1.10 Stray Capacitances in Devices – Pulse and Digital Circuits


The semiconductor diode is used as a switch. In a forward-biased diode, there is a diffusion capacitance CD that depends on the forward current of the diode. However, while considering the equivalent circuit of the diode at high frequencies, this capacitance will have a negligible say. But in a reverse-biased diode, there exists a transition capacitance CT which is inversely proportional to the diode reverse-bias voltage and is taken into consideration at high frequencies. Hence, the high-frequency equivalent circuit of a reverse-bias semiconductor diode is given in Fig. 1.52. The stray capacitances limit the switching speed. Here, Rr is the reverse resistance of the diode.

Similarly, we have stray capacitances in transistors between a pair of terminals, called inter-electrode capacitances. These influence the switching speed of the devices. These capacitances also have a role in the high-frequency response of the amplifiers.