The methods to determine flat-band voltage VFB in semiconductor of a MOS structure

One of the most important parameters of the MOS (metal-oxide-semiconductor) structure is the flat-band voltage V_FB in semiconductor. This voltage influences the threshold voltage V_T, which is the fundamental parameter of any MOS device. Hence, the V_FB voltage has a great practical importance and the precise and accurate determination of its value is very important. In this paper, the measurement techniques for V_FB voltage determination are presented. Most of these methods are based on electric measurements of capacitance-voltage C(V_G) characteristics of MOS structure. The results show considerable spread of V_FB values, which exceeds the tolerances allowed by the technology of modern, scaled down MOS integrated circuits. Also, the photoelectric method of the V_FB voltage determination is described. This method, called LPT (Light Pulse Technique), is based on simultaneous illumination (modulated light) of the semitransparent gate and polarization of the substrate of the MOS structure. The expected accuracy of the LPT method is ± 10 mV, while the accuracy of above mentioned electric methods is rarely better than ± 50 mV. There is no other method with an accuracy similar to the accuracy of the LPT method. It causes that, despite many advantages of the LPT method, the determination of the absolute accuracy of this method is still problematic.