Electron tunneling spectroscopy is used to study drain-source current spectra of metal-oxide-semiconductor field-effect transistors (MOSFETs). Measured at liquid helium temperature (4.2 K), experimental results reveal that as drain-source voltage (V-ds) increases, the first derivative of drain-source current (or conductance) first decreases, then increases to a maximum and finally decreases again at higher Vds, which is different from the monotonous decreasing feature described by the conventional MOSFET theory. In addition, the measured MOSFET spectra show that there are fine features on the second derivative spectra, and these features may be used to extract trap information. (c) 2005 American Institute of Physics.