A method for making a microelectronic device comprising, on a same substrate (100, 200), at least one electro-mechanical component provided with a mobile structure of a monocrystalline semi-conductor material and means for actuating and/or detecting the mobile structure, and with at least one transistor, the method comprising the following steps of: a) providing a substrate provided with at least a first semi-conducting layer including at least one region in which a channel area of said transistor is provided, - forming an insulating layer onto the substrate, the insulating layer (106, 206) being placed on said first semi-conducting layer, - making at least one first opening in said insulating layer uncovering the first semi-conducting layer, - removing an area of said insulating layer to form a second opening in said insulating layer, said second opening uncovering said region in which a channel area of said transistor is provided, - forming a gate dielectric area in said second opening, - forming at least a semi-conducting sacrificial area by epitaxy growth so as to form a monocrystalline semi-conductor material at the bottom of the first opening, and then, - forming said second semi-conducting layer by growth, so as to form a monocrystalline semi-conductor material on said semi-conducting sacrificial area and a polycrystalline semi-conductor material area on said gate dielectric area, b) etching the second semi-conducting layer (112, 212) so as to form at least one pattern of said mobile structure of said component in an area of monocrystalline semi-conductor material (112a, 212a) of the second semi-conducting layer, and at least one pattern (M'1) of gate of said transistor on a gate dielectric area located facing said given region, c) removing said semi-conducting sacrificial area. a) providing a substrate provided with at least a first semi-conducting layer including at least one region in which a channel area of said transistor is provided, - forming an insulating layer onto the substrate, the insulating layer (106, 206) being placed on said first semi-conducting layer, - making at least one first opening in said insulating layer uncovering the first semi-conducting layer, - removing an area of said insulating layer to form a second opening in said insulating layer, said second opening uncovering said region in which a channel area of said transistor is provided, - forming a gate dielectric area in said second opening, - forming at least a semi-conducting sacrificial area by epitaxy growth so as to form a monocrystalline semi-conductor material at the bottom of the first opening, and then, - forming said second semi-conducting layer by growth, so as to form a monocrystalline semi-conductor material on said semi-conducting sacrificial area and a polycrystalline semi-conductor material area on said gate dielectric area, - forming an insulating layer onto the substrate, the insulating layer (106, 206) being placed on said first semi-conducting layer, - making at least one first opening in said insulating layer uncovering the first semi-conducting layer, - removing an area of said insulating layer to form a second opening in said insulating layer, said second opening uncovering said region in which a channel area of said transistor is provided, - forming a gate dielectric area in said second opening, - forming at least a semi-conducting sacrificial area by epitaxy growth so as to form a monocrystalline semi-conductor material at the bottom of the first opening, and then, - forming said second semi-conducting layer by growth, so as to form a monocrystalline semi-conductor material on said semi-conducting sacrificial area and a polycrystalline semi-conductor material area on said gate dielectric area, b) etching the second semi-conducting layer (112, 212) so as to form at least one pattern of said mobile structure of said component in an area of monocrystalline semi-conductor material (112a, 212a) of the second semi-conducting layer, and at least one pattern (M'1) of gate of said transistor on a gate dielectric area located facing said given region, c) removing said semi-conducting sacrificial area.