Off-axis electron holography is used to measure electrostatic
potential profiles across a silicon p-n junction,
which has been prepared for examination in the transmission electron
microscope (TEM) in two different specimen geometries using focused ion
beam (FIB) milling. Results are obtained both from a conventional
unbiased FIB-milled sample and using a novel sample geometry that
allows a reverse bias to be applied to an FIB-milled sample in
situ in the TEM. Computer simulations are fitted to the results to
assess the effect of TEM specimen preparation on the charge density and
the electrostatic potential in the thin sample.