MOS Capacitance Voltage Analysis
- Let us see how the various parameters of a MOS device such as insulator thickness, substrate doping, and VT can be determined from the C-V characteristics (Fig.a).
- First, the shape of the C-V curve depends upon the type of substrate doping. If the high-frequency capacitance is large for negative gate biases and small for positive biases, it is a p-type substrate, and vice versa.
- From the lowfrequency C-V curve for p-type material, as the gate bias is made more positive (or less negative), the capacitance goes down slowly in depletion and then rises rapidly in inversion.
- As a result, the low-frequency C-V is not quite symmetric in shape. For n-type substrates, the C-V curves would be the mirror image of Fig a.
- Fig. a contains high frequency and low-frequency CV curves showing impact of fast interface states;
- Fig b contains energy levels in the band gap due to fast interface states;
- Fig c contains equivalent circuit of MOS structure showing capacitance components due to gate oxide Ci , depletion layer in the channel Cd, and fast interface states Cit.