ELECTRONICS

Q.1: What is a bipolar junction transistor? Why it is so named?
Ans: BJT: A BJT consist of two back to back P – N junctions manufactured in a single piece of a semiconductor crystal. These two junctions give rise to three regions called emitter, base and collector. Junction transistor is simply a sandwich of one type of semiconductor material between two layers of the other type. The two junctions are emitter- base (E / B) and collector – base (C / B) junction.
It is so named because –
i. It is smaller and light weight.
ii. It has no heater requirement or heater loss.
iii. It has rugged construction.
iv. It is more efficient since less power was absorbed by the device itself.
v. It is instantly available for use, requiring no warm – up period.
vi. Here lower operating voltages were possible.
Q.2: Describe the construction of a transistor.
Ans: Transistor construction: A transistor is a three layer semiconductor device consisting of either two n - type and one p - type layers of material. The former is called an n-p-n transistor while the later is called a p-n-p transistor. The emitter layer is heavily doped, the base layer is lightly doped and the collector layer is only lightly doped. The output layers have widths much greater than the inner p or n type material. The first layer is called emitter and is indicated by capital E, the mid layer is called base and is indicated by B and the third layer is called collector and indicated by capital C.
Q.4: Mention the rules for biasing a transistor.
Ans: Rules for biasing a transistor:
1. Base – Emitter junction is always forward biased.
2. Collector – Base junction is always reverse biased.
Q.5: What do you mean by transistor configuration?
Ans: In a circuit a transistor can be connected in three different ways. This is called transistor configuration. There are three types of transistor configuration –
1. Common base configuration: In a common base configuration mid layer of the transistor is common to outer two layers that is the base is common to both the input and output sides of the configuration. In addition, the base is usually the terminal closest to or at ground potential.
The input set for the common – base amplifier relates an input current to an input voltage for various levels of output voltage. The output set relates an output current to an output voltage for various levels of input current. The output or collector set of characteristics has three basic regions of interest the active, cutoff, and saturation regions.
In the active region the base emitter junction is forward biased, whereas the collector base junction is reverse biased. In the cutoff region the base emitter and collector base junctions of a transistor are both reversed biased. In the saturation region the base emitter and collector base junctions are forward biased.











2. Common emitter configuration: The most frequently encountered transistor configuration for the p-n-p and n-p-n transistors is called the common emitter configuration. In a common emitter configuration the emitter is common to both the input and output terminals. Two sets of characteristic are the input or base emitter circuit and the output or collector emitter circuit.
In the active region of a common emitter amplifier the base emitter junction is forward biased, whereas the collector base amplifier is reversed biased.









3. Common collector configuration: The third and final transistor configuration is the common collector configuration. In a common collector configuration the collector is common to both the base and emitter terminals. The common collector configuration has high input impedance, opposite to that of the common base and common emitter configurations.
A common collector circuit configuration is provided with the load resistor connected from emitter to ground. It can be designed using the common emitter characteristics. The output characteristics of the common collector configuration are the same for the common emitter configuration.





Q.6: Explain the operation of a transistor in common base (CB) configuration and draw input and output characteristics.
Or,
Explain the operation of a transistor in common base (CB) configuration and draw input and output characteristics. Identify cutoff region, saturation region and active region on the output characteristics.
Ans: The operation of a transistor in common base (CB) configuration: For the operation of a transistor, the transistor should be properly biased. The general rule for the biasing of a transistor is that emitter base junction should be forward biased and collector base junction should be reverse biased. The circuit diagram for a biased p-n-p transistor is given bellow.









Here battery VEE makes the emitter base junction forward biased and battery VCC makes the collector base junctions reverse biased. Since emitter base junction is forward biased, the depletion region is reduced, due to applied voltage, resulting a heavy flow of majority carriers from p to the n type material. On the other hand, since collector base junction is reverse biased, depletion region increased which stops the flow of majority carriers. The huge number of injected charge carriers will be minority carriers in the n type material and will see an easy path to flow through collector base reverse biased junction to the collector. Only a few will go through the high resistive path to the base terminal. The direction of current flow through the transistor is shown by arrow sign.
Applying Kirchhoff’s law of current flow to the transistor we obtain,
IE = IC + IB
That is the emitter current is the sum of collector and base currents. The collector current is composed of two components the majority and the minority components. This can be written as
IC = IC majority + ICO minority.
The minority component ICO is very small.
Input and output characteristics of a transistor in common base configuration: For a common base configuration IE = IC + IB. To fully describe the behavior of a three terminal device requires two sets of characteristics – the input characteristics and the output characteristics.
The input characteristic for common base transistor is shown bellow which relates the input current IE to input voltage VBE for various levels of output voltage, VCB. This curve is identical to a forward biased p-n junction.

