Why is the body of FD-SOI FETs undoped?
The reason for not doping is not really because the body is &quot;thin&quot;, but rather, it's because the body is isolated from other devices on the same chip by the insulating oxide layer.
In non-SOI devices, the channel is formed by inverting the carrier population of the doped body, and the channel is isolated from the substrate by the reverse-biased junction formed by that. For example, in an N-channel enhancement-mode MOSFET, the body is P-doped, with an excess of holes. The channel doesn't form until the gate is brought positive enough with respect to the substrate to attract sufficient electrons immediately under it to bridge the gap between the source and the drain. Between the channel and the rest of the P-doped body, there is an &quot;intrinsic&quot; region that is devoid of charge carriers, isolating it from the substrate.
In the SOI devices, the body is already permanently isolated, so its doping, if any, is strictly used to control the specific characteristics of that transistor.