Patent ID: 7394279

Claim:
A method of measuring a surface voltage of an insulating layer, the method comprising: measuring first surface voltage values of a first insulating layer each time that a surface charge amount of the first insulating layer increases by a first amount in a light state where the first insulating layer is provided with light; measuring second surface voltage values of the first insulating layer each time that the surface charge amount of the first insulating layer increases by the first amount in a dark state where the first insulating layer is not provided with the light; comparing the first surface voltage values with the second surface voltage values to obtain libraries concerning an accumulation region, a depletion region and an inversion region, the first surface voltage value being substantially the same as the second surface voltage value in the accumulation region, a difference between the first surface voltage and the second surface voltage increasing in the depletion region, the difference between the first surface voltage and the second surface voltage being constantly maintained at a predetermined positive value in the inversion region; measuring third surface voltage values of a second insulating layer substantially the same as the first insulating layer each time that a surface charge amount of the second insulating layer increases in the light state where the second insulating layer is provided with the light, the surface charge amount of the second insulating layer being repeatedly increased by a second amount in the depletion region, the surface charge amount of the second insulating layer being repeatedly increased in the accumulation region and the inversion region by a third amount substantially larger than the second amount; measuring fourth surface voltage values of the second insulating layer substantially the same as the first insulating layer each time that a surface charge amount of the fourth insulating layer increases in the dark state where the fourth insulating layer is not provided with the light, the surface charge amount of the fourth insulating layer being repeatedly increased by the second amount in the depletion region, the surface charge amount of the fourth insulating layer being repeatedly increased in the accumulation region and the inversion region by the third amount; and obtaining data concerning a region within which a difference between the fourth surface voltage and the third surface voltage increases by comparing the third surface voltage values with the fourth surface voltages.