A1 Circuits with Diodes

Topic

The diode is the simplest semiconductor device: it allows current to flow easily in one direction while virtually blocking it in the opposite direction, up to the point where a breakdown voltage is reached. The junction between two regions of differing doping—as described by the semiconductor physics model—produces a current-versus-voltage curve that is not a straight line (like that of a resistor) but an exponential curve: below a forward voltage of a few tenths of a volt (in silicon), it barely conducts, whereas above that threshold, the current rises very rapidly.

This property makes the diode the central component of any rectifier—the circuit that converts alternating current into direct current. A full-wave rectifier produces a pulsating signal that repeats at twice the frequency of the original mains supply; this signal is smoothed by filtering it with a capacitor—utilizing the energy-storage capability described by the reactive components model—where the capacitor's size determines the amount of residual ripple at the output: the larger the capacitor, the lower the ripple.

The Zener diode is a special type designed to operate stably in its reverse breakdown region, maintaining a virtually constant voltage across its terminals over a specific current range; it is the simplest existing voltage regulator. LEDs are diodes that, rather than dissipating recombination energy as heat, emit it as light; the color of this light depends directly on the material's bandgap, a property established by the semiconductor physics model. Schottky diodes, characterized by a lower forward voltage drop and a much faster response time than conventional diodes, are typically used in circuits designed to protect against reverse polarity connections or voltage transients. That is why this module—which opens with a junction that allows current to flow in only one direction—concludes by showing that this same basic asymmetry (conducting in one direction while blocking in the other) is what, depending on how it is utilized, yields four devices as distinct from one another as a rectifier, a regulator, a light source, and a reverse-connection protector.

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