Table of Contents
A material is considered a semiconductor if its electrical conductivity falls somewhere between that of a metal (like copper) and that of an insulator (such as glass). Transistors, solar cells, light-emitting diodes (LEDs), quantum dots, and digital and analogue integrated circuits are all examples of semiconductor-based modern electronics.
Diode – Di + ode
Di means two and ode means electrode. So physical contact of two electrodes is known as diode and its important function is alternative current to direct current.
1. INTRINSIC SEMICONDUCTOR
An intrinsic semiconductor is one that is sufficiently pure that impurities have little to no impact on how it conducts electricity. In this instance, all carriers are produced as a result of excited electrons crossing from the full valence band into the empty conduction band via thermal or optical processes. Thus, an intrinsic semiconductor has an equal number of electrons and holes. In an electric field, electrons and holes flow in the opposite directions, but because of their opposing charges, they also contribute to current flowing in the same direction. In an intrinsic semiconductor, however, because electrons and holes have different effective masses, hole current and electron current are not always equal (crystalline analogues to free inertial masses).
The temperature has a significant impact on the carrier concentration. The material becomes an insulator when the valence band is fully filled at low temperatures. The number of carriers rises with temperature, increasing conductivity in an inverse relationship. The behaviour of most metals, which tend to become less conductive at higher temperatures due to increased phonon scattering, differs from that of intrinsic semiconductors in that they do not exhibit this characteristic.
Both silicon and germanium are tetravalent, i.e. each has four electrons (valence electrons) in their outermost shell. Both elements crystallize with a diamond-like structure, i.e. in such a way that each atom in the crystal is inside a tetrahedron formed by the four atoms which are closest to it. Each atom shares its four valence electrons with its four immediate neighbours, so that each atom is involved in four covalent bonds.
2. EXTRINSIC SEMICONDUCTOR
A semiconductor that has been doped with impurities to change the variety and quantity of free charge carriers is known as an extrinsic semiconductor. A semiconductor that has been doped, or into which a doping agent has been introduced, is referred to as an extrinsic semiconductor. This semiconductor differs from an intrinsic (pure) semiconductor in terms of its electrical properties. An intrinsic semiconductor is doped by adding doping atoms, which alters the semiconductor’s electron and hole carrier concentrations at thermal equilibrium. Extrinsic semiconductors are either n-type or p-type semiconductors depending on the predominant carrier concentrations present.
In a pure or intrinsic conductor, the holes and electrons are produced thermally. However, there aren’t many of these. By carefully introducing impurities into the semiconductor, it is possible to greatly increase the number of charge carriers. As a result, an extrinsic semiconductor is created. Doping is the name given to this process. Donor impurities and acceptor impurities are the two main categories of impurities. Donor impurities are composed of atoms with five valence electrons, such as arsenic. Atoms with three valence electrons, such as gallium, make up acceptor impurities.
The two types of extrinsic semiconductor are
N-type semiconductors are extrinsic semiconductors with a higher electron concentration than hole concentration. The term “n-type” refers to the electron’s negative charge. Electrons make up the majority of carriers in n-type semiconductors while holes make up the minority. By adding donor impurities to an intrinsic semiconductor, N-type semiconductors are produced.
In an n-type semiconductor, the Fermi energy level is greater than that of the intrinsic semiconductor and lies closer to the conduction band than the valence band. Arsenic has 5 valence electrons, however, only 4 of them form part of covalent bonds. The 5th electron is then free to take part in conduction. The electrons are said to be the majority carriers and the holes are said to be the minority carriers.
P-type semiconductors have a higher hole concentration than electron concentration in comparison to n-type semiconductors. The term “p-type” describes the hole’s positive charge. Electrons make up the minority of carriers in p-type semiconductors, while holes are the majority. By adding acceptor impurities to an intrinsic semiconductor, P-type semiconductors are produced. Fermi energy levels in P-type semiconductors are lower than the intrinsic Fermi energy level.
