Thyristor V-I, avalanche and forward breakdown

The thyristor (SCR) is a four-layer PNPN device with anode A, cathode K and gate G. Its V-I characteristic shows anode current $I_A$ against anode-cathode voltage $V_{AK}$.

  Symbol          Structure
                     A
     A               |
     |             +---+
    _V_            | P |
     |\            +---+ J1
     | \ G         | N |
     K             +---+ J2
                   | P |---- G
                   +---+ J3
                   | N |
                   +---+
                     |
                     K

              IA
               ^
               |  |<- ON state (VT = 1-2 V)
               |  |
         IL ---|--|
         IH ---|--+.
               |    `.   Ig2 > Ig1 > Ig=0
               |      `-.___      ___
     -VBR      |            `----'   |
  -----+-------+---------------------+--> VAK
       |       |  forward blocking  VBO
       |       |  (leakage current)
       | reverse blocking
       | (leakage current)
       v reverse avalanche

• Avalanche (reverse) breakdown: in reverse bias, $J_1$ and $J_3$ block. When the reverse voltage reaches the reverse breakdown voltage $V_{BR}$, carriers gain enough energy to knock out more carriers (avalanche multiplication), and the reverse current rises sharply. This normally destroys the device, so it is operated below $V_{RRM}$.
• Forward breakover voltage $V_{BO}$: in forward bias with gate open, $J_2$ blocks. $V_{BO}$ is the forward voltage at which $J_2$ breaks down by avalanche and the SCR switches from forward blocking to the ON state without any gate signal. With gate current, the breakover voltage becomes smaller.