We have studied cross-sectioned n- and p-metal-oxide-semiconductor field effect transistors with gate lengths approaching 60 nm using a scanning capacitance microscope (SCM). In a homogeneous semiconductor, the SCM measures the depletion length, determining the dopant concentration. When imaging a real device there is an interaction between the probe tip and the built-in depletion of the p-n junction. With the help of a device simulator, we can understand the relation between the SCM images and the position of the p-n junction, making the SCM a quantitative tool for junction delineation and direct measurement of the electrical channel length.