Metallic trilayers are successfully used to fabricate spintronic devices. To pursue an analogue of layer-structured spintronic materials, we demonstrate that it is possible to obtain attractive magnetic properties in hole-doped Mn/Si trilayers. We found that by forming a trilayer structure, with Mn-occupying interstitial sites, and by introducing a layer of holes in between, the sample can be a half-metal. This new finding should open a viable route to grow Si-based half-metallic spintronic materials because doping Mn at interstitial sites significantly reduces the formation-energy barrier as compared with the transition metal element occupying substitutional sites. An argument is also presented that a finite width of an Mn layer may not destroy the half-metallic property. (Some figures may appear in colour only in the online journal)