Conventional silicon on insulator (SOI) MOSFETs are operated with a floating potential in the substrate. This results in different electrical characteristics compared to standard MOSFETs which are fabricated using bulk technology. The substrate potential in standard MOSFETs is controlled via a body contact. In conventional SOI MOSFETs, however, it is determined by the bias condition.
The sensitivity with respect to the drain current is crucial . In short-channel MOSFETs with a high channel doping the electric field in the pinch-off region can get very large even for moderate drain bias. When the electric field is large enough impact ionization arises. The electrons generated by impact ionization move to the drain whereas the generated holes move to the substrate and rise the body potential. When the body potential increases, the threshold voltage is reduced due to the body effect [149,150] thus resulting in a positive feedback which increases the channel current.
The body effect creates an undesired kink in the output characteristics as shown in Fig. 4.12. When a contact is added to the substrate the onset of impact-ionization becomes clearly visible in the substrate current as shown in Fig. 4.13. Consideration of self-heating slightly reduces the strength of this effect. One can clearly see the onset of impact ionization. For comparison the continuous line shows a simulation result without impact ionization.