Semiconductors are those materials whoseconductivity lies between insulators and conductors. At 0 Kelvin semiconductorsbehave as insulators but at room temperature some of the valence electrons movetowards the conduction band. This is property by which we can distinguishsemiconductors from insulators and conductors. For conductors the valence bandand conduction band overlaps and for the insulators the energy gap differenceis 7eV but for the semiconductors it is approximately 1.107eV. Therefore atroom temperature the valence electrons get enough energy to move from valenceband to the conduction band.
There are two types of semiconductors
· Intrinsic semiconductors
· Extrinsic semiconductors
Intrinsic semiconductors are those semiconductors inwhich no impurity is added. For example silicon semiconductor containing allthe silicon atoms and not even a single impurity is added is intrinsic siliconsemiconductor.
In this type of semiconductors all the conductivityis due to the rupturing of covalent bond. In other words due to heating whenthe covalent bond breaks and move towards the conduction band the valenceelectrons move towards the conduction band. For a single structure of silicon thereis 5×1022/cm3 and only one of every billion atom isionized.
Extrinsic semiconductors are those semiconductors inwhich impurity is added and this process is called doping.
Typesof extrinsic semiconductors:
There are two types of extrinsic semiconductors
P-type semiconductors are that semiconductor inwhich trivalent impurity is added i.e. the impurity atom belongs to the 3-Agroup of the periodic table which has three electrons in the last shell. Forexample aluminum is selected.
It only formbond with the three valence electrons of silicon and three single bonds will beformed. There will be one vacant space named as hole. This hole will also helpfor the conductivity. An electron from the other silicon atom will come to fillthis space and a hole will be generated there. Similarly another electron fromany other silicon atom will come to fill this gap and hole will be generatedagain. In this way this process will continue and the electrons will movetowards the conduction band. One thing should be remembered that the electronsand hole move in the opposite directions. During this process temperature(heat) will be provided to the semiconductors for the rupturing (breaking) ofcovalent bond of silicon to silicon atoms.
The charge carriers which are in excess are named asmajority charge carriers. In p-type material holes will be the minority chargecarriers
The charge carriers which are lesser in amount arecalled minority charge carriers. In p-type materials electrons are minoritycharge carriers.
Thermal ionization (ionization by providing heat)results in the formation of free electrons and holes and due to this ionizationthe number of electrons is equal to the number of holes. These holes andelectrons move randomly throughout the silicon lattice and during this processsome of the electrons may fill some of the holes. This process is calledrecombination and due to this processes some if the free electrons and holesmay disappear. So the number of holes and the number of free electrons may notremain equal to the free number of electrons. The rate of recombination isdirectly proportional to the free number of electrons and holes and it dependsupon the ionization rate.
N-type semiconductors are that semiconductor inwhich a pentavalent impurity is added i.e. the impurity belongs to the 5-Agroup of the periodic table which has five electrons in the valence shell. Forexample phosphorus is selected.
Four of thevalence electrons of the impurity will form bond with the four valenceelectrons of the silicon atom and the fifth electron will be free. This fifthelectron will move towards the conduction band and form a bound positive ion.Remember that there is a difference between hole and a bound positive ion. In thehole there is a free space for the formation of a single bond but in the boundpositive ion all the four electrons are bonded to the silicon and this fifthelectron was free and has moved to the conduction band. There is no free spacefor the formation of any other bond. This fifth electron will towards theconduction band. In the mean while temperature will be provided to the siliconstructure due to which rupturing of covalent bond will continue and it willprovide free electrons to the conduction band. Meanwhile hole will also begenerated and the electrons from the other silicon atoms will come to fill thisvacant space. Same process will continue hole will move downward and theelectrons will move upward.
The charge carriers which are in excess are named asmajority charge carriers. In n-type material electrons will be the minoritycharge carriers
The charge carriers which are lesser in amount arecalled minority charge carriers. In n-type materials holes are minority chargecarriers.
Now we will discuss the formation of a pn-junction.Pn junction is formed by adding trivalent impurity at one side of the siliconstructure and a pentavalent impurity at the other side of the pn junction.
Remember that pn junction cannot be formed byjoining p-type and n-type semiconductors.
