This article presents a 100 mm diameter, flat, silicon gradient index (GRIN) lens with high resistivity silicon combined with a deep reactive-ion-etched (DRIE) three-layer antireflection (AR) structure for 160-355 GHz applications. The patterning of the silicon wafer with subwavelength posts or holes changes the refractive index to create AR layers and a radial index gradient. These subwavelength structures are nonresonant and achromatic. Structures that vary in size and distance from the optical axis create a parabolic index profile. DRIE was proven to be viable at THz frequencies by this work, and cleaning and chemical activation of the wafers combined with wafer bonding to eliminate air gaps was important to significantly improve transmittance.