  We present a novel scheme to engineer the entanglement in a fermionic system,
which is modeled by a minimally three-site Hubbard model. It is found that, in
this type of system, we can have two free parameters. One is used to tune the
entanglement and the other to switch on or off the above tuning function. The
whole process is much like what happens in a traditional transistor switch,
where the main current between the emitter and collector can be turned on or
off by the base current.
