We have studied channel thickness dependence of InGaAs quantum-well field-effect transistors (QWFETs) with high- gate dielectrics. Device performances of ultrathin 5- and 10-nm-channel QWFETs with gate length down to sub-50-nm regime have been investigated. Thinning down the channel improves subthreshold characteristics and reduces the short-channel effect. The 5-nm channel devices exhibit a reduced subthreshold swing (SS) of around 100 mV/dec and drain-induced barrier lowering (DIBL) of 128 mV/V compared to 10-nm channel devices (SS of 140 mV/dec and DIBL of 275 mV/V). However, the drawback for thinner channel devices is that the effective channel mobility also decreases. At inversion charge density of , 10-nm channel devices exhibit mobility of 1860 versus mobility of 1460 for 5-nm channel devices.