Finfet Device - Fin thickness vs Channel length
In MOSFET one wants the gate electric field to control the channel and drain electric field should have a lesser effect on the channel. Otherwise it will not work as expected and will show a set of unwanted effects called Short-channel Effects. 

In order to reduce the effect of drain field on channel, the gate length (or channel length) is usually made (45x) longer than oxide thickness for a planar structure. In case of a FinFET, the gate electric field is decided by the thickness of silicon fin (Tsi) and hence it is made smaller than the channel length or gate length (Lg).