Answer: In reverse bias the Zener current rises sharply at the breakdown voltage \(V_Z\) while the voltage stays almost constant; placed in reverse across the load with a series resistor, it holds the load voltage at \(V_Z\).
V-I characteristics (describe the graph): \(V\) on the x-axis, \(I\) on the y-axis.
- Forward region (first quadrant): like an ordinary silicon diode; negligible current up to about 0.7 V, then current rises steeply.
- Reverse region (third quadrant): only a tiny leakage current until the reverse voltage reaches \(V_Z\). At \(V_Z\) breakdown occurs (Zener effect for heavily doped diodes below about 5 V, avalanche effect above about 6 V) and the curve becomes almost vertical: current increases from \(I_{Z(min)}\) (knee current) to \(I_{Z(max)}\) while the voltage stays nearly constant at \(V_Z\). Beyond \(I_{Z(max)}\) the diode is damaged by heat.
Voltage regulator circuit: unregulated DC input \(V_{in}\) → series resistor \(R_S\) → Zener diode connected in reverse bias (cathode to positive) in parallel with the load \(R_L\).
\[I_S=\frac{V_{in}-V_Z}{R_S},\qquad I_S=I_Z+I_L,\qquad V_o=V_Z.\]
Line regulation: if \(V_{in}\) rises, \(I_S\) rises; the extra current flows through the Zener (\(I_Z\) increases), the drop across \(R_S\) increases, and \(V_o\) stays at \(V_Z\). If \(V_{in}\) falls, \(I_Z\) decreases and again \(V_o\) stays constant.
Load regulation: if \(R_L\) decreases (\(I_L\) increases), \(I_Z\) decreases by the same amount, so \(I_S\) and the drop across \(R_S\) remain the same and \(V_o=V_Z\).
Condition: regulation holds as long as \(I_{Z(min)}\le I_Z\le I_{Z(max)}\).
