A p-n junction diode is reverse biased when the p-side is joined to the negative terminal and the n-side to the positive terminal of a battery. The applied voltage adds to the built-in barrier, so the barrier height becomes (V0 + V), the depletion region gets wider, and almost no current flows except a very small reverse saturation current in microamperes carried by minority carriers. Memory hook: "Reverse bias = Repel and Resist" (charges are pulled away from the junction, so the diode blocks current).
In reverse bias the p-side connects to the negative terminal and the n-side to the positive terminal. The barrier rises to (V0 + V), the depletion region widens, and only a tiny microampere reverse current flows due to minority carriers.
Your doubts, answered
In reverse bias, is the p-side connected to positive or negative?
The p-side goes to the negative terminal and the n-side goes to the positive terminal of the battery. This is the opposite of forward bias. An easy check: in reverse bias, each side is connected to the terminal of the opposite sign to its majority charge (p-side has holes = positive carriers, so it goes to negative).
Why does almost no current flow in reverse bias?
The applied voltage points in the same direction as the built-in barrier potential, so the total barrier height rises to (V0 + V). This pulls the majority carriers away from the junction (holes toward the negative terminal, electrons toward the positive terminal), so the depletion region widens and the diffusion current is suppressed almost completely. The diode acts like an open switch (very high resistance).
Does the depletion region get wider or narrower in reverse bias?
It gets wider. The external field adds to the built-in field and sweeps more mobile carriers away from the junction, leaving behind more immobile charged ions. More ions means a wider depletion region and a taller barrier. Forward bias does the opposite and narrows it. (NEET 2020 tested exactly this.)
What carries the small reverse current, majority or minority carriers?
Minority carriers. The tiny reverse current (order of microamperes) is the reverse saturation current, driven by the drift of minority carriers (electrons in the p-side, holes in the n-side). Many students wrongly say majority carriers, which was the trap in NEET 2024. It is thermally generated, so it depends on temperature, not on the applied reverse voltage.
Why is reverse saturation current almost constant with voltage?
The reverse current is limited by how many minority carriers are available at the junction, not by the applied voltage. So once these few carriers are being swept across, increasing the reverse voltage barely changes the current. It stays nearly constant (saturates) until the reverse voltage reaches the breakdown voltage Vbr, where the current then rises sharply.
⚠️ The NEET trap ✗ The small reverse current (in microamperes) is due to majority charge carriers crossing the junction. ✓ The small reverse saturation current is due to the drift of MINORITY carriers (electrons from p-side, holes from n-side). Majority carriers are pushed away from the junction in reverse bias. 🧠 Reverse bias blocks the majority, so only the few minority carriers can leak across. Remember: Reverse = Minority runs the current.
Real NEET questions
NEET 2020
The increase in the width of the depletion region in a p-n junction diode is due to:
A · Both forward bias and reverse bias
B · Increase in forward current
C · Forward bias only
D · Reverse bias only ✓
Solution: Step 1: In reverse bias the applied field points the same way as the built-in field. Step 2: This combined field sweeps more mobile carriers away from the junction, exposing more immobile ions. Step 3: More exposed ions means the depletion region widens. Forward bias narrows it. So the width increases due to reverse bias only. Answer: D.
NEET 2024
Consider statements A and B: A. For a solar cell, the I-V characteristic lies in the fourth (IV) quadrant. B. In a reverse biased p-n junction diode, the current measured in microamperes is due to majority charge carriers.
A · A is incorrect but B is correct
B · Both A and B are correct
C · Both A and B are incorrect
D · A is correct but B is incorrect ✓
Solution: Step 1: A solar cell operates in the fourth quadrant of its I-V graph, so statement A is correct. Step 2: In reverse bias the majority carriers are pushed away; the tiny microampere current is the reverse saturation current carried by MINORITY carriers, not majority. So statement B is incorrect. Answer: D (A correct, B incorrect).
ReNEET 2026
Three identical p-n junction diodes D1, D2 and D3 are connected across a battery (each in series with a 1 kohm resistor). If the depletion widths are W1, W2, W3, then the correct option is:
A · W1 > W2 > W3
B · W3 = W1 > W2
C · W3 > W2 > W1 ✓
D · W2 > W1 = W3
Solution: Step 1: Forward bias narrows the depletion region, reverse bias widens it, and an unbiased junction is in between. Step 2: D1 is forward biased so W1 is smallest, D2 is unbiased so W2 is intermediate, D3 is reverse biased so W3 is largest. Step 3: Ordering gives W3 > W2 > W1. Answer: C.
What is reverse bias of a p-n junction diode in simple words?
It means connecting the p-side to the negative terminal and the n-side to the positive terminal of a battery. This raises the barrier, widens the depletion region, and stops almost all current, so the diode behaves like a switch that is OFF.
What is the effective barrier height in reverse bias?
The effective barrier height becomes (V0 + V), where V0 is the built-in potential and V is the applied reverse voltage. The applied voltage adds to the barrier, making it harder for majority carriers to cross.
What is reverse saturation current?
It is the very small, nearly constant current (in microamperes) that flows in reverse bias due to the drift of minority carriers. It depends on temperature but stays almost constant with applied voltage until breakdown.
What is breakdown voltage?
It is the critical reverse voltage Vbr at which the reverse current suddenly rises sharply. If this current is not limited by the external circuit, the diode can be destroyed by overheating. Zener diodes are made to work safely in this region.
Which type of carrier gives the reverse current?
Minority carriers, meaning electrons in the p-region and holes in the n-region. The majority carriers are pushed away from the junction in reverse bias, so they do not contribute.
