Transition capacitance and Diffusion capacitance

 Transition capacitance (CT):

A capacitor has two metal plates and an insulating material (dielectric) between them. The metal plates allow electric current, but the dielectric does not. It only allows an electric field. When voltage is applied, charges collect on the plates but cannot cross the dielectric. These trapped charges create an electric field, which stores electric charge. The ability to store charge is called capacitance.Capacitance increases when plate size is larger and decreases when the distance between plates is larger. 

A reverse biased p–n junction diode acts like a capacitor. The p-type and n-type regions act as the plates. The depletion region acts as the dielectric. Charges in the depletion region do not move, but they create an electric field and store charge. This creates junction (transition) capacitance. When reverse voltage increases, the depletion region becomes wider. Wider depletion region means less stored charge. So, capacitance decreases when reverse voltage increases. In forward bias, transition capacitance is very small and usually ignored. Transition capacitance is also called depletion capacitance, junction capacitance, or barrier capacitance.

The change of capacitance at the depletion region can be defined as the change in electric charge per change in voltage. CT = dQ / dV Where, CT = Transition capacitance dQ = Change in electric charge dV = Change in voltage The transition capacitance can be mathematically written as, CT = ε A / W Where, ε = Permittivity of the semiconductor A = Area of plates or p-type and n-type regions W = Width of depletion region.

Diffusion capacitance (CD) :

Diffusion capacitance (CD) occurs in a forward biased p–n junction diode.It is also called storage capacitance.In forward bias, diffusion capacitance is much larger than transition capacitance, so it is considered.It occurs due to stored minority carriers near the depletion region.Forward bias causes electrons and holes to cross the junction and accumulate before recombining.This stored charge on both sides acts like a capacitor.     

                                                                                                                                                           Diffusion capacitance increases with current and voltage.Smaller depletion width gives higher diffusion capacitance.Its value is usually in nF to μF range.