Draw and explain the V-I characteristics of a Zener diode

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.