The Fermi energy level lies closer to the valence band than the conduction band in a p- type semiconductor. Gallium has 3 valence electrons, however, there are 4 covalent bonds to fill. The 4th bond therefore remains vacant producing a hole. The holes are said to be the majority carriers and the electrons are said to be the minority carriers.
PN JUNCTION DIODE
When the N and P-type semiconductor materials are first fused together, there is a significant density gradient between the two sides of the junction, which causes some of the free electrons from the donor impurity atoms to start migrating across the newly formed junction to fill up the holes in the P-type material, producing negative ions.
1. FORWARD BIAS CONDITION PN JUNCTION DIODE
Forward bias condition occurs when the battery’s positive terminal is connected to the P-type and its negative terminal to the N-type of a PN junction diode.
The applied potential in external battery acts in opposition to the internal potential barrier which disturbs the equilibrium.
As soon as equilibrium is disturbed by the application of an external voltage, the Fermi level is no longer continuous across the junction.
Under the forward bias condition the applied positive potential repels the holes in P type region so that the holes move towards the junction and the applied positive potential repels the electrons in N type region so that the electrons move towards the junction.
When the applied potential is more than the internal barrier potential the depletion region and internal potential barrier disappear.
VI Characteristics of PN JUNCTION DIODE
As the forward voltage increased for VF < Vo, the forward current IF almost zero because the potential barrier prevents the holes from P region and electrons from N region to flow across the depletion region in opposite direction.
For VF > Vo, the potential barrier at the junction completely disappears and hence, the holes cross the junction from P to N type and electrons cross the junction to opposite direction, resulting large current flow in external circuit.
Here, the voltage cutoff or threshold voltage VF below which the current is very small is a feature to be noted. At this voltage, the junction’s current starts to rapidly increase as the potential barrier is broken. In comparison to silicon, germanium has a voltage cut of 0.7V.
2. UNDER REVERSE BIAS CONDITION PN JUNCTION DIODE
Forward bias condition occurs when the battery’s negative terminal is connected to the P-type and its positive terminal to the N-type of a PN junction diode.
The majority of the P side’s carriers produce holes, which are drawn to the battery’s negative terminal, while the majority of the N side’s carriers produce electrons, which are drawn to the battery’s positive terminal.
As a result, the mobile charge carriers are depleted, increasing the width of the depletion region. And As a result, the applied reverse bias produces an electric field that is parallel to the electric field of the potential barrier. As a result, the potential barrier that results is higher, obstructing the movement of majority carriers in both directions. Under reverse bias, the depletion width W is proportional.
VI characteristics PN JUNCTION DIODE
Theoretically no current flow in the external circuit. But in practice a very small amount of current of the order of few microamperes flows under reverse bias.
Electrons forming covalent bonds of semiconductor atoms in the P and N type regions may absorb sufficient energy from heat and light to cause breaking covalent bonds. So electron hole pairs continuously produced.
As a result, the minority carriers, holes in the N region and electrons in the P region, stray over to the junction and flow in the opposite direction of where the majority carriers are located, creating a small reverse current. Reverse saturation current Io is the name of this current. Because the minority carrier is made of thermally broken covalent bonds, the size of this current is temperature dependent.
1. When P-N junction is reverse biased the depletion region act as an insulator or as a dielectric medium and the p-type an N-type region have low resistance and act as the plates.
2. Thus this P-N junction can be considered as a parallel plate capacitor.
3. This junction capacitance is called as space charge capacitance or transition capacitance and is denoted as CT .
4. Since reverse bias causes the majority charge carriers to move away from the junction , so the thickness of the depletion region denoted as W increases with the increase in reverse bias voltage.
5. This incremental capacitance CT may be defined as
CT = dQ/dV,
Where dQ is the increase in charge and dV is the change or increase in voltage.
6. The depletion region increases with the increase in reverse bias potential the resulting transition capacitance decreases.
7. The formula for transition capacitance is given as CT = Aε/W, where A is the cross sectional area of the region, and W is the width.
1. When the junction is forward biased, a capacitance comes into play , that is known as diffusion capacitance denoted as CD. It is much greater than the transition capacitance.
