• 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 = (V_in - V_Z)/R_s. If load R_L is connected: I_L = 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.