Monolithic FETs including a channel region in a first semiconductor material disposed over a substrate. While a mask, such as a gate stack or sacrificial gate stack, is covering a channel region, a semiconductor spacer of a semiconductor material with a band offset relative to the channel material is grown, for example on at least a drain end of the channel region to introduce at least one charge carrier-blocking band offset between the channel semiconductor and a drain region of a third III-V semiconductor material. In some N-type transistor embodiments, the carrier-blocking band offset is a conduction band offset of at least 0.1 eV. A wider band gap and/or a blocking conduction band offset may contribute to reduced gate induced drain leakage (GIDL). Source/drain regions couple electrically to the channel region through the semiconductor spacer, which may be substantially undoped (i.e. intrinsic) or doped. In some embodiments, the semiconductor spacer growth is integrated into a gate-last, source/drain regrowth finFET fabrication process.