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Modeling of the impurity-gradient effect in high-voltage MOSFETs

Y. Maekawa, K. Fukushima, A. Tanaka, H. Kikuchihara, M. Miyake, H.J. Mattausch, M. Miura-Mattausch
Hiroshima University, JP

Keywords: LDMOS, surface-potential model, impurity gradient, capacitance


Two major high-voltage MOSFET types are commonly used by semiconductor industries. One has a symmetrical structure, and the other has a laterally-diffused asymmetric structure called LDMOS. Here our focus is the LDMOS structure, where an impurity gradient from the source to the drain is present. We have considered the impurity gradient along the channel explicitly in the solution of the Poisson equation. The result shows that the impurity gradient makes the potential distribution along the channel steeper than for a uniform impurity concentration. This steeper potential increase causes additional charge storage, which induces additional capacitances in comparison to the case with homogeneous impurity concentration. Consequently, we can demonstrate that the anomalous spikes of LDMOS capacitances are only enhanced by the impurity gradient, but that they are originally caused by the high resistive drift region at the drain side. The spike enhancement due to the impurity gradient is drastically reduced for increasing the drain voltage.
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