PNjunction incase battery is not attached:
When the battery is not attached to the pn junctionfollowings processes can be happen
· Diffusion of charges & formation ofdepletion region:
· Electric field direction
Diffusionof charges & formation of depletion region:
After the formation of pn junction the electrons arein abundance in n-type side and holes are in abundance in p-type. So theelectrons of n-type material will diffuse towards the p-type side and combinewith the holes and form the neutral atoms. Due to this the number of electronswill decrease in n-side. When the numbers of electrons decrease in the n-sideit will give rise to positive charge. Similarlywhen the electrons combine with the holes it will reduce the number of holes inp-side. Due to decrease of number of holes the negative charge will appear atp-side of the junction. Remember that the positive and negative charges due tothe depletion region are different from the holes and electrons of p &n-side of the junction respectively. These positive and negative charges arecalled mobile ions and they are formed due to the decrease in the number ofholes and electrons in the p and n-side of the junction respectively due to thediffusion of charges.
You can see in the depletion region the holes(represented with +) are migrated towards n-type region.
And the electrons (represented with -) are migratedtowards p-type region. This will produce electric field. In case of silicon thedepletion region will be of 0.6 to 0.7V. In case of germanium it will be about0.2 to 0.3V. The electric field will be directed from positive towards negativeas shown in the diagram. This electric field will allow the electrons to movefrom right to left and holes to move from left to right as electrons flow inthe opposite direction of the electric field and holes move in the direction ofelectric field.
PNjunction incase battery is attached:
When the battery is connected to the pn junctionthen there are two possibilities.
· Forward biased pn junction
· Reverse biased pn junction
Forwardbiased pn junction:
When the positive terminal of the battery isconnected to the p-side of the junction and negative terminal of the battery isconnected to the n-side of the junction then it is called forward biased pnjunction or forward biased diode.
In the forward biased following things happen.
· Decrease of depletion region
· Drift current
· Diffusion current
Decreaseof depletion region:
When the diode is forward the depletion region willdecrease. Now it is important to know that how the depletion region willdecrease. The negative side of the battery will repel the electrons and thepositive side of the battery will repel the holes. When the electrons of then-type region try to cross the potential barrier (depletion region) they cannotbecause they don’t have enough energy to cross it. They will combine with thepositive mobile ions and neutralize them. Similarly the holes of p- side regionwill try to cross the potential barrier. In trying to do so the holes willcombine with the negative mobile ions and neutralize them. This neutralizationwill decrease the depletion region and now the holes of the battery can beattracted towards the negative terminal of the battery and the electrons can beattracted towards the positive terminal of the battery.
The current which flows due to the majority chargecarriers is called diffusion current. In the forward biased pn junction thecurrent flows due to majority charge carriers i.e. in p-type the holes and inn-type the electrons.
It is the diffusion current in the forward biased pnjunction and with a very small change in the voltage we can see a big change inthe current. This can be illustrated by the graph given below. You can see inthe graph with the increase of roughly 2 volts the current can be increasedfrom 5mA to 50 mA. This increases the importance of the junctions. The resistanceis added to limit the current.
Drift current is the current which flows due tominority charge carriers so I will explain it there in the reverse biased pnjunction.
Reversebiased pn junction:
When the negative terminal is connected to thep-side of the junction and positive terminal of the battery is connected to then-side of the junction then it is called reverse biased pn junction.
In the reverse biased pn junction following thingscan happen
· Increase in the depletion region
· Drift current
· Diffusion current
Increasein the depletion region:
When the junction is in reverse biased conditionthen the positive side of the battery will attract the electrons towards itselfand negative side of the battery will attract the holes towards itself. Due tothis the holes and the electrons in their respective areas will decrease andthe mobile ions will increase due to which the depletion region will increase.Due to increase in the depletion region the electric field strength willincrease and this will support the flow of minority charge carriers.
The flow of electric current due to minority chargecarriers is called drift current. In the reverse biased pn junction driftcurrent flows. Because in the p side region there are free electrons but in avery small amount and in the n side region the holes are in a very smallamount. These both are called minority charge carriers. The electrons flow inthe opposite direction of the electric field and the holes flow in thedirection of electric field. The increased electric field will provide theminority charge carriers to flow. The p-type minority charge carriers(electrons) will flow from left to right and the n-type minority chargecarriers (holes) will flow from right to left. This current is also calledleakage current.
You can see in the negative cycle of the graph thatthe voltage is continuously increasing but it has no effect on the current andthe current is same. Every diode has a limit up to which it can bear thevoltage after that the junction break down and this point is commonly known asbreakdown point. After the break point the current abruptly increases.
Diffusion current is the current which flows due tothe majority charge carriers. This will not flow in the reverse biased pnjunction.
By: Bilal Siddique Bajwa