• Tags: Zener, breakdown, regulation
• Difficulty: Moderate

A Zener diode is a heavily doped p-n junction with a precisely controlled breakdown voltage $V_Z$ (typically 2-200 V). Two breakdown mechanisms: (1) Zener breakdown ($V_Z$ < 6 V): intense electric field in the thin depletion region directly pulls electrons from covalent bonds (field ionization). (2) Avalanche breakdown ($V_Z$ > 6 V): minority carriers gain enough kinetic energy to ionize atoms through collisions, creating an avalanche of carriers. In a voltage regulator circuit: Zener in reverse bias, series resistor $R_s$. As long as $V_{in}$ > $V_Z$: $V_{out}$ = $V_Z$ (constant). The series resistor absorbs voltage fluctuations: $I_s$ = $\frac{V_in - V_Z}{R_s}$. If load $R_L$ is connected: $I_L$ = $\frac{V_Z}{R_L}$, $I_Z$ = $I_s$ - $I_L$. For regulation to work: $I_Z$ must remain positive (otherwise $V_{out}$ drops below $V_Z$). JEE problems: calculate $R_s$ range for regulation, or find $I_Z$ for given $V_{in}$ and $R_L$.