# Bulk Resistance, rB , of a Diode

### What is the Bulk Resistance of a Diode?

The Bulk Resistance, rB, of a diode is the approximate resistance across the terminals of the diode when a forward voltage and current are applied across the diode.

The bulk resistance represents the resistance of the p and n materials of the p-n junction of the diode. Its value is dependent on the doping level and the size of the p and n materials.

The bulk resistance is not a fixed resistance but a dynamic one. It changes according to the amount of forward voltage and current going through the diode at any particular time.

The bulk resistance is then used to calculate the total
resistance that is present in a diode circuit.

### How to Calculate the Bulk Resistance of a Diode

The bulk resistance of a diode can be calculated at any given time by ohm's law:

**RB= ∆VF/∆IF**

where VF is the forward voltage dropped across the silicon diode and IF is the forward current flowing through the diode.

**Note:** For silicon diodes, the barrier voltage for current flow to occur is above the point of
0.7V. So we
assume that the diode current, IF is zero when the forward voltage of the silicon diode is
exactly 0.7V. For germanium diodes, the barrier voltage is above the 0.3V point.

### Example Calculations

**1st Example**

If a silicon diode has a forward voltage drop of 1.1V for a forward diode current, IF, of 1 A, what
is the bulk resistance, RB?

Again, since this is a silicon diode, current flow doesn't start until the voltage across the diode surpasses the barrier voltage, which is above 0.7V. Therefore, we assume the current is initially 0A at 0.7V.

So the calculation is:

**RB= ∆VF/∆IF= (1.1V - 0.7V)/(1A - 0A)= 0.4Ω **

So the bulk resistance is 0.4Ω.

**2nd Example**

If a silicon diode has a forward voltage drop of 0.9V for a forward diode current, IF, of 1.5A, what
is the bulk resistance, RB?

Again, since this is a silicon diode, current flow doesn't start until the voltage across the diode surpasses the barrier voltage, which is above 0.7V. Therefore, we assume the current is initially 0A at 0.7V.

So the calculation is:

**RB= ∆VF/∆IF= (0.9V - 0.7V)/(1.5A - 0A)= 0.13Ω**

So the bulk resistance is 0.13Ω.

Related Resources

How Does Current Flow Through a Diode?

What is a Diode Connected in Forward Biased?

What is a Diode Connected in Reverse Biased?

What is the Leakage Current of a Diode?