THE INFLECTION POINT OF THE CAPACITANCE-VOLTAGE AND THE FLAT BAND VOLTAGE OF METAL-OXIDE-SEMICONDUCTOR GATE STACK

This work demonstrates a new method for determining flat band voltage (V_FB). This method is based on the point of inflection in the capacitance-voltage curve. This method does not require the knowledge of material or experimental parameters, and can be used on high interface and border trap density metal-oxide-semiconductor (MOS) structures at all frequencies.

The MOS structure is treated as a serial connection of two capacitors: the capacitance of the oxide and the capacitance of the depletion layer. In accumulation, when the applied gate voltage is lower than the flat-band voltage in a p-type capacitor (or higher in n-type), there is no depletion layer and the only relevant capacitance is that of the oxide. In depletion, when V_G is between the threshold voltage and the flat-band voltage, the MOS capacitance is obtained from the serial connection of the oxide capacitance and the capacitance of the depletion layer. The transition between these two regimes occurs at V_FB. We suggest that this inflection point in the C-V curve corresponds to the flat-band situation.

This hypothesis was tested by using a second derivative of C-V simulation of an ideal capacitor and measured MOS capacitors. Our work reveals that there is an excellent correlation between the derived inflection points and the flat-band voltages which were calculated using the "flat-band capacitance method".

The second derivative method to determine V_FB is quick and simple. This method does not require prior knowledge of the parameters of the sample, such as substrate doping, measurement temperature, the capacitors’ area and parameters of the oxide and the semiconductor. Most importantly high density of interface traps and border traps do not affect the determination of V_FB.