Draw the characteristics of Zener diode and Explain application of a Zener diode as a voltage regulator

Zener diode: a heavily doped silicon PN junction diode designed to operate in the reverse breakdown region without damage. Symbol: diode with bent (Z-shaped) cathode bar.

              I_F (mA)
                |        /
                |       /   forward: like normal Si diode,
                |      /    knee ~ 0.7 V
   -Vz          |    _/
 ---+-----------+---------------- V_F
    |  reverse  | 0  0.7 V
    |  leakage  |
    |  (very small, uA)
    |  <- knee at Vz (I_ZK)
    |
    |  breakdown region: voltage stays ~ Vz
    |  while current rises (I_ZK ... I_ZM)
    v I_R (mA)

• Forward bias: behaves like an ordinary diode; negligible current up to cut-in voltage (~0.7 V for Si), then current rises steeply.
• Reverse bias below V_Z: only a tiny reverse saturation (leakage) current flows.
• Breakdown at V_Z: at the Zener voltage the reverse current increases sharply while the voltage across the diode remains almost constant at V_Z. The curve is nearly vertical between the knee current I_ZK (minimum current to stay in regulation) and I_ZM (maximum current set by power rating P_Z = V_Z I_ZM).
• Mechanism: for V_Z below about 5 V, Zener breakdown (high field across a very thin depletion layer pulls electrons out of covalent bonds); above about 6 V, avalanche breakdown (carriers gain enough energy to ionise atoms by collision).
• Dynamic resistance r_Z = ΔV_Z/ΔI_Z is very small, which is why it is used as a voltage regulator/reference.

Zener diode as a voltage regulator (shunt regulator).

        Rs        I_L
 +o---/\/\/---+--------+
   ^   I ->   |        |
   |   I_Z    |       R_L   V_o = V_Z
  Vin        _|_ cathode up |
   |         /_\ Zener      |
 -o-----------+--------+

A series resistor R_S is connected from the unregulated DC input V_in to the Zener diode, which is reverse-biased (cathode to +) and placed in parallel with the load R_L. Output V_o = V_Z.

• \( I = \dfrac{V_{in}-V_Z}{R_S} = I_Z + I_L \).
• Line regulation (V_in changes, R_L fixed): if V_in rises, I rises; since V_Z is constant, the extra current flows through the Zener (I_Z increases) and the extra voltage drops across R_S. V_o stays at V_Z. The reverse happens if V_in falls.
• Load regulation (R_L changes, V_in fixed): I is constant; if I_L increases, I_Z decreases by the same amount, so V_o stays at V_Z.
• Condition: the diode must stay in breakdown, I_ZK < I_Z < I_ZM; R_S is chosen so that \( R_S = \dfrac{V_{in(min)}-V_Z}{I_{L(max)}+I_{ZK}} \) and the Zener power \( V_Z I_{Z(max)} \le P_Z \).

Uses: reference voltage in power supplies, regulated low-power DC supplies, over-voltage protection.