What is a Diode? PN Junction, Forward/Reverse Bias, LEDs & Rectification

A **Diode** is a specialized two-terminal semiconductor device that acts as a **one-way check valve for electric current**. It allows current to flow freely in one direction (Forward Bias), while providing virtually infinite resistance to current attempting to flow in the reverse direction (Reverse Bias). ### Diode Terminals: - **Anode ($A$, Positive terminal)**: Current enters here. - **Cathode ($K$, Negative terminal)**: Current exits here. - On standard physical cylindrical diodes (like the 1N4001/1N4007), a **silver or white ring band** marks the **Cathode** end.

Diodes are manufactured by joining **P-type silicon** (rich in positive charge holes) with **N-type silicon** (rich in free electrons), forming a **PN Junction**: ### 1. Forward Bias (Conducting): When the positive battery terminal connects to the Anode ($P$) and negative to Cathode ($N$), external voltage overcomes the internal **barrier potential** ($0.7\,\text{V}$ for Silicon, $0.3\,\text{V}$ for Schottky). The depletion zone shrinks and heavy current conducts with a constant $0.7\,\text{V}$ drop across the diode. ### 2. Reverse Bias (Blocking): When positive connects to Cathode ($N$) and negative to Anode ($P$), electrons and holes are pulled away from the junction, widening the depletion barrier. Current is blocked completely (leakage is negligible, $<1\,\mu\text{A}$).

A **Light Emitting Diode (LED)** is a specialized diode designed to release photon energy (light) when electrons fall across its bandgap. Unlike incandescent light bulbs, LEDs have **zero internal resistance once conducting**. Connecting an LED directly to a $5\,\text{V}$ battery will cause infinite current flow and instantly vaporize the silicon die! ### LED Current-Limiting Resistor Formula: $$R = \frac{V_{\text{supply}} - V_{\text{forward}}}{I_{\text{LED}}}$$ ### Typical Forward Voltage Drops ($V_f$) by Color: - **Red / Amber**: $1.8\,\text{V} - 2.0\,\text{V}$ - **Green / Yellow**: $2.1\,\text{V} - 2.4\,\text{V}$ - **Blue / White**: $3.0\,\text{V} - 3.3\,\text{V}$ - Target Current ($I_{\text{LED}}$): Typically $10\,\text{mA}$ to $20\,\text{mA}$ ($0.015\,\text{A}$). *Example*: Sizing a resistor for a Red LED ($V_f = 2.0\,\text{V}$) on a $5\,\text{V}$ Arduino pin at $15\,\text{mA}$: $$R = \frac{5.0\,\text{V} - 2.0\,\text{V}}{0.015\,\text{A}} = \frac{3.0\,\text{V}}{0.015\,\text{A}} = 200\,\Omega \quad (\text{Use standard } 220\,\Omega)$$

A **Full-Wave Bridge Rectifier** arranges 4 diodes in a diamond configuration to convert both positive and negative AC half-cycles into pulsating positive DC: - During the positive half-cycle, diodes $D_1$ and $D_2$ conduct. - During the negative half-cycle, diodes $D_3$ and $D_4$ conduct, steering current into the load in the exact same direction. - A reservoir capacitor placed across the output smooths the remaining ripples into steady DC.