Analytical and Optimization Based Modeling Techniques to Assess the Performance of Submicron SiC MESFETs.
In the first part of the thesis, a detailed analytical mathematical model describing I − V characteristic of a submicron SiC MESFET has been presented. Poisson’s equation with appropriate boundary conditions is solved to determine potential distribution inside the channel. The location of Schottky barrier gate with corresponding depletion layer width where the carrier’s velocity gets saturated is evaluated. It has been demonstrated that the depletion modification underneath the Schottky barrier gate causes finite output conductance in the saturation region of operation. I − V characteristics of submicron SiC MESFET have been modeled and compared with conventional velocity saturation techniques, where the depletion layer after the onset of current saturation has been treated as constant. It is noted that the proposed depletion layer modification technique, when incorporated in the device modeling, gave ∼15.9% improvement in the modeled out characteristics of a submicron SiC MESFET.
In the second part, an improved empirical model has been presented to simulate DC and pulsed I − V characteristics of SiC MESFETs. A comparative analysis has been carried out by employing swarm optimization technique and it has been established that the proposed model, dependent upon the device characteristics, is ∼7-24% better than its counterparts. Based on simulated characteristics, numerous parameters defining the device geometrical structure have been estimated to a good degree of accuracy. It has been shown that the developed technique is versatile enough and can be a useful tool for device simulation softwares.
In the third part, a technique has been developed to estimate intrinsic small signal parameters of submicron SiC MESFETs, designed for high power microwave applications. In the developed technique, small signal parameters are extracted by involving drain-to-source current, Ids instead of Schottky barrier depletion layer expression. It has been demonstrated that in SiC MESFETs, the depletion layer gets modified due to intense transverse electric field and/or self-heating effects, which are conventionally not taken into account. Thus, assessment of AC small-signal parameters by employing depletion layer expression loses its accuracy for devices meant for high power applications. A set of expressions for AC small signal elements have been developed using Ids and their dependence on device biasing have been discussed. The validity of the proposed technique has been demonstrated using experimental data.