ELECTRONICS

Q.8: what are the majority carrier and minority carrier of p – type and n - type material?
Ans: Majority carrier and minority carrier of p – type and n - type material: In the intrinsic state, the number of free electrons in Ge or Si is due only to those few electrons in the valence bands that have acquired sufficient energy from thermal or light sources to break the covalent bond or to the few impurities that could not be removed. The vacancies left behind in the covalent bonding structure represent our very limited supply of holes. In an n – type material, the number of holes has not changed significantly from this intrinsic level. The net result therefore is that the number of electrons for outweighs the number of holes. For this reason –

In an n – type material the electron is called the majority carrier and the hole is the minority carrier.

Fig: N-type Fig: P – type

For p – type material the number of holes or outweighs the number of electron. Therefore –

In a p – type material the hole is the majority carrier and the electron is the minority carrier.

Q.9: What do you mean by bias?

Ans: Bias: The term bias refers to the application of an external voltage across the two terminals of the device to extract a response. In no – bias situation there is no external voltage applied that is applied voltage is zero and current is zero amperes.

Q.10: Describe semiconductor diode/P-N junction with diagram or the following condition –

1. No biasing


2. Forward biasing


3. Reverse biasing.

Ans: No biasing: Under no – bias condition any minority carriers (holes) in the n – type material find themselves within the depletion region and they can pass quickly into the p – type material for the greater attraction of the layer of negative ions and the less is the opposition offered by the positive ions in the depletion region of the n – type material. Depletion region

P-type N-type

Fig: No bias condition

The majority carriers of the n – type material overcome the attractive forces of the layer of positive ions in the n – type material and the shield of negative ions in the p – type material to migrate into the area beyond the depletion region of the p – type material. The number of majority carriers is so large in the n – type material but there are a small number of majority carriers with sufficient kinetic energy to pass through the depletion region into the p – type material. The same type of discussion can be applied to the majority carriers (holes) of the p – type materials. As a result, the current under no bias conditions is zero.

Reverse bias: If an external potential is applied across the p – n junction such that the positive terminal is connected to the n – type material and the negative terminal is connected to the p – type material makes the diode reverse bias. As a result, the numbered of uncovered positive ions in the depletion region of the n – type material will increase due to the large number of free electrons drawn to the positive potential of the applied voltage. Depletion region

P-type N-type

Fig: Reverse bias condition

For same region the number of uncovered negative ions will increase in the p – type material. As a result the width of the depletion region increase and established a great barrier for the majority carriers to overcome and effectively reducing flow to zero.

The number of minority carriers entering the depletion region will not change resulting in minority – carrier flow vectors of the same magnitude indicated no applied voltage.

Forward bias: A forward bias or “on” condition is established by applying the positive potential to the p – type material and the negative potential to the n – type material. Depletion region

Fig: Forward bias condition

The application of forward bias potential will “pressure” electrons in the n – type and holes in the p – type material to recombine with the ions near the boundary and reduce the width of the depletion region. The resulting minority carrier flow of electrons from the p – type material to the n – type material has not changed in magnitude but the reduction in the width of the depletion region has resulted in a heavy majority flow across the junction. An electron of the n – type material now “sees” a reduced barrier at the junction due to the reduced depletion region and a strong attraction for the positive potential applied to the p – type material. The depletion region will continue to decrease in width until a flood of electrons can pass through the junction, resulting in an exponential rise in current in the forward bias.

the third post

Q.4: Define energy band, energy gap, energy level.

Ans: Energy band: In the isolated atomic structure there are discrete energy levels associated with each orbiting electron. As the atoms of a material are brought closer together to form the crystal lattice structure, there is an interaction between atoms that will result in the electrons of a particular orbit of an atom having slightly different energy levels from electron in the same orbit of an adjoining atom. The net result is an expansion of the discrete levels of possible energy states for the valence elections to that of energy band.

