Diodes

What is a Diode?

A diode is a two-terminal semiconductor device that allows current to flow easily in one direction (forward) while blocking it in the opposite direction (reverse). Think of it as a one-way valve for electricity. The two terminals are called the anode (+) and cathode (−).

The PN Junction

A diode is made by joining P-type semiconductor (excess holes) and N-type semiconductor (excess electrons). At the boundary — the PN junction — electrons and holes recombine, creating a thin depletion region with no free carriers. This region acts as an insulating barrier.

• •P-type — semiconductor doped to have extra 'holes' (missing electrons) that act as positive charge carriers.
• •N-type — semiconductor doped to have extra free electrons as negative charge carriers.
• •Depletion region — the thin zone at the junction where carriers have recombined, creating an electric field that opposes further current flow.

Forward Bias

When the anode is made more positive than the cathode by at least the forward voltage (V_f), the depletion region shrinks, carriers flood across the junction, and current flows freely. For silicon diodes, V_f ≈ 0.7 V. For germanium diodes, V_f ≈ 0.3 V. Once forward-biased, the diode acts almost like a closed switch with a small constant voltage drop.

Reverse Bias

When the cathode is more positive than the anode, the depletion region widens, and only a tiny leakage current flows (nanoamps to microamps). The diode effectively blocks current. If the reverse voltage exceeds the breakdown voltage (V_BR), the diode suddenly conducts heavily in reverse — this is destructive for regular diodes but is used intentionally in Zener diodes.

The I-V Characteristic Curve

If you plot current (I) vs. voltage (V) for a diode, you get a distinctive curve: almost zero current below V_f, then current rises exponentially once V_f is reached. In reverse, current is near zero until breakdown voltage. For most circuit analysis, the simplified model 'V_f = 0.7 V when on, infinite resistance when off' works well enough.

Common Types of Diodes

• •Standard rectifier diode (1N400x series) — general purpose, handles 1 A. Used in power supplies.
• •Signal diode (1N4148) — small, fast, low current. Used for logic, clamping, signal detection.
• •Schottky diode — lower forward voltage (~0.2–0.4 V), very fast switching. Used in high-frequency and power circuits.
• •Zener diode — designed to conduct reliably in reverse at a specific breakdown voltage. Used for voltage regulation and reference.
• •Schottky vs. standard: Schottky is faster and wastes less voltage but handles less reverse voltage.

Rectification — Converting AC to DC

The most common application of diodes is rectification: converting alternating current (AC) into direct current (DC). Since diodes only pass current in one direction, they block the negative half of an AC waveform.

• •Half-wave rectifier — single diode passes only the positive half-cycle. Simple but wastes half the power.
• •Full-wave bridge rectifier — four diodes arranged in a bridge pass both half-cycles (flipping the negative one). Much more efficient.
• •After rectification, a smoothing capacitor reduces the ripple to produce a more steady DC voltage.