A graphene layer, a method of forming the graphene layer, a device including the graphene layer, and a method of manufacturing the device are provided. The method of forming the graphene layer may include forming a first graphene at a first temperature using a first source gas and forming a second graphene at a second temperature using a second source gas. One of the first and second graphenes may be a P-type graphene, and the other one of the first and second graphenes may be an N-type graphene. The first graphene and the second graphene together form a P—N junction.