on the pmosfet is removed to expose the pmosfet region 14 as shown in fig2 b a compressively strained film 20 is then formed on the pmosfet region 14 in preferred embodiment of the present invention the amorphous layer 18 can be used as a mask to form the compressively strained film 20 by selective epitaxy the selective epitaxy means a film is selectively grown on a surface with a specific morphology in this preferred embodiment the semiconductor substrate of the pmosfet region 14 has an lattice surface morphology while that of the nmosfet region 16 is an amorphous layer 18 accordingly the selective epitaxy occurs only on the pmosfet region 14 but poly or amorphous film of poor quality may sometimes grow on the amorphous layer 18 in preferred embodiment of the present invention the methods for forming the compressively strained film 20 comprise forming a sige film by plasma enhanced chemical vapor deposition pecvd ultra high vacuum chemical vapor deposition uhv cvd rapid thermal chemical vapor deposition rtcvd or molecular beam epitaxy mbe compressive stress of the sige film on the si substrate relates to its ge concentration increasing the ge concentration of the sige film will enhance the hole mobility of the pmosfet when the ge concentration is higher the critical thickness of the sige film is thinner if a thickness of the sige film is thicker than the critical thickness the sige will become relaxed and defects will occur on the other hand.