Solved past question for electrical engineering NDii eec124

         EEC 124

Past question and answer

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1) Describe the basic principle of operation of a bipolar junction transistor including me why majority carriers crossing into the base from the emitter pass to the collector and why the collector current is nearly equals to the emitter current.

Answer
Don’t panick when you see a question like this in the exam hall. This is what you just have to do…. And guess, you just have to answer it step by step..,.. As simple as that.
So let’s go..

BASIC PRINCIPLE OF A BIPOLAR TRANSISTOR

Firstly, a bipolar transistor consists of both NPN and PNP transistor. It also have an N part and a P type material.
After then draw both PNP and NPN junction transistor

It has three layers, three terminal and two junction
-The emitter is heavily doped in a bipolar transistor in order to carry charge carriers from it region to the base region. The charge carriers here are called majority charge carrier while the few charge carriers are called minority(in a PNP junction transistor the emitter is heavily doped so as to pass holes to the base while in a NPN transistor it is heavily doped and forward biased so as to pass electrons to the base region. It contains majority charge carrier

-the base region at the middle is moderately doped so as to be able to control the charge carriers coming from the emitter to the collector. Only 0.5% of charge carriers coming from the emitter remain in the base region why 99.5% is found in the collector. The base region contains minority charge carriers. (For PNP transistor electrons are the minority charge carriers here,, while in NPN transistor the holes are the minority charge carriers.)

-the collector is lightly doped so as to be able to receive charge carriers controlled by the base region. It also contain majority charge carriers.
This will help you to understand,
( Take the expression as a cup which is filled, assuming you fill it with an emitter, may be tap water. Then you use your hand to control it to another cup, which must be empty in other to receive water from the filled cup. At the end you will observe that, the filled cup controlled to the empty cup, still have small water)

Second question says, why majority carriers crossing the base from the emitter region to the collector region.
ANSWER: Because the base is forward bias and heavily doped to pass majority carrier to the base. And the Base is moderately doped so that majority carrier crossing the base may be controlled to the collector which is lightly doped to receive majority carriers from the base.

Third question says, why is collector current nearly equals to the emitter current
Answer G
The question has been answered from the previous page sn’t it?

Because out of 100% of charge carriers coming only 0.5%are left in the base while 95.5% are found in the collector region
These expression makes the collector current approximate to the emitter current.
NOTE. Majority carriers are found in the emitter and collector region while minority carriers are found in the base region.

b) The application of an external voltage to a PN junction diode can influence the drift of holes and electrons. With the aid of a diagram explain this statement, and also how the direction and magnitude of the applied voltage affect the depletion layer.
ANSWER

When an external voltage is applied to a p-n junction making the p-type material positive with respect to the n-type material, as shown in the p-n junction is forward biased.The applied voltage opposes the contact potential, and in effect, closes the depletion layer. Holes and electrons can now cross the junction and current will flow.

An increase in the applied voltage above is required to narrow the depletion layer about 0.2V for germanium and 0.6V for silicon, as a result rapid rise in the current flow. Graphs depicting the current-voltage relationship for forward bias pn junction.

When an external voltage is applied to a P-N junction making the p-type material negative with respect to the n-type material the p-n junction is reversed bias. The application voltage is now in the same sense with the contact potential and opposes the movement of holes and electron due to opening up the depletion layer.
Thus in theory, no current flows. However at normal room temperature certain electrons in the covalent bond lattice acquire sufficient energy from the heat available to leave the lattice, generating mobile electrons and holes. This process is called the electron hole generation by thermal excitation are called minority carrier and this will be attracted by the applied voltage. Thus, In practice, a small current of few micro amperes for germanium and less than one micro amperes for silicon, at normal room temperature, flows under, reverse bias condition.
As the magnitude of the reverse voltage is increased a point is reached where a large current suddenly start to flow. The voltage at which this occur is called the breakdown voltage

c) A pure piece of silicon is doped with (I) pentavalent impurity and (ii) trivalent impurity. Explain the effect this have on the form of conduction.

