MOS interface-state density measurements using transient capacitance spectroscopy

The use of a deep-level transient capacitance technique for characterizing the interface properties of an MOS transistor (MOSFET) is discussed. A formulation to calculate interface-state densities is extended from the previous work. Experimental results done with both MOSFET\´s and MOS capacitors are shown to illustrate the advantages of using a transistor-type structure. The use of MOSFET\´s provides not only the capability of probing the interface-state densities throughout the bandgap but also eliminates the effects of minority-carrier generation at the interface. The interface-state densities for variously processed MOS structures were investigated. For hydrogen-annealed MOSFET\´s and MOS capacitors on and orientations of n-type substrates, the interface-state densities were shown to peak near the energies close to the band edges, these corresponding to the measurement temperature where the freeze-out of bull majority carriers occurs. The ability to measure mobile-ion-induced interface states was discussed. The present technique, being a direct differential measurement, has several advantages over the conventional technique. It may provide a higher sensitivity and more reliable data on the densities of states. Moreover, the measurement of the densities of states does not necessarily require a determination of surface potential.