2. During forward biased the potential barrier is reduced. The charge carriers moves away from the junction and recombine.
3. The density of the charge carriers is high near the junction and reduces or decays as the distance increases.
4. Thus in this case charge is stored on both side of the junction and varies with the applied potential. So as per definition change in charge with respect to applied voltage results in capacitance which here is called as diffusion capacitance.
5. The formula for diffusion capacitance is CD = τID / ηVT , where τ is the mean life time of the charge carrier, ID is the diode current and VT is the applied forward voltage, and η is generation recombination factor.
6. The diffusion capacitance is directly proportional to the diode current.
7. In forward biased CD >> CT . And thus CT can be neglected.
1.What is a PN Junction? How is it formed?
A PN Junction diode is created in a semiconductor material if one half is doped with P-type impurities while the other half is doped with N-type impurities. The PN Junction is the line dividing the two zones (or halves).
2. What is meant by diffusion capacitance (CD)?
Diffusion (or) storage capacitance (Cp), which exists in forward bias junctions, typically has a value that is much higher than the capacitance Cr found in reverse bias junctions. The rate of change of the injected charge with the applied voltage is another definition of this
Cp = (dQ/dW),
dQ -» represents the change in the number of minority carriers stored outside the depletion region when a change in voltage across the diode,
dv is applied.
3.What is Zener diode?
A PN junction diode with specialised design is a zener diode. a PN junction diode that is reverse biassed and heavily doped. The Zener diode is an operating in the breakdown region reverse biassed heavily doped PN junction diode. It is also known as a breakdown diode or voltage regulator diode.
4. List out the applications of LED.
LEDs are more popularly used in display clocks , audio and video equipments ,traffic lights.
It is also used as light source in optical fibre communication.
5. Define transition capacitance of P-N diode.
When a diode is reverse biased , the holes in the p- side and the electrons in the n-side drift away from the junction , thereby uncovering more immobile charges. As a result the thickness of depletion increases . this leads to capacitance effect across the region called transition capacitance.
6. Distinguish between shunt and series voltage regulator.
In a series regulator the regulating element is in series with the load and the regulation is done by varying the voltage across the series element.
In a shunt regulator the regulating element is in shunt with the load and the regulation is done by varying the current across the shunt element.
7. Draw the VI Characteristics of Zener diode.
8. Derive the ripple factor of full wave rectifier.
An indicator of how well a rectifier converts ac to dc is the ripple factor. That is, it is the proportion of the ac component’s rms value to the dc value.
Ripple factor= Vr(rms)/Vdc
9. Define peak inverse voltage in a diode.
Peak inverse voltage is the maximum negative voltage which appears across a non conducting reverse biased voltage.
10. What is Drift Current?
The electrons are accelerated in a specific direction by the effect of the externally applied electric field. They move along at the same speed as the drift. A current known as the drift current will result from this movement of electrons.
11. What is barrier potential at the junction?
Due to the presence of immobile positive and negative ions on opposite sides of the junction an electric field is created across the junction . The electric field is known as the barrier potential.
12.What is a Rectifier?
A rectifier is a device which converts a.c. voltage to pulsating d.c. voltage, using one or more Pn junction diodes. Its types
i)half wave rectifier
ii)full wave rectifier
13. What is meant by drift current?
The drift velocity of the charge carriers is equal to the product of the mobility of the charge carriers and the applied electric field intensity E when an electric field is applied across a semiconductor material. The holes move in the direction of the battery’s negative terminal while the electrons move in the direction of the positive terminal. Drift current is the result of the combined effect of the charge carriers moving.
14. Define Hall effect?
A metal or semiconductor carrying current I is placed in a transverse magnetic field B, which induces an electric field E in a direction perpendicular to both I and B. This phenomenon is referred to as the Hall effect.
15.Give some application of Hall Effect.
i. hall effect can be used to measure the strength of a magnetic field in terms of electrical voltage.
ii. it is used to determine whether the semiconductor is p – type or n- type material
iii. it is used to determine the carrier concentration
iv.it is used to determine the mobility.