especially in the case of bipolar devices, and it cannot be neglected. Due to the
existent uncertainty about this parameter in SiC epi-layers, the OCVD method reveals itself a
practical way to overcoming this limit.
In detail, by using our self-consistent model, that exploits an improved method of the traditional
OCVD technique, it is possible to characterize the carrier lifetime into 4H-SiC epitaxial layer of a
generic diode under test, obtaining the spatial distributions of the minority carrier concentration and
carrier lifetime at any injection regime. The overall model performances are compared to both
device simulations and experimental results performed on Si and 4H-SiC rectifier structures with
various physical and electrical characteristics. From the comparisons, the model results to have
good predictive capabilities for describing the spatial–temporal variation of carriers and currents
along the whole epi-layer, proving contextually the validity of the used approximations and
allowing also to resolve some ambiguities reported in the literature, such as the stated
inapplicability of the OCVD method on thick epitaxial layers, the reasons of the observed non linear
decay of the voltage with time, and the effects of junction properties on voltage transient.
Finally, with the imposition of right boundary conditions, it is possible to use the versatility of
the developed model for extending the analysis and obtaining a physical insight of any arbitrary switching condition of 4H-SiC power diodes. [edited by author] | en_US |
