A method for forming a semiconductor device, the method comprising: forming a first dummy gate part extending across a first fin and a second dummy gate part extending across a second fin, said first dummy gate part extending in a first gate trench section in an insulating layer and said second dummy gate part extending in a second gate trench section in the insulating layer, removing said first dummy gate part selectively to said second dummy gate part, wherein each of the first and second fins (10, 20) is formed by a layer stack comprising a first layer (16, 26) and a second layer (18, 28) on the first layer, the first layer comprising Si 1-x Ge x and the second layer comprising Si 1-y Ge y, wherein 0≤ x≤1 and 0≤y≤1 and x≠y, forming a silicon capping layer on the portion of the first fin exposed in said first gate trench section, performing an oxidation process to oxidize the silicon capping layer and to oxidize an outer thickness portion of said portion of the first fin such that a trimmed fin portion comprising laterally trimmed portions (116, 118) of said first and second layers (16, 18) remains inside the oxidized outer thickness portion, subsequent to performing the oxidation process, removing the second dummy gate part while the oxidized silicon capping layer and the oxidized outer thickness portion covers said trimmed fin portion, removing the oxidized silicon capping layer and the oxidized outer thickness portion from said trimmed fin portion, removing the laterally trimmed portion of the first layer exposed in the first gate trench section and the portion of the first layer exposed in the second gate trench section, and forming a final gate structure (60, 70) around the laterally trimmed second layer portion in the first gate trench section and around the second layer portion in the second gate trench section. forming a first dummy gate part extending across a first fin and a second dummy gate part extending across a second fin, said first dummy gate part extending in a first gate trench section in an insulating layer and said second dummy gate part extending in a second gate trench section in the insulating layer, removing said first dummy gate part selectively to said second dummy gate part, wherein each of the first and second fins (10, 20) is formed by a layer stack comprising a first layer (16, 26) and a second layer (18, 28) on the first layer, the first layer comprising Si 1-x Ge x and the second layer comprising Si 1-y Ge y, wherein 0≤ x≤1 and 0≤y≤1 and x≠y, forming a silicon capping layer on the portion of the first fin exposed in said first gate trench section, performing an oxidation process to oxidize the silicon capping layer and to oxidize an outer thickness portion of said portion of the first fin such that a trimmed fin portion comprising laterally trimmed portions (116, 118) of said first and second layers (16, 18) remains inside the oxidized outer thickness portion, subsequent to performing the oxidation process, removing the second dummy gate part while the oxidized silicon capping layer and the oxidized outer thickness portion covers said trimmed fin portion, removing the oxidized silicon capping layer and the oxidized outer thickness portion from said trimmed fin portion, removing the laterally trimmed portion of the first layer exposed in the first gate trench section and the portion of the first layer exposed in the second gate trench section, and forming a final gate structure (60, 70) around the laterally trimmed second layer portion in the first gate trench section and around the second layer portion in the second gate trench section.