Analysis of the Drain Breakdown Mechanism in Thin-Film SOI MOSFET's

One important trend in recent years is the reduction of the silicon film thickness in SOI devices. As results of scaling this parameter several benefits have been obtained such as the elimination of the kink effect, the suppression of short-channel effects, improved subthreshold characteristics, the enhancement of carrier mobility, suppression of punchthrough and drain current overshoot and so on. These advantages mean that thin film SOI MOSFET's show great promise as high density and speed devices for future ULSI circuits. However, as was already seen in recent experiments, it is a common fact that thinner film SOI devices exhibit lower drain breakdown voltages than thicker film SOI MOSFET's. Because thin film SOI devices seem to quite resistant to minority carrier accumulation and also suitable to the reduction of the peak electric field near the drain as had been seen in a lot of previous simulation works for understanding the kink elimination or the suppression of drain-current overshoot, it is difficult to explain the drain breakdown mechanism using the so-called parasitic bipolar action just like that in a bulk Si MOSFET directly. For a long time, there were no better models (especially analytical models)explaining the drain breakdown mechanism of thin film SOI devices. Lately, Yoshimi et al. suggested that the drain structure, rather than source structure, would play a major role in determining the drain breakdown voltage and that holes could accumulate near the source in the thin film devices at a higher drain bias. Although this is a reasonable explanation to the drain breakdown mechanism up to now, a more clear description of the physical process of the drain breakdown and a comprehensive analytical model will be needed for the analysis and design of SOI devices and circuits. In this paper, the drain breakdown mechanism in thin film SOI devices has been analyzed in detail. The objectives of our work are to study the physical factor of determining the drain breakdown voltage and get a clear analytical model for understanding the drain breakdown phenomena. With the proposed analytical model, which takes into account the dependence of the drain breakdown voltage on biases and geometries such as film thickness and channel length of a SOI device, we can explain the breakdown phenomena of SOI MOSFET's and understand the relation between the kink effect and drain breakdown very well. With a quasi-two-dimensional method, we have investigated the distribution of the lateral electric field near the drain region. It can be seen that the lateral electric field strength and the impact ionization factor M, which are the dominant factors influencing the impact ionization process, increase significantly with the decrease of SOI film thickness. Because the generation rate of minority carriers increases, rather than decreases, with the reduction of the film thickness of a SOI device, we suggested that a lot of minority