Question

physically what is diode bulk resistance?

Answer 1

Solution:
Diode is a semiconductor device that consist of a p-type and n-type semiconductor material joined together to form a junction, thus they are also called as pn junction diode.
The basic function of a Diode is to act like a switch. It provides conduction of electricity in one direction and blocks the conduction of electricity in the reverse direction.
The conduction of electricity by the diode is dependent on the biasing voltage provided at the terminals of the diode.
Internally, when a pn junction is formed, the junction forms a barrier potential. This barrier potential could be overcomed only when a positive voltage is supplied to the p-type semiconductor terminal with respect to the n-type semiconductor material.

For, different semicondutor material the value of this barrier potential is different, for Silicon = 0.7 V and Germanium = 0.3 V

When the pn junction is forward biased, it allows conduction of electricity, considering that biasing voltage is more than barrier potential. As the biasing voltage increases, the flow of current or conduction of electricity through the diode increases, as the number of charge carriers excited by the applied bias potential increases.
Thus, when diode is modelled as a simple resistor in the forward bias mode, the value of its resistance changes as the value of the applied forward voltage changes. This resistance of the diode as a whole in the forward bias mode is called the bulk resistance of the diode. And as we can see is dynamic in nature.

Mathematically it is given by:





So, in the forward bias mode, till the applied bias potential or terminal voltage reaches the barrier potential voltage , the forward current is zero, and the diode bulk resistance is infinity.
After the terminal voltage becomes greater than the barrier potential voltage, there is some current flow and thus the resistance is finite. This finite resistance is captured by the bulk resistance of the diode.
Thus, phusically we can say that bulk resistance is the measure of the resistance offered by the pn junction material as a whole when seen from the diode terminals to the flow of current through it at various values of applied forward voltage.