Abstract: The invention relates to apparatus and method for determining the carrier concentration profile of the carrier concentration in semiconductor materials, multi-layer semiconductor structures and semiconductor devices. According to the invention, applied between an ohmic contact and a rectifying contact formed on the semiconductor are a d.c. voltage resulting in a reverse bias, and two periodic excitation signals having frequencies .OMEGA..sub.1 and .OMEGA..sub.2. Frequency .OMEGA..sub.1 is greater than .OMEGA..sub.2 such that .OMEGA..sub.1, .OMEGA..sub.2, their difference (.OMEGA..sub.2 -.OMEGA..sub.2) and their sum (.OMEGA..sub.1 +.OMEGA..sub.2) are in the same order of magnitude. Two components from the semiconductor's electronic response to the excitation are selected for analysis. The first component is the response to the frequency .OMEGA..sub.1 or .OMEGA..sub.2, and the second component is a first order intermodulation product of one of .OMEGA..sub.1 and .OMEGA..sub.2.