4.2 Body and Bipolar Effect

Drift-diffusion simulations show a remarkable influence of impact-ionization on the drain current (Fig. 4.2).

Figure 4.2: Output characteristics of the SOI (Device 1) obtained by drift-diffusion simulations with and without impact-ionization.

The increase of the drain current can be partially attributed to the kink-effect [58]: the holes generated by impact-ionization are drawn into the floating body where they raise the potential. Fig. 4.3 shows the lateral potential distribution in the middle of the silicon film.

Figure 4.3: Distributed potential of the SOI (Device 1) obtained by drift-diffusion simulations with impact-ionization turned on. The cutline through the device is located at a depth of $y = 100 \hspace {.35ex} \textrm {nm}$.

The increased body potential leads via the body effect to an increased drain current. The second contribution to the current increase is due to the bipolar effect. The increased body potential acts as a forward bias to the source-body diode. Electrons are injected from source to the body, diffuse through the body, and are collected by the drain.

Simulating the device without impact-ionization yields a comparatively small shift in the body potential as shown in Fig. 4.4. In this simulation condition the kink in the output characteristic does not appear.

Figure 4.4: Distributed potential of the SOI (Device 1) obtained by drift-diffusion simulations with impact-ionization turned off. The cutline through the device is located at a depth of $y = 100 \hspace {.35ex} \textrm {nm}$.

M. Gritsch: Numerical Modeling of Silicon-on-Insulator MOSFETs PDF