A Tunnel Field-Effect Transistor (TFET) includes a source region in a semiconductor substrate, and a drain region in the semiconductor substrate. The source region and the drain region are of opposite conductivity types. The TFET further includes a gate stack over the semiconductor substrate, with the source region and the drain region extending to opposite sides of the gate stack. The gate stack includes a gate dielectric over the semiconductor substrate, and a ferroelectric layer over the gate dielectric.