Let’s now see what happens when we apply an external voltage to the diode. This process is called biasing.
1️⃣ Forward Bias
In forward bias, the positive terminal of the battery is connected to the P-side, and the negative terminal to the N-side.
Now, think of what’s happening internally:
- The positive terminal repels holes in the P-side towards the junction.
- The negative terminal pushes electrons in the N-side towards the junction.
- As a result, the depletion region becomes thinner, and the barrier potential decreases.
When the applied voltage becomes equal to or greater than the barrier potential (0.7 V for silicon), the junction becomes conductive.
Electrons now have enough energy to cross the barrier, recombine with holes, and current begins to flow through the diode.
This current is called Forward Current, and the diode is said to be in its conducting state.
So, in simple words — in forward bias, the diode allows current to flow.
2️⃣ Reverse Bias
Now let’s reverse the connections — connect the positive terminal to the N-side, and the negative terminal to the P-side.
What happens?
- The positive terminal pulls electrons away from the junction.
- The negative terminal pulls holes away from the junction.
- The depletion region widens, and the barrier potential increases.
As a result, no significant current flows. Only a tiny current (called leakage current) flows due to the minority charge carriers.
This current is extremely small and often negligible.
So, in reverse bias, the diode acts like an open switch — it blocks current.
What’s Next?
Now that we understand how a diode behaves under forward and reverse bias conditions, the next step is to study how the current through the diode changes with applied voltage.
In the next blog, we will study:
- V–I characteristics of a diode
- Cut-in (threshold) voltage
- Forward and reverse current behavior
- Reverse saturation current
- Breakdown region
👉 Click below to continue to the next part:
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