Classification of Materials
Materials can be classified in many ways, Here we will consider the property of the electric carriers for the classification of materials.
Materials similar to copper, aluminum, silver, etc. are good conductors of electricity,
while substances like plastic, rubber, wood, etc. are poor conductors of electricity, they are called insulators.
In addition, there are some materials like silicon and germanium, whose conductivity lies between the conductor and the insulator such materials are called semiconductors
These materials include 4 electrons in the outermost shell of their atomic construction, these four electrons are called valence electrons.
They form a bond with other valence electrons of an adjoining atom, these bonds are called covalent bonds.
The energy levels interacted with the valence electrons blend into each other, this merger forms a valence band.
The energy band established due to the merger of energy levels associated with free electrons is called the conduction band.
When the valence electron gets energy, it jumps from the valence band to the driving band and becomes free.
While jumping, it has to cross the energy gap
The difference between the energy difference is the separation of the valence band and the conduction band, it is called the forbidden gap
Classification of materials based on energy Gap
In conductors like copper, aluminum, etc., the valence band and the conduction band overlap,
And there are many electrons available for conduction at room temperature.
Insulators like plastic, wood, etc. have a large prohibited difference (about 6 EVs) between the valence band and the conduction band, And these materials cannot operate electricity.
The forbidden difference in semiconductors is much (about 1 EV) as compared to insulators.
At absolute zero temperature, these materials behave as the correct insulator.
But at room temperature, for example, the Germanium has 0.72 eV and silicon for 1.12 EV.
As the temperature rises, these materials can conduct heavily.
Types of Semiconductors:
1. Internal (intrinsic) semiconductor:
These are pure semiconductors.
2. External semiconductor:
These are obtained by adding impurities to pure semiconductors.
Effect of temperature on the conduction of semiconductors (intrinsic)
- As we have seen before, At absolute zero temperature internally semiconductors, the outermost shell of all atoms is completely filled,
And valence electrons are tightly bound by parents’ atoms.
For this reason, no free electrons are available at full zero temperature, and the semiconductor is treated as a perfect insulator.
- However, at room temperature, some covalent bonds break due to heat energy.
These electrons, which break the bond and come out as free electrons, are available for conduction.
- The empty space left behind by electrons forms an incomplete covalent bond and is called a hole.
A valance electron from a neighboring atom can fill this hole and make another hole.
- So each valance electron leaves behind a hole that is captured by another valance electron, and the process continues.
Thus, the hole can be considered as an electric carrier.
- An electron is a negatively charged particle and the hole is charged positively. The overall concentration of free electrons and holes in an internal semiconductor is always equal.
- If an external voltage applies to the internal semiconductor, the free electrons move towards the positive terminal of the battery, And holes move towards the negative terminal of the battery.
- The current which flows from the positive to the negative terminal of the battery, and is external to the battery, is called a Conventional Current.
- The Conventional Current direction is thus opposite to the direction of electron flow. Internal semiconductors are never used independently.
- To change the properties of internal semiconductors, a small amount of other material is added to them. The process of adding some selected impurities to crystal internal semiconductors to improve conductivity is called doping, thus the semiconductor formed is called an external semiconductor.
- There are two types of impurities, donor impurities, and acceptable impurities, and therefore there are two types of external semiconductors as ‘P-type and N-type semiconductors.
- For a pure semiconductor, pentavalent impurity (having five valence electrons) or by adding donor impurity, an N-type semiconductor is formed, Examples of donor impurities are arsenic, phosphorus, etc. This type of semiconductor contains many free electrons.
- For an impurity that accepts a pure semiconductor, a P-type semiconductor is made by adding trivalent impurity (having only three valence electrons) Examples of trivalent impurity are gallium, boron, etc.This type of semiconductor has many holes.
When a voltage is applied to a semiconductor, try to drift towards the positive terminal of the free electron battery.
Due to this flow, a current flows in the semiconductor called the current silos and the velocity at which electrons flow is called drift velocity.
Note: The direction of the traditional present is always opposite to the direction of the flowing electrons.
Due to the lack of uniform doping in semiconductors, a uniform concentration of charged particles is possible.
For example, the concentration of electrons on one side of the surface is greater than the density on the other.
So electrons will move in favor of less concentration from the side of greater concentration.
This transport of electrons results in the flow of the present called the diffusion current.
What is P-N junction and Its Characteristics?
The two P-type and ‘N-type materials are chemically combined with a definite fabrication technique to form a p-n junction, which is called a diode.
