MOS flat-band capacitance method at low temperatures

The expression C_FB=C_ox×(ε_si /L_D)/[C_ox+(ε_si /L_D)] (where L_D is the Debye length), commonly used for the flatband capacitance of the MOS structure, is invalid in the temperature range below 100 K. Consequently, significant error may be encountered when the flatband capacitance method is used to extract the flatband voltage, V _FB, which is of considerable interest for both the modeling and characterization of MOS devices. To extend this method to low-temperature CMOS applications one has to use a more general model that can be obtained by applying Fermi-Dirac statistics and taking into account the impurity freeze-out effect. It is shown that when the temperature dependence of V_FB is extracted using this approach, the experimental data for n⁺ polysilicon gate MOS capacitors are in good agreement with a simple method