What Is a p-n Junction
A p-n junction is the interface between a p-type and an n-type region within a single semiconductor crystal. NCERT introduces it as "the basic building block of many semiconductor devices like diodes, transistor, etc." A clear understanding of junction behaviour is the prerequisite for analysing the working of all later devices in the chapter.
The junction is not made by clamping two finished blocks together. The most convenient method, described in both NCERT and NIOS, is to take a single semiconducting crystal and introduce donor (pentavalent) impurities on one side and acceptor (trivalent) impurities on the other side. NCERT describes starting from a thin p-type silicon wafer and converting part of it into n-Si by adding a small, precise quantity of pentavalent impurity. The wafer then contains a p-region, an n-region, and a metallurgical junction between them.
Before the two regions interact, recall their carrier populations. In the n-region electrons are the majority carriers and holes the minority; in the p-region holes are the majority and electrons the minority. This asymmetry across the junction is what drives everything that follows.
| Region | Dopant | Majority carrier | Minority carrier | Fixed ion left after diffusion |
|---|---|---|---|---|
| n-region | Pentavalent (donor) — As, Sb, P | Electrons | Holes | Positive donor ion (immobile) |
| p-region | Trivalent (acceptor) — B, Al, In | Holes | Electrons | Negative acceptor ion (immobile) |
Diffusion of Majority Carriers
Two processes occur during the formation of a p-n junction: diffusion and drift. Diffusion comes first. Because the electron concentration is high on the n-side and low on the p-side, and the hole concentration is high on the p-side and low on the n-side, a steep concentration gradient exists across the junction.
Driven by this gradient alone — not by any external voltage — holes diffuse from the p-side to the n-side ($p \rightarrow n$) and electrons diffuse from the n-side to the p-side ($n \rightarrow p$). This motion of majority carriers across the junction constitutes the diffusion current. As NIOS adds, each electron that crosses into the p-region meets a hole and the pair recombines, eliminating one free electron and one hole near the junction.
The Depletion Region and Immobile Ions
Each diffusing carrier leaves something behind. When an electron diffuses from $n \rightarrow p$, it leaves behind an ionised donor on the n-side. This donor ion carries a positive charge and is immobile — it is bonded to the surrounding lattice atoms and cannot move. As electrons keep diffusing, a layer of positive space charge builds up on the n-side of the junction.
Symmetrically, when a hole diffuses from $p \rightarrow n$, it leaves behind an ionised acceptor on the p-side — a negative, immobile charge. A layer of negative space charge therefore builds on the p-side. Together these two layers of fixed ion cores form the depletion region, so named because the electrons and holes that crossed initially depleted the region of its free charges.
What the depletion region actually contains
A common error is to picture the depletion region as full of electrons and holes piled up at the junction. The opposite is true. The depletion region has been emptied of mobile charge carriers; what remains are only the immobile ionised donor cores (positive, on the n-side) and immobile ionised acceptor cores (negative, on the p-side). NIOS confirms these are the "fixed donor and acceptor ions," not majority or minority carriers.
Depletion region = fixed ion cores + no free carriers. Thickness ≈ one-tenth of a micrometre (NCERT); about 0.5 μm (NIOS).
Barrier Potential and the Built-in Field
With positive space charge on the n-side and negative space charge on the p-side, an electric field is established across the depletion region, directed from the positive charge towards the negative charge — that is, from the n-side to the p-side. This internal field is the engine of the drift process and the source of the junction's potential barrier.
The loss of electrons from the n-region and the gain of electrons by the p-region produces a difference of potential across the junction. The n-material becomes positive relative to the p-material. Because this potential difference tends to prevent further movement of electrons from the n-region into the p-region, it is called the barrier potential (also the built-in potential, $V_0$). NIOS notes that with no external field this barrier prevents diffusion of charge carriers across the junction.
Once you can apply an external voltage to this barrier, the junction becomes a working diode. See Semiconductor Diode and Its I–V Characteristics.
Diffusion–Drift Equilibrium
The built-in field acts on any carrier near the junction. An electron that strays onto the p-side, or a hole that strays onto the n-side, is swept across by the field to its majority zone. This field-driven motion of carriers is called drift, and it gives rise to a drift current that flows in the direction opposite to the diffusion current.
The two currents do not start out equal. NCERT is precise about the sequence: initially the diffusion current is large and the drift current is small. As diffusion continues, the space-charge regions extend, the electric field strength rises, and the drift current grows. This process continues until the diffusion current exactly equals the drift current. At that instant the junction is formed and equilibrium is reached.
| Property | Diffusion current | Drift current |
|---|---|---|
| Carriers responsible | Majority carriers | Minority carriers |
| Driving cause | Concentration gradient | Built-in electric field |
| Direction | p → n (conventional) | Opposite to diffusion current |
| At equilibrium | Equal in magnitude, opposite in direction → net current is zero | |
"Zero net current" does not mean "no current"
At equilibrium NCERT states there is "no net current" — but this is the result of two non-zero currents cancelling, not the absence of carrier motion. The diffusion current is carried by majority carriers moving down the concentration gradient; the drift current is carried by minority carriers swept by the field. They are equal and opposite, so they sum to zero. NIOS phrases the equilibrium as "zero, as equal and opposite charges are crossing the junction" — not "zero because no charges cross."
Diffusion = majority carriers; drift = minority carriers. Net current = 0 by cancellation, not by stoppage.
Barrier Values and the Unbiased Junction
The barrier potential is a characteristic of the semiconductor material. NIOS gives its value as about 0.3 V for germanium and about 0.7 V for silicon. These are the equilibrium built-in potentials with no external bias applied. The polarity of the barrier is always such as to oppose the further diffusion of majority carriers, which is precisely what holds the equilibrium in place.
This is also the most directly examined number on this subtopic. NEET 2021 asked whether the potential barrier of a p-n junction lies between 0.1 V and 0.3 V; the statement was marked incorrect because the silicon barrier is nearly 0.7 V. Memorising the two material values, and which value goes with which element, is the single highest-yield fact here.
Why can a p-n junction not be made by physically joining a p-type slab to an n-type slab? (NCERT Example 14.3)
No junction forms. Any slab, however flat, has surface roughness much larger than the inter-atomic crystal spacing (about 2 to 3 Å). Continuous contact at the atomic level is therefore impossible, and the gap behaves as a discontinuity for the flowing charge carriers. A genuine junction must be created within one crystal by doping one side with donors and the other with acceptors, so that diffusion and the depletion region can develop across a continuous lattice.
p-n junction formation in one screen
- A junction is made inside a single crystal by doping one side n-type (donors) and the other p-type (acceptors), not by pressing two slabs together.
- Diffusion of majority carriers down the concentration gradient: holes p → n, electrons n → p, giving the diffusion current.
- Diffusing carriers leave behind immobile ionised cores — positive donors on the n-side, negative acceptors on the p-side — forming the depletion region (no free carriers; thickness ≈ 0.1–0.5 μm).
- The space charge sets up a built-in field (n → p) and a barrier potential: about 0.3 V for Ge, 0.7 V for Si, opposing further diffusion.
- The field drives a drift current (minority carriers) opposite to the diffusion current; the junction reaches equilibrium when the two are equal, giving zero net current.