a method of measuring total charge of an insulating layer on a semiconductor substrate includes applying corona charges to the insulating layer and measuring a surface photovoltage of the insulating layer after applying each of the corona charges . the charge density of each of the corona charges is measured with a coulombmeter . a total corona charge required to obtain a surface photovoltage of a predetermined fixed value is determined and used to calculate the total charge of the insulating layer . the fixed value corresponds to either a flatband or midband condition .