Diode are the electronic components which conducts current in one direction(unidirectional).A PN junction is known as a semiconductor diode. Silicon and germanium are used in manufacturing the diode. The schematic symbol of diode is represented by
The arrow in the symbol indicates the direction of easier conventional current flow. A diode has the two terminals when it is connected in a circuit.
- If the external circuit is trying to push the conventional current in the direction of arrow, the diode is forward biased.
- If the conventional current is trying to flow opposite to arrowhead, the diode is reverse biased.
From the above figure:
- If the arrowhead of diode symbol is positive with respect to bar of the symbol, the diode is forward biased.
- If the arrowhead of the diode symbol is negative with respect bar ,the diode is reversed biased.
Formation of PN junction
When a P-type semiconductor is suitably joined to N-type semiconductor ,the contact surface is called pn junction.
When PN junction is formed ,the free electrons near the junction in the n region to diffuse across the junction into the p region. As the free electrons diffuse across the junction ,they combine with holes and leave a layer of positive charges(pentavalent ions) in the n-region and a layer of negative charges (trivalent ions) in the p-region. This forms a barrier potential as shown in figure. This action continues until the voltage of the barrier stops further diffusion. For the silicon, barrier potential is about 0.7V and for germanium it is about 0.3V.
Biasing of the PN junction Diode
The terms bias refers to the use of a d.c voltage to establish certain operating conditions for an electronic device. There are two bias conditions for the PN junction diode i.e.
- Forward bias
- Reverse biias
Forward Biasing of PN junction Diode
when a external d.c voltage applied to the PN junction diode is in such a direction that it cancels the potential barrier permitting the current flow , called forward biasing.
The positive terminal of the battery is connected to P-type and negative terminal to N-type. The applied forward potential establishes an electric filed which acts against the filed due to potential barrier. This results in weakened of the resultant filed and the barrier height is reduced at the junction. As the potential barrier voltage is very small(0.1V to 0.3V),therefore a small forward voltage is sufficient to completely eliminate the barrier. When the potential barrier is eliminated by the forward voltage ,junction resistance becomes almost Zero and a low resistance path is established for the entire path. Hence the current flows in the circuit called the forward current.
- A potential barrier is reduced and at some forward voltage (0.1 V to 0.3V) is is completely eliminated.
- The junction offers low resistance to current flow.
Reversed Biasing of PN junction Diode
when a external d.c voltage applied to the PN junction diode is in such a direction that potential barrier is increased, blocking the current flow , called reverse biasing.
The negative terminal of the battery is connected to P-type and positive terminal is connected to N-type. The applied forward potential establishes an electric filed which acts in same direction as the filed due to potential barrier. This results in increase of the resultant filed and the barrier height is increased at the junction. The increased in potential barrier prevents the flow of charge carriers across the junction. Hence a high resistance path is established for the entire circuit and current does not flow.
- The potential barrier is increased.
- The junction offers very high resistance to current flow.
Characteristics curve of PN junction Diode
When the external voltage is Zero, the potential barrier at the junction does not permit current flow. The circuit current in this case is indicated by the point O in figure.
With the forward bias to the PN junction diode i.e. p -type connected to positive terminal and n-type connected to negative terminal, the potential barrier is reduced. when the potential barrier eliminates at certain voltage current starts to flow in the circuit. The current increases with the increase in forward voltage. Thus the rising curve OB is obtained with forward bias. Form the figure OA ,the current increases very slowly and the curve is non-linear because the external voltage is used in overcoming the potential barrier. Once the external voltage exceeds the potential barrier voltage, the PN junction diode behaves like an ordinary conductor. Therefore the current starts to rises very sharply with increase in external voltage(AB) and is almost linear.
With the reverse bias to the PN junction diode i.e. p -type connected to negative terminal and n-type connected to positive terminal, the potential barrier is increased at the diode junction. There is no flow of current as the junction resistance becomes very high. A very small current(μA) flows in the circuit with reverse bias. This current is due to the minority charge carries.
Uses of diode
- Voltage Regulators