Physics · Semiconductor Electronics : Materials, Devices And Simple Circuits · NEET
| Battery connection | p-side to + , n-side to - (P to Plus) | p-side to - , n-side to + |
| Depletion region width | Decreases (thinner) | Increases (wider) |
| Barrier potential | Decreases | Increases |
| Current | Large, in milliamperes (majority carriers) | Very small reverse saturation current (minority carriers) |
| Resistance | Low (few ohms to hundreds of ohms) | Very high (mega-ohms) |
| Diode state | ON, conducts after knee voltage | OFF, blocks until breakdown |
For forward bias, connect the positive terminal of the battery to the p-side and the negative terminal to the n-side. A simple rule is 'P to Plus'. For reverse bias, do the opposite: positive terminal to the n-side and negative terminal to the p-side. In forward bias the external voltage opposes the built-in barrier voltage, so it helps carriers cross. In reverse bias the external voltage adds to the barrier, so it stops carriers.
It gets smaller (thinner) in forward bias. The applied voltage pushes majority carriers toward the junction, so they fill in some of the depletion region and reduce its width. The barrier potential also drops. In reverse bias the opposite happens: majority carriers are pulled away from the junction, the depletion region gets wider, and the barrier potential rises. Remember: forward = thin barrier = current flows; reverse = thick barrier = current blocked.
Forward bias gives low resistance (roughly a few tens to a few hundred ohms), so a large current flows once the voltage crosses the knee voltage (about 0.7 V for silicon, 0.3 V for germanium). Reverse bias gives very high resistance (of the order of mega-ohms), so only a tiny reverse saturation current (microamperes or nanoamperes) flows. This is why a diode acts like a one-way switch.
Yes, but it is extremely small. A tiny current called the reverse saturation current flows because minority carriers (a few electrons in the p-side and a few holes in the n-side) still drift across the junction. This current is of the order of microamperes (Ge) or nanoamperes (Si) and is nearly independent of the reverse voltage until breakdown. So for NEET, treat reverse current as almost zero unless the question is about reverse saturation current or breakdown.
Below the knee voltage, the applied voltage is too small to overcome the barrier potential, so current is very small. Once the forward voltage crosses the knee (threshold) voltage, the barrier is nearly cancelled and the depletion region becomes very thin, so a large number of majority carriers cross the junction. That is why the forward V-I curve stays flat and then shoots up steeply after the knee voltage. This is exactly what NEET 2026 tested.
Two statements are given below: A. When the forward bias voltage across a p-n junction diode increases above a certain threshold voltage, the diode current increases significantly. B. This current is called reverse saturation current. Choose the correct answer.
In a p-n junction diode, change in temperature due to heating:
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Forward bias connects p-side to positive (P to Plus), thins the depletion region, and allows a large current; reverse bias connects p-side to negative, widens the depletion region, and blocks current (only a tiny reverse saturation current flows).
In forward bias the applied voltage opposes the built-in barrier, so the net barrier decreases. In reverse bias the applied voltage adds to the barrier, so the net barrier increases and current is stopped.
The knee (threshold) voltage is about 0.7 V for a silicon diode and about 0.3 V for a germanium diode. Forward current rises sharply only after the applied voltage crosses this value.
Forward current is mainly due to majority carriers (electrons from n-side, holes from p-side) crossing the junction. The small reverse saturation current is due to minority carriers drifting across the junction.
Because it has low resistance and conducts in forward bias but very high resistance and blocks current in reverse bias. This lets current pass in only one direction, which is used in rectifiers.
