The PN Junction Diode

If you push two blocks of oppositely doped silicon together — one N-type, one P-type — something interesting happens right at the boundary, without any external power applied at all. That boundary is called a PN junction, and it’s the single most important structure in all of semiconductor electronics: the ancestor of the diode, the transistor, and ultimately every logic gate in a modern chip.

What happens when N-type meets P-type

Recall from Doping: N-Type and P-Type that N-type silicon has a surplus of mobile free electrons, and P-type silicon has a surplus of mobile holes. The instant these two regions are joined:

Free electrons near the junction on the N-side notice a huge concentration of holes just across the border and diffuse across to fill them (and vice versa for holes diffusing into the N-side).
As electrons and holes cross and recombine near the boundary, they leave behind the fixed, charged dopant atoms that can’t move — positively charged phosphorus ions on the N-side, negatively charged boron ions on the P-side.
This strip of exposed fixed charge, now stripped of mobile carriers, is called the depletion region. The exposed ions create an internal electric field that opposes further diffusion, and the process quickly reaches equilibrium.

🧩 Think of it like… two rooms — one packed with people (electrons on the N-side), one nearly empty (holes on the P-side) — connected by an open doorway. People wander across into the empty room until enough of them have crossed that a kind of "social pressure" builds up right at the doorway, discouraging more people from crossing. That equilibrium zone right around the doorway, where the crowd has thinned out on both sides, is the depletion region.

Cross-section of the junction at equilibrium

P-typeN-typeDepletion region

⊕⊕⊕⊕⊕

e⁻e⁻e⁻e⁻e⁻

⊖⊖⊖⊕⊕⊕

built-in electric field (points N → P)

AnodeCathode

The P-side terminal of a diode is called the anode, and the N-side terminal is the cathode — terms you’ll see on every diode datasheet and schematic symbol.

Forward bias: opening the gate

If you connect a battery’s positive terminal to the P-side (anode) and negative terminal to the N-side (cathode), you’re pushing electrons and holes toward the junction, opposing the built-in field. Once the applied voltage is large enough — about 0.6–0.7 V for silicon — it overcomes the built-in barrier, the depletion region narrows dramatically, and current flows easily through the diode.

Reverse bias: widening the gate

Connect the battery the other way — positive to the N-side, negative to the P-side — and you pull carriers away from the junction. The depletion region widens, the barrier grows, and almost no current flows (aside from a tiny “leakage current” from a small number of thermally generated carriers). This is what makes a diode a one-way valve for current.

The diode equation

The relationship between voltage across the diode and the current through it is captured by the Shockley diode equation:

I = I_0\left(e^{V/V_T} - 1\right)

Here’s what each symbol means:

$I$ is the current flowing through the diode.
$I_0$ is the reverse saturation current — the tiny leakage current that flows under reverse bias, determined by the doping levels and geometry of the specific diode.
$V$ is the voltage applied across the diode (positive in forward bias, negative in reverse bias).
$V_T$ is the thermal voltage, about 25.85 mV at room temperature (300 K), coming from $V_T = kT/q$ , where $k$ is Boltzmann’s constant, $T$ is absolute temperature, and $q$ is the electron charge.

Because $V$ appears in an exponent, forward current rises extremely steeply once $V$ exceeds a fraction of a volt — this is why silicon diodes appear to have almost no current at all below about 0.6 V, then shoot up sharply just above it. In reverse bias ( $V$ negative), the exponential term vanishes and $I$ settles at just $-I_0$ , the small leakage current.

Why the PN junction matters beyond diodes

A standalone diode is useful for rectifying AC power or protecting circuits, but the PN junction’s real significance is structural: it’s the fundamental building block inside the Bipolar Transistor (which uses two junctions back-to-back) and it also appears inside the MOSFET, even though the MOSFET operates on a different core principle (an electric field controlling a channel, rather than direct current injection across a junction).

Key takeaways

Joining N-type and P-type silicon creates a depletion region — a zone stripped of mobile carriers by an internal electric field.
Forward bias narrows the depletion region and allows current to flow; reverse bias widens it and blocks current.
The diode equation $I = I_0(e^{V/V_T}-1)$ describes this exponential, one-way current behavior.
Silicon diodes typically “turn on” around 0.6–0.7 V forward bias.
The PN junction is the foundational structure behind the Bipolar Transistor and the MOSFET.

The Bipolar Transistor