Body self bias in fully depleted and non-fully depleted SOI devices

One major advantage in SOI processing is the ease of device isolation. By using mesa etching or field oxidation the silicon film can be selectively removed, thereby electrically isolating the bodies of SOI devices from one another. Each body can be individually biased or in most cases, left floating. However there are anomalies associated with a floating body, namely threshold voltage shifting, kink in the current characteristics, and degradation of output resistance in the saturation region. Such anomalies arise because the floating body potential changes with the external biases, which in turn changes the threshold voltage. The amount of body potential shifting is also dependent on the technology and the material qualities. To eliminate these anomalies different schemes have been proposed to bias the body. These schemes add to the process complexity and increase the device area. It is also unclear if such schemes are effective in biasing the body of a fully depleted device, in which case the presence of a quasi-neutral body is lacking. In this paper we have developed a model to predict the body potentials of both Fully Depleted (FD) and Non-Fully Depleted (NE) SOI devices. We found that contrary to general belief the kink anomaly also occurs in FD SOI. However the kink effect diminishes as the film thickness is reduced. For thin silicon film SOI devices, output resistance for floating body long channel SOI devices can be larger than 1 M/spl Omega/-/spl mu/m.