MOSFET: Time-Dependent Capacitance Measurements
During C-V measurements, if the gate bias is varied rapidly from accumulation to inversion, the depletion width can momentarily become greater than the theoretical maximum for gate biases beyond Fr.This phenomenon is known as deep depletion, and causes the MOS capacitance to drop below the theoretical minimum, Cmin, for a transient period.
Time-dependent MOS capacitance:
After a time period characteristic of the minority carrier lifetime, which determines the rate of generation of the minority carriers in the MOS device, the depletion
width collapses back to the theoretical maximum and the capacitance recovers to Cmin that is shown in figure. This capacitance transient, C-t, forms the basis of a powerful technique to measure the lifetime, known as the Zerbst Technique.
- We can also determine MOS parameters such as the fast interface state density, Dit and mobile ion charges, Qm, from C-V measurements.
- The term fast interface state refers to the fact that these defects can change their charge state relatively fast in response to changes of the gate bias.
- As the surface potential in a MOS device is varied, the fast interface states or traps in the band gap can move above or below the Fermi level in response to the bias, because their positions relative to the band edges are fixed.
- The capacitance Ci = ξi /d in accumulation or strong inversion (at low frequencies) gives us the insulator thickness, d.
- The minimum MOS capacitance, Cmin is the series combination of Q and the minimum depletion capacitance, Cdmin = ξs/Wm, corresponding to the maximum depletion width.