MOS Capacitor and Ideal C-V Characteristics A MOS capacitor , also known as a capacitor-controlled device (CCD), is a semiconductor device that acts as a...
A MOS capacitor, also known as a capacitor-controlled device (CCD), is a semiconductor device that acts as a capacitor when a voltage is applied to it. The ideal C-V characteristics of a MOS capacitor reveal some very interesting and useful properties of this device.
Capacitance:
The capacitance of a MOS capacitor is determined by the area of the depletion region (A_d) between the gate and source terminals, as well as the thickness of the gate oxide film (t_ox).
The key concept is that the capacitance is not constant but depends on the applied voltage.
At zero gate voltage (V_gs = 0), the capacitance is equal to the gate-source overlap capacitance (C_gs), which is typically much smaller than the channel capacitance (C_ch).
As V_gs is increased, the C_gs increases while the C_ch remains relatively constant.
This behavior is highly non-linear and follows a Capacitance-Voltage (C-V) relationship.
Ideal C-V characteristics:
The ideal C-V characteristics of a MOS capacitor are shown in the plot below.
The C-V curve is non-linear due to the varying capacitance.
At low gate voltages, the C-V curve is almost linear and follows the behavior of a capacitor.
As V_gs is increased, the C-V curve becomes increasingly non-linear, eventually approaching a constant value at a high gate voltage.
The ideal C-V characteristics provide valuable insights into the internal workings of a MOS capacitor, including the relationships between gate voltage, channel width, and capacitance.
Examples:
A MOS capacitor with a C_gs of 100 fF and a C_ch of 10 nF when V_gs = 0.
The ideal C-V characteristics of a MOS capacitor can be simulated or measured and used to understand the behavior of this device