Types of electronic components , pn junction connection & uses
Most of electronic components & devices are made of semiconductors which are known to be sensitive to environmental conditions such as light , heat , pressure , radiation and chemical pollution , That is why they are used as sensors or means for measuring external stimuli , Electronic components & devices are the building blocks for all the electronic systems .
Types of electronic components
- Simple components : Such as resistor ( R ) , induction coil ( L ) , capacitor ( C ) .
- Complex components : such as pn junction ( diode ) and transistor .
- Specialized components : such as optoelectronic and current control devices .
The pn junction
Explaining the way of working :
In the p-type crystal , the concentration of holes ( p ) is greater than the concentration of electrons ( n ) , while concentration of holes in the n-type crystal is lower than the concentration of electrons .
Some holes diffuse from the p-type region to the n-type region , Also , some electrons diffuse from the n-type region ( high concentration for electrons ) to the p-type region ( low concentration for electrons ) which is called diffusion current .
The transfer of some electrons from the n-type region do not cover all the holes from the positive donor ions and the transfer of some holes from p-type region do not cover all the electrons from the negative acceptor ions , This results in a middle region free of charge called transition ( depletion ) region with positive ions on one side and negative ions on the other side .
When the crystal of n-type loses some of its electrons , it acquires a positive voltage while the crystal of p-type acquires negative voltage due to transferring electrons to it , thus an electric field is set up and its direction from n-crystal ( positive voltage ) to p-crystal ( negative voltage ) , that generates electric current called drift current which is opposite to the diffusion current .
By continuous transferring of electrons and holes from high concentration to low concentration , the potential difference between the two crystals increases up to a certain value where the diffusion current = Drift current where there is no transferring of electrons from n to p and the potential difference is called the voltage barrier of pn junction .
Diffusion current is the current which resulted from the diffusion of positive holes from region ( p ) to region ( n ) and diffusion of electrons from region ( n ) to region ( p ) at the contact position of the two crystals , The depletion ( transition ) region is the region which does not contain charge carriers but at its two sides the crystal ( n ) and crystal ( p ) in pn junction .
Drift current is the current which is resulted from the internal electric field between positive ions in the n region and negative ions in the p region at the two sides of the contact region of the pn junction and it is opposite to the diffusion current .
The voltage barrier of pn junction is the least potential difference at the two sides of the p and n contact position which is enough to prevent the diffusion of more holes and free electrons to the low concentration region .
When the voltage barrier of pn junction = 0.3 V , It means that the least potential difference at the two sides of the p and n contact position which is enough to prevent the diffusion of more holes and free electrons to the low concentration region = 0.3 V .
Connection of pn junction
The pn junction is connected by two methods in the electric circuit :
Forward bias of pn junction
P-type crystal is connected to the positive terminal of the battery and n-type crystal is connected to the negative terminal of the battery , The direction of the external field ( due to battery ) is opposite to the direction of the internal field in the transition region and therefore , it weakens it .
Voltage of pn junction becomes less than the barrier voltage , Thickness of the transition region decreases , ( holes and electrons repel with the poles of the battery and move closer to the separating surface ) , Resistance of the junction ( R ) is small , High intensity electric current passes through the junction .
Reverse bias of pn junction
P-type crystal is connected to the negative terminal of the battery and n-type crystal is connected to the positive terminal of the battery , The direction of the external field ( due to battery ) is in the same direction of the internal field in the transition region and therefore , it strengthens it .
Voltage of pn junction becomes greater than the barrier voltage , Thickness of the transition region increases , ( holes and electrons attract with the poles of the battery and move far away from the separating surface ) , Resistance of the junction ( R ) is large , The electric current intensity is very weak almost vanished , I = 0 .
Uses of pn junction
- Used as a switch : When the pn junction is connected in the forward direction , it allows the flow of the electric current in the circuit , thus it acts as a closed switch , While when it is connected in the reverse direction it does not allow the current to flow through the circuit and it acts as an opened switch .
- Rectification of a alternating current : It means converting the alternating current ( which is changeable in intensity and direction ) into a unidirectional current which is used in charging the car battery and the cell phone , We can convert the AC current into DC current by using many pn junctions .
Ohmmeter can be used to make sure that the pn junction is functioning , such that the diode has a small resistance in the forward direction and a large resistance in the reverse direction , Ohmmeter can be used to differentiate between the pn junction and the ohmic resistance :
- In case of pn junction : the reading of ohmmeter is very large in the forward direction and very small in the reverse direction .
- In case of an ohmic resistance : the reading of ohmmeter does not change if the direction of current is reversed .
We can compare between the pn junction and the ohmic resistance :
Pn junction consists of two attached crystals n and p , The charge carriers are free electrons or holes , The electric current intensity passes in one direction and does not pass in the reverse direction , By increasing the temperature , the resistance decreases and the conductivity increases .
Ohmic resistance consists of coil of wire of suitable resistivity such as tungsten or nichrome , The charge carriers are free electrons , The electric current intensity passes in two directions , By increasing the temperature , the resistance increases and the conductivity decreases .
To tune up a TV or radio onto a certain station , we need to adjust the value of a capacitor to set the frequency of the receiver to the frequency of the selected broadcast station , this condition is called resonance .
In modern receivers , the capacitor is replaced by a reverse biased pn junction , The width of the transition region increases with increasing reverse bias , The increase of the width of the transition region means an increase of the fixed ionic charge on both sides of the transition region with reverse voltage , This is tantamount to capacitor action .
Thus , we can change the value of the capacitor by controlling the reverse voltage , This is called electronic tuning ( and the device is called a varactor ) .