Transition Capacitance

AN INTRODUCTION TO “TRANSITION CAPACITANCE” BY VIKAS CHANDRA III-SEM

JABALPUR 17/12/2008

In the reverse bias region we have the transition or depletion region capacitane (CT), whereas in the forward bias region we have the diffusion (CD) or storage capacitance. The transition capacitance represents the change in charge stored in the depletion region with respect to a change in junction voltage. The increase in the level of reverse bias caused the width of the depletion region, W to increase. An increase in the width of the depletion region, W is accompanied by additional uncovered ions in the space charge or transition region.

Because positive ions exist on one side of the junction and negative ions on the other, the transition capacitance, CT is analogous to a parallel plate capacitor for which we have

CT=εA/w

Farads

where A= Junction area ε= Permittivity of the semiconductor We must note that w is a function area of the reversed biased voltage so that transittion capacitance CT is voltage dependent. For a step graded junction, the width of the depletion region, W is inversely proportional to the square root of the reverse bias voltage. Under forward biased condition, the value of transittion capacitance is to small compared to diffusion capacitance that it is generally neglected . Similarly in a reverse biased diode, a small amount of carrier diffusion exists,

but this capacitance is negligible when compared to transition capacatance.

Transition & Diffusion capacitance verse applied bias for a silicon diode