A method for making semiconducting layer includes: providing a carbon nanotube film; providing a conductive substrate and applying an insulating layer on the conductive substrate; laying the carbon nanotube film on a surface of the insulating layer, and placing the carbon nanotube film under a scanning electron microscope; adjusting the scanning electron microscope with an accelerating voltage ranging 5˜20 KV, and taking photos of the carbon nanotube film with the scanning electron microscope; obtaining a photo of the carbon nanotube film, wherein the photo shows the plurality of carbon nanotubes and a background, a plurality of first carbon nanotubes have lighter color than a color of the background, a plurality of second carbon nanotubes have deeper color than the color of the background; and removing the plurality of first carbon nanotubes.