Between successive energy band there are forbidden regions where no electron can reside. This forbidden region is called energy gap.

Fig: Discrete levels in isolated atom

Energy level: In the atoms electrons are distributed around the nucleus in different well defined orbits. Each orbit is associated with a certain amount of energy. The energy of an electron in an orbit for away from the nucleus having higher energy, than an electron in an orbit close to the nucleus. A small region near each orbit where electrons reside is called energy level.

Q.5: Brief the characteristic of a semiconductor.

Ans: Characteristic of a semiconductor: Pure semiconductors materials show some properties as listed below:

1. At extremely low temperature, pure semiconductor is insulator.


2. As the temperature rises from absolute zero, an increasing number of valence electrons break covalent bond and become free to conduct electricity.


3. Resistivity of semiconductor materials decrease with increasing temperature.


4. The energy gap between valence and conduction bands of semiconductor materials are in the order of 1ev.


5. Resistivity of a semiconductor material lies between conductor and insulator.

Q.6: Draw the atomic structure of P – type and n – type material. Or, Describe P – type and n – type material with diagram.

Ans: N – type material: If a pure semiconductor material is doped with pentavalent impurity atoms then the resulting semiconductor material is called n – type semiconductor material.

Atomic structure of n – type material where silicon is doped with pentavalent antimony is shown in bellow:

Here, the four covalent bonds are still present. An additional fifth electron, due to the impurity atom which is unassociated with any particular covalent bond. This remaining electron, loosely bound to its parent atom, is relatively free to move with in the newly formed n – type material.

P – type material: If a pure semiconductor material is doped with trivalent impurity atoms then the resulting semiconductor material is called P – type semiconductor material. The P – type material is formed by doping a pure germanium or silicon crystal with impurity atoms having three valence electrons such as boron, gallium and indium. The effect of one of these elements, boron on a base of silicon is indicated in fig:

Now an insufficient number of electrons to complete the covalent bonds of the newly formed lattice. The resulting vacancy is called a hole and is represented by a small circle or a plus sign, indicating the absence of a negative charge.

DEFINATION AND OTHER ABOUT ELECTRONICS

Q.1: What do you mean by semiconductor?

Ans: Semiconductor: Semiconductor is special class of elements having conductivity between that of a good conductor and an insulator.

Semiconductor materials are two classes:

1. Single - crystal: Single crystal semiconductors such as germanium (Ge) and silicon (Si) have a repetitive crystal structure.


2. Compound: Compound semiconductors such as gallium arsenide (GaAs), cadmium sulfide (Cads), gallium nitride (GaN), and gallium arsenide phosphide (GaAsP) are constructed of two or more semiconductor materials of different atomic structures.

Semiconductor materials have a negative temperature coefficient. Pure semiconductor is an insulator. Three semiconductors used most frequently in the construction of electronic devices are Ge, Si, and GaAs.

Q.2: What do you mean by doping?

Ans: Doping: The characteristics of semiconductor materials can be altered significantly by the addition of a small controlled amount of impurity atoms into the pure semiconductor. The process by which this can be happened is called doping.

Q.3: Define valence electrons and covalent bonding.

Ans: Valence electron: Every atom is composed of three basic particles: the electron, the proton and the neutron. In the lattice structure neutrons and protons form the nucleus and electrons appear in fixed orbits around the nucleus. The Bohr model for the three materials is provided in fig bellow. Valance electron

Fig: Silicon and Germanium

In the fig silicon has 14 orbiting electron, germanium has 32 orbiting electrons. Here germanium and silicon has four electrons in the outermost shell which are referred to as valence electrons.

So we can say, in an atomic structure the outer most shell is known as valence electron which can remove from that shell.

Covalent bonding: In the atomic structure when valence electron and their parent atom create bonding by the sharing of electrons, is called covalent bonding. Covalent bond is stronger between the valence electrons and parent atom.

In a pure silicon or germanium crystal four valence electrons of one atom from a covalent bonding arrangement with four adjoining atoms in the fig.

GaAs is a compound semiconductor, there is sharing between the two different atoms, as shown in above.

ELECTRONICS BLOG FOR STUDENTS

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