Answer
Arsenic, antimony and phosphorus have five valence electrons (pentavalent) and when a semiconductor such as silicon is doped with one of these substance, some impurity atoms are incorporated in the tetrahedral structure resulting in N- type material. The fifth valence electrons is not rigidly bonded and is free to conduct, the impurity Atom donates a charge carrier. The resulting material is called n-type material.
It has an excess conduction: This means that the resulting n-type material contain free electro

indium, aluminum and boron have three valence electrons and when a semiconductor such as silicon is doped with one of these substance, some if the semiconductor atoms are replaced by semiconductor impurity resulting into P-type semiconductor. One of the four bond associated with the semiconductor material is deficient by one electron and this deficiency is called a Hole.
It has a deficit conduction: This is when holes give rise to conduction when it capture free electron from neighboring atom.

d) Explain with the aid of a suitable diagram the three states of a scr (silicon controlled rectifier)

Answer
The scr has three conducting state
1) forward blocking state: The anode is positive with respect to the cathode
-J1 is forward biased I.e it is on
-j2 is reversed bias. I.e it is off
-J3 forward biased i.e it is on

• The gate is not triggered
  Therefore scr is not on

2)reverse blocking state: The anode is negative with respect to the cathode.
-J1 is reverse biased I.e it is off
-j2 is forward biased. I.e it is on
-J3. reverse biased i.e it is off
-The gate is not triggered
Therefore the scr is not on

3)forward conducting state:The anode is positive with respect to the cathode.
J1 is forward biased I.e it is on
-j2 is forward biased. I.e it is on
-J3 forward biased i.e it is on

• The gate us triggered
  So, therefore scr is on.

Question 2) Draw the circuit for CC, CB and CE configuration of pnp bipolar transistor, showing correct polarities and direction of various current.
Answer
Current gain =

2b) A bipolar junction transistor operate with a collector current of 1.2A and a base collector of 50mA. What will be the value of the emitter current be.

Answer
From kirchoff’s law when viewing a common base, one will derive that
IE=IB+IC
when, IB=0.05A, IC=1.2A
IE=0.05A + 1.2A.
=1.25A

2c) Explain why the emitter region of a bipolar junction is heavily doped.

Answer
Emitter region is always heavily doped so as to carry or supply enough charge carriers such as electrons and holes to cross the base to the collector region.

3a) Explain briefly the action of a pn junction(I)on open circuit(no external voltage) (ii) when provided with a forward biased (iii) when provided with a reversed biased. Draw the characteristics graph for a forward biased and reversed biased condition.

Answer
When it is open circuited.
We assume that separate block of p-type and n-type materials are pushed together. Also assume that a hole is positive charge carrier at the junction the donated electrons in the n-type material, called the majority carrier diffuse into the p-type material (diffusion is from an area of high density to an area of lower density) and the acceptors hole in the p-type material diffuse into the n-type material. Because the n-type material has lost electrons, it acquires a positive potential with respect to the p-type material and thus tends to prevent further movement of electrons. The p-type material has gained electrons and becomes negatively charge with respect to the n-type materials and hence tends to retain holes. Thus after a short while, the movement of electrons and holes stop due to potential difference across the junction, called the potential barrier. The area in the region on the junction becomes depleted of holes and electron due to electron-hole recombination called depletion layer
Meaning of some terms used
Diffusion:mobile holes move from the n-type material to the p-type material.

When an external voltage is applied to a p-n junction making the p-type material positive with respect to the n-type material, as shown in the p-n junction is forward biased.The applied voltage opposes the contact potential, and in effect, closes the depletion layer. Holes and electrons can now cross the junction and current will flow.

An increase in the applied voltage above is required to narrow the depletion layer about 0.2V for germanium and 0.6V for silicon, as a result rapid rise in the current flow. Graph depicting the current-voltage relationship for forward bias pn junction.

When an external voltage is applied to a P-N junction making the p-type material negative with respect to the n-type material the p-n junction is reversed bias. The application voltage is now in the same sense with the contact potential and opposes the movement of holes and electron due to opening up the depletion layer.

3b) Explain zener and avalanche breakdown.

Answer
Zener breakdown :this form of breakdown occurs in heavily doped junctions thereby narrowing the depletion layer. The breakdown voltage