Search This Blog

Friday, February 18, 2011



The electrical symbol representation of a four layer pnpn diode is shown in the above figure  and its VI characteristic is also  shown in the above figure

Explain why VI characteristic of the pnpn diode behaves as represented in figure
When the voltage is applied in the reverse direction, the two outer junctions of the switch are reverse biased. At an adequately large voltage, breakdown will occur at these junctions, as indicated, at;the “reverse avalanche” voltage VRA.
When a forward voltage is applied, only a small forward current will flow untill the voltage attains the re akover voltage VBO. The corresponding current is ‘BO If the voltage V, which is ipplied through a resistor is increased beyond VBO, the diode will switch from its OFF (blocked) state to its ON (saturation) state and will operate in the saturation region. The device is then said to latch, if the voltage is now reduced, the switch will remain ON untill the current has decreased to ‘H• This current and the corresponding voltage VH are called holding current and voltage, respectively. The current IN is the minimum current required to hold the switch in its ON state.

 Can the above device be used as an electronic switch? If so why?
The pnpn didoe when biased with the anode positive has two stable states. One is a very high resistance state, typically of the order of 100 M Ohms, and the other a very low resistance state, typically less tha 10 Ohms. When reverse biased, this device acts like a typical pn diode, having a very low leakage current. When external voltage is applied to make the anode positive with respect to the cathode, junctions eJ1 and J3 are forward biased and the centre junction J2 is reverse-biased. The externally impressed voltage appears principally across the reverse-biased junction, and the current which flows through the device is small. As the impressed voltage is increased, the current increases slowly untill a voltage caused the firing or breakover voltage VBQ is reached where the current increases abruptly and the voltage across the device decreases sharply. At this breakover point the pnpn diode switches from its OFF state to the ON state.

No comments:

Post a Comment