Semiconductor –Part-III : Semiconductor –Part-III P-n junction diode
P-n junction diode under forward bias
P-n junction diode under reverse bias
Characteristics of p-n junction diode
Rectifier
Junction transistors
Semiconductor devices : Semiconductor devices The devices formed with the suitable combination of n-type and p-type semiconductors are known as semi-conductor devices e.g. semiconductor diodes and transistors.
P-n junction diode : P-n junction diode A single piece of a semiconductor material (either Si or Ge) whose one portion is doped with pentavalent impurity and the other portion is doped with trivalent impurity behaves as p-n junction. In fact , the boundary dividing the two halves or portions of such a semiconductor is called a junction and the arrangement is known as p-n junction.
P-n junction diode : P-n junction diode P-n junction diode or semiconductor diode consists of a p-n junction having metallic contacts at both the ends of the p-n junction .
Depletion layer and potential barrier.
Forward bias : Forward bias When a battery of e.m.f. greater than the barrier potential (vb) is connected across a p-n junction diode in such a way that the positive terminal of the battery is connected to p-region and the negative terminal of the battery is connected to the n-region of the junction diode, then the p-n junction diode is said to be forward biased. The thickness of depletion layer and the junction resistance decreases.
Circuit Diagram for forward bias : Circuit Diagram for forward bias Hole
Electron P----n
Reverse bias : Reverse bias A p-n junction is said to be reverse biased when the positive terminal of a battery is connected to the n-region and negative terminal is connected to the p- region of the p-n junction diode.
In reverse biasing , the thickness of depletion layer and the junction resistance increases.
Circuit Diagram for reverse bias : Circuit Diagram for reverse bias Hole
Electron n p
Forward bias characteristic : Forward bias characteristic The battery voltage at which the forward current starts increasing rapidly is known as knee voltage(vk) .After knee voltage, the junction diode behaves almost like a conductor. The variation of current with voltage applied across the junction diode is almost linear. However, there is a limit of current that can pass through the diode without damaging it. A high current through the diode produces a large amount of heat which may burn the diode.
Reverse bias characteristic : Reverse bias characteristic As the reverse voltage is increased to a certain value, called break down voltage, large amount of covalent bonds in p and n regions are broken. As a result of this, large electron-hole pairs are produced which diffuse through the junction and hence there is a sudden rise in the reverse current. Once break down voltage is reached, the high reverse current may damage the junction diode.
Current, static and dynamic resistance of a junction diode : Current, static and dynamic resistance of a junction diode The current in a p-n junction diode is given by
I=Io(eev/kT-1)
Rs=V/I
Rd=V/I
Rs-Static resistance
Rd-Dynamic resistance
K-Boltzmann constant
Io-Saturation current
T-Temperature
Junction diode as rectifier : Junction diode as rectifier A device which converts alternating current into direct current is known as rectifier. The process of converting a.c. into d.c. is known as rectification.
PRINCIPLE- Junction diode conducts only when forward biased and it does not conduct when reverse biased.
There are two types of rectifier-Half wave rectifier and Full wave rectifier.
Half and full wave rectifier : Half and full wave rectifier Half wave rectifier-The rectifier which converts only one half of a.c.into d.c. is called half wave rectifier.
Full wave rectifier-Full wave rectifier rectifies both halves of a.c. input signal.
Home work questions : Home work questions 1.Explain why there is a very small current across the junction, when a p-n junction diode is reverse biased. Does it depend on the applied voltage?
2.The resistance of p-n junction diode decrease when forward biased. Explain why?
3.In the following circuit, which of the diodes is forward biased and which is reverse biased. Why?