gate sidewall spacers are created by a two - step procedure in fabricating a field - effect transistor using a protective material such as silicon nitride to prevent gate - electrode oxidation . in the first step , a layer of insulating material is conformally deposited and then substantially removed except for small spacer portions adjoining the sidewalls of a doped non - monocrystalline semiconductor layer destined to become the gate electrode . the second step consists of performing an oxidizing heat treatment to increase the thickness of the spacer portions . no significant gate dielectric encroachment occurs . also , the spacers achieve a profile that substantially avoids electrical shorts .