As We have seen earlier, the movement of charge carriers from high concentration to the low concentration area to achieve uniform concentration over the material is called a diffusion process.
In a P-N Junction, there is a large number of electrons on the N side, whereas on the P side the concentration of electrons is very low.
Due to this non-uniform concentration, diffusion starts, and electrons start moving from N Side towards the P side. Similarly, the holes from the p-region diffuse into the N-region across the junction.
When the P-N junction is formed, the N-side donor atoms accept additional holes, and they become positively charged immobile ions. Similarly, P-side acceptor atoms Accept additional electrons and they become Negatively charged immobile ions.
The formation of immobile ions near the junction is shown in Fig. In this region, there exists a wall in which there are no mobile charge carriers. hence is called a depletion region or depletion layer.
Thus, under thermal equilibrium, the depletion region gets widened up to an extent where no more electrons or holes can cross the junction. This depletion region acts as the barrier due to which the hole and electrons cannot diffuse further. This condition is shown in Fig.
Thus, at the junction, there are immobile positive and negative ions, due to which an electric field called barrier potential or cut-in voltage is created at the junction.
The barrier potential depends on different factors like:
(a) Type of semiconductor,
(b) The donor impurity added,
(c)The acceptor impurity added,
(d) The surrounding temperature.
The barrier potential for Silicon is 0.7 V and the barrier potential for Germanium is 0.3 v
What is P-N Junction Diode?
The P-N junction forms a device which is known as a Diode. The symbol of a diode is shown below. The P-type electrode connection is called an Anode and N-type electrode. The connection is called a Cathode Junction
Biasing of a Diode
Biasing means applying an external DC voltage to a device. The P-N junction diode can be reverse biased or forward biased, depending upon the polarity of the battery connected across it. The forward biasing of a P-N junction diode is shown below
The condition is not possible as long as the applied forward voltage is less than barrier potential.
When the applied voltage becomes more than the barrier potential, the negative terminal of
the battery pushes free electrons from N to the P region and the positive terminal of The battery pushes the holes from the P to N region.
If the applied forward voltage is further increased, it overcomes the barrier potential. The depletion region gets reduced, and the majority of carriers can pass through the junction, which causes a current to flow. Refer Fig.
This current is called the forward current I.
Forward V-I Characteristic of P-N Junction Diode
The V-I characteristics and the circuit for forward-biased diode are as shown in fig
The forward characteristic may be divided into two parts:
Region A to B: This is the region where VF is less than the cut-in voltage and the current flow is very small, so If is assumed to be zero. (The cut-in voltage is 0.3 V for Germanium diode and for Silicon diode it is 0.7 V).
Region B to F: As the applied forward voltage (V) increases, at point B it exceeds the cut-in voltage V, the depletion region reduces to zero and there is a sudden increase in I.
As shown in Fig, the curve B to F is exponential in nature.
Forward Resistance of Diode
The resistance offered by the diode in the forward biased condition is called forward resistance, which is very small in nature. There are two types of forwarding resistances’ that are static and dynamic.
Static forward resistance (Rf): This is the forward resistance offered by the P-N junction diode when a forward DC voltage is applied to it. The static forward resistance (Rf) at a point on the forward characteristic, can be calculated by taking the ratio of the dc voltage applied across the P-N junction to the DC current flowing through the P-N Junction at that point.
Thus, Rf = Vdc/Idc at a particular point on the forward characteristic.
Dynamic forward resistance (RF): This is the forward resistance offered by the
On P-N diode when a forward AC voltage is applied to it. The dynamic resistance ‘RF’ is the reciprocal of the slope of the forward characteristic. Thus, Rf=Δ Vf /ΔIf
P-N Junction Diode in Reverse Biased Mode
A diode is said to be reverse-biased when the positive terminal of the battery is connected to
The cathode and the negative terminal are connected to the anode, as shown in Fig. 1.13.
The free electrons in the N region are attracted by the positive terminal and the holes in the P region are attracted by the negative terminal of the battery,
due to which the depletion region widens and hence the barrier potential increases,
shown as in Fig.
Therefore, very small reverse current flows due to the minority carriers which constitute the holes in the N-region and the electrons in the P-region.
As they are very less in amount, the reverse current is very small. The reverse current depends upon the temperature and not on the reverse voltage applied.
It is represented as Io. It is a few microamperes in Germanium and a few nano amperes in Silicon.
Breakdown in a Reverse Biased Diode
Though we have mentioned earlier that the reverse current is independent of the value of the applied reverse voltage, if this applied reverse voltage is increased beyond a particular value, it may damage the diode. It is called a reverse breakdown of the diode which is caused due to the following two effects: (a) Avalanche breakdown (b) Zener
(a) Avalanche breakdown:
- As the magnitude of the reverse bias voltage is increased, the kinetic energy of the minority carriers gets increased. While traveling, the minority carriers collide with the stationary atoms, which in turn results in breaking some covalent bonds and generating free electrons.
- These electrons act as minority carriers. Again, they get accelerated by the strong reverse bias field, thereby increasing the collision and also the number of Free Electrons. This is known as carrier multiplication.
- This process continues, leading to a very swift multiplication effect and giving rise to a large reverse current in just a few microseconds. This effect is called as “Avalanche breakdown effect”.
- The Avalanche breakdown effect causes a reverse breakdown in the p-n junction. Due to large power dissipation, the junction temperature increases and may destroy the semiconductor device permanently.
- To limit this reverse current, a series resistance of exactly calculated value must be used in series with the diode for its protection.
(b) Zener breakdown:
- This type of breakdown occurs in heavily doped p-n junctions in which the depletion The region is very narrow.
- All the applied reverse voltage appears across the depletion layer. The electric field is voltage per unit distance. It is very intense in the depletion region.
- Therefore, it can pull the electrons out of the valance band by breaking the covalent bonds and producing free electrons. This process is known as the Zener effect. Numerous such free electrons can result in a large reverse current through the diode.
- Due to the large current, there is large power dissipation. This increases the junction temperature beyond a certain limit and may damage the diode permanently. To limit the current, a series resistance is added to the circuit to protect the diode.
Reverse Characteristics of Diode
The graph of reverse voltage VR versus the reverse current IR is shown in Fig. 1.15(a). It is the reverse characteristic of a diode.
As we can see from the Fig. 1.15 (a), at a constant temperature, if the reverse voltage is increased the reverse saturation current (I) remains constant showing that it is independent of the reverse voltage.
But if the reverse voltage is increased beyond the breakdown voltage, a large current
Flows through the diode. Typically, the breakdown voltage is about 50 V to 100 V.
Very high resistance of about a few hundred-kilo ohms is connected in series with the diode for protection.
Reverse Resistance of Diode
Reverse static resistance (Rr,): It is equal to the ratio of applied reverse DC voltage to the reverse DC saturation current I, It is very large in Mega ohms.
Reverse dynamic resistance (rr.): Under AC conditions, r, is defined as the ‘ratio of change in reverse voltage applied to the corresponding change in the reverse current’.
Reverse Resistance =(ΔVf/ΔIf )=(1/Slope of Graph) =(V2-V1/I0-I1) =(V2/I0)
V-I Characteristics of the Typical Silicon and Germanium Diodes
The combined forward and reverse V-I characteristics of the typical Silicon and Germanium diodes are as shown in Fig.
Diode Current Equation
The relation between the applied voltage “V across the diode and current I flowing through the diode can be expressed mathematically by the diode current equation
which is given as, where,
V = Applied voltage across the diode in volts
I = current flow through the diode in amperes n = 2 for Silicon P-N junction diode
= 1 for germanium P-N junction diode
Io =Reverse saturation current flow through the diode in ampere
The term e indicates that the diode current equation is exponential in nature and it is applicable for all the conditions of the diode operating modes (i.e. whether a diode is forward biased, reverse-biased, or in unbiased condition).Vt is the voltage equivalent of temperature in volts. It is given by the following equation.
VT = K x T volts
K Boltzmann’s constant 8.62 x 10 eV/°K.
T Temperature in °K (Kelvin) .
The equation V Kx T indicates that the current flow through the diode also depends upon the ambient temperature. Consider, for example, room temperature 25°C.
Then temperature in °K= 25 °C + 273. Therefore, T= 298°K
VT= 8.62 x 10x 298 K VT = 25.68 mV
Effect of Temperature on V-I Characteristics.
As the temperature increases,
- Cut-in voltage decreases, i.e., the diode turns on at smaller voltages.
- Reverse saturation current (io) increases. The reverse current almost doubles at 10 °C every rise in the temperature.
- The reverse breakdown voltage increases.
- The VT(voltage equivalent of temperature) also increases.
These effects are shown in Fig.