PNPN diode is a PN diode with three terminal & four layer devices. In this diode, there is two transistor: NPN Transistor & PNP Transistor
In figure, this is four layer diode with anode A at the outside p region & cathode K at the outside n region.
The forward biased condition of this diode can be considered in two separate states:
- High impedance (Forward Blocking) state
- Low Impedance (Forward Coducting) state
In figure given below, the forward I-V characterstic switches from blocking to conducting state at a critical peak forward voltage Vp.
- As initial positive voltage v junction j1 & j3 are forward biased and junction j2 is reverse biased.
- The forward voltage in the blocking state must appear across the reverse biased junction j2.
- After switching to the conducting state, the voltage from A to K is very small & we conclude that all three junctions are forward biased.
- In reverse blocking mode as v negative j1 & j3 are reverse biased and j2 is forward biased.
- The supply of electrons & holes to j2 is restricted by the reverse biased junction on the either side, the device current is limited to a small saturation current arising from the thermal generation of EHP near j1 & j3.
- The current remains small in the reverse blocking condition until avalanche breakdown occurs at a large reverse bias.
- The mechanism by which this device" often called a Shockley Diode" switches from forward blocking state to forward conducting state.