Type of BJT (bipolar junction transistor)

Type of BJT (bipolar junction transistor) :  

  NPN and PNP:

NPN: A small input current and a positive voltage applied @ its base (with VB>VE) allows a larger current to how from collector to emitter.
PNP: A small output current and a negative voltage @ its base (with VB>VE) allows much larger current to flow from emitter to collector. 

    Working : 

• When no voltage is applied at transistor’s base, electrons in the emitter are prevented from passing to the collector side because of the PN junction.
• If a negative voltage is applied to the base, things get even worse as the PN junction between the base and emitter becomes reverse biased resulting in the formation of a depletion region that prevents current flow. 

• If a positive voltage (>O.6V) is applied to the base of an npn transistor, the pn junction between the base and emitter becomes forward-biased. During forward bias, escaping electrons are drawn to the positive base.
• Some electrons exit through the base, but because the p-type base is so thin. the onslaught of electrons that leave the emitter get close enough to the collector side that they begin jumping into the collector. Increasing the base voltage increases the emitter-to- collector electron flow.
• Recall, positive current flow is in the direction opposite to the electron flow ->current flows from collector to emitter.

 Circuit :

• Normally OFF.
• No current passes from collector to emitter when base is not activated.

• When VB > VE we have an operating circuit.
• Current passes from collector to emitter when base is activated.

     experiment :   

• Turning the switch on/off turns the LED on/off.