In a 
thin film transistor (1), a gate insulating layer (4) is formed on a gate 
electrode (3) formed on an insulating substrate (2). Formed on the gate insulating layer (4) is a 
semiconductor layer (5). Formed on the 
semiconductor layer (5) are a source 
electrode (6) and a drain 
electrode (7). A protective layer (8) covers them, so that the 
semiconductor layer (5) is blocked from an 
atmosphere. The semiconductor layer (5) (
active layer) is made of, e.g., a semiconductor containing polycrystalline ZnO to which, e.g., a group V element is added. The protective layer (8) thus formed causes decrease of a 
surface level of the semiconductor layer (5). This eliminates a depletion layer spreading therewithin. Accordingly, the ZnO becomes an n-type semiconductor indicating an 
intrinsic resistance, with the result that too many free electrons are generated. However, the added element works on the ZnO as an accepter 
impurity, so that the free electrons are reduced. This decreases a 
gate voltage required for removal of the free electrons, so that the 
threshold voltage of the 
thin film transistor (1) becomes on the order of 0V. This allows practical use of a 
semiconductor device which has an 
active layer made of 
zinc oxide and which includes an protective layer for blocking the 
active layer from an 
atmosphere.