PN JUNCTION DIODE

 PN junction diode:

                       A PN device is known as a diode. This is a unidirectional device (i.e.) the current flow in one direction but not in the opposite direction. These unilateral conduction characteristics of PN junction diode permits it to do the job of rectification. So PN junction diode is act as a rectifier.

 Construction:

                        A semiconductor diode or PN junction diode is made by combining P and N materials to form a PN junction.

             The P type material has high concentration of  holes while the N type material has high concentration of free electrons. Therefore at the junction there is a tendency for free electrons to diffuse over the P side and holes to N side. This process is called diffusion PN junction diode has two layer, two terminal and a junction. Terminal taken from the P side is known as Anode and the terminal taken from the N side is known as Cathode.

Working principle:

 The DC voltage across a PN junction can be applied in two ways, namely

             1. Forward biasing

             2. Reverse biasing

Forward biasing:

                        When external dc voltage applied to the PN junction is in such a direction that it cancels the barrier potential thus permitting current flow it is called forward biasing. A forward bias of PN junction diode is established by applying positive potential of the battery (voltage) is connected to Anode and it's negative is connected to Cathode as shown in figure 

            Under the forward bias condition the applied positive potential repels the holes in S type region so that the hole move towards the junction and the applied negative potential repels the electrons in the N type region and the electrons move towards the junction. Hence the width of the depletion region is decreased. As barrier potential is very small therefore a small forward voltage is completely eliminates the potential barrier. Once the barrier potential is eliminated by the forward voltage junction resistance becomes almost zero and the conventional current flows from positive terminal of the battery across the p to N junction and to the negative terminal of the battery and is called forward current or Anode current.  

Reverse biasing:

                        When the external dc voltage applied to PN Junction is in such a  direction that barrier potential is increased it is called reverse biasing.  

                        If an external voltage is applied to PN junction such that positive terminal of the battery is connected to Cathode and the negative terminal of the battery is connected to the Anode a reverse bias condition is established.

                       Under reverse bias condition the applied positive potential attracts the electrons in the N type reign so that the electrons move towards the battery and the applied negative potential attracts the holes in P type region so that the holes move towards the battery. Hence the width of the depletion region is increased. The net effect is widening of the depletion region and the barrier potential rises. The increased barrier potential prevents the flow of charge carriers across the junction. Thus a high resistance path is established for the entire circuit and hence the current dose not flow. However the minority carriers will cross the junction at reverse bias voltage and contribute to reverse current (Ir). The minority carrier current reaches its saturation value at even very low reverse bias voltage because of the low concentration of minority carriers. The current that exists under reverse bias condition is called reverse saturation current.

V-I characteristics:

                          The volt amphere characteristics of a PN junction diode is shown in figure. It is a graph between the voltage applied across its terminals and the current that flows through it. It tells ur how much diode current flows for a particular value of diode voltage.   

                        Under forward bias condition as the forward voltage (Vf) increase for forward voltage less than the barrier potential(Vb) the forward current. (If) is almost zero because the potential barrier prevents the electrons flow from N region and holes from Pregion to flow across the depletion region in the opposite direction. For PN junction diode the forward current ir very small up to Vf=Vb=0.7V for silicon and 0.3V for germanium called cut in voltage or threshold voltage or knee voltage. For Vf>Vb the potential barrier at the junction completely disappears and hence the holes across the junction from P type to N type and the electrons cross the junction in the opposite direction resulting in relatively large current flow in the external circuit.                        

                       Under reverse bias a small amount of current in few micro amphere flows in the circuit. This is called reverse current and is due to the minority carriers. To these minority carriers a very small current called leakage current flows in the reverse direction. The leakage current stays at a  small value as  the reverse voltage is increased a large number of free electrons are formed which is called as an avalanche of free electrons. This leads to the break down of the junction leading to very large reverse current. The reverse voltage at which the junction break down occurs is known as break down voltage. As the reverse bias voltage in increased above the break down voltage there is a sudden rise of reverse current and a sudden fall of the resistance of depletion region. This may damage the junction permanently due to excessive heat.