A PN junction is said to be forward biased when the externally applied voltage is in such a direction that it cancels the potential barrier of a diode, hence, allowing the current flow.

Forward Bias of PN Junction Diode:

Forward bias is the condition that allows current through the PN junction Diode. The voltage source is connected in such a way that it produces a Forward Bias.

In order to forward bias a PN junction, the positive terminal of the battery is connected to to the p-type material and negative terminal of the battery is connected to the n-type material of PN junction diode as shown in the figure below.

Forward bias of PN Junction Diode

Note that the -ve side of VBIAS is connected to the n-region of the diode and the +ve side is connected to the p-region. This is one requirement for forward bias.

A second requirement is that the bias voltage, V(bias), must be greater than the barrier potential.

Barrier Potential is an internal potential a semiconductor material, in case of Silicon-based diode it is 0.7v and in case of Germanium, it is 0.3v. It means in order to forward bias the PN junction diode applied voltage should be greater than 0.7 for silicon and 0.3V for germanium.

What happens during forward bias of PN Junction Diode?

A fundamental picture of what happens when a PN junction diode is forward-biased is shown below.

Working of diode

As we know the N-type material is consist of Electrons and the P-type material is consist of Holes.

When forward voltage is applied to a PN junction, the free electrons in the n-type move towards the junction, leaving behind positively charged atoms.

When the P-type material is connected with a positive terminal of battery it transfers the holes (positive charge carrier), which travels from p-type material to the N-type material through (Junction).

When the N-type material is connected with a negative terminal of battery it transfers the free electrons (negatively charged carriers), which travels from n-type material to the P-type material through (junction).

These free electrons are attracted towards the positive terminal of the diode while the holes are attracted towards the negative terminal of a diode.

Forward Current in PN Junction

When VBIAS is applied across the junction in the forward bias, a current will flow continuously through this junction. That current can be calculated through the given formula.

IS = Saturation Current (10-9 to 10-18 A)
VT = Volt-equivalent temperature (≈ 26mV at room temperature)
n = Emission coefficient (1 ≤ n ≤ 2 for Si ICs)
Actually, this expression is approximated.

VI Characteristics of PN Junction Diode in Forward Bias:

When a forward-bias voltage is applied across a diode, there is current that current is called the forward current.

When the forward-bias voltage is increased to a value where the voltage across the diode reaches approximately 0.7 V (barrier potential), the forward current begins to increase rapidly, as illustrated in Figure given below. As you continue to increase the forward-bias voltage, the current continues to increase very rapidly, but the voltage across the diode is constant up to 0.7v for silicon and 0.3v for germanium.

PN junction diode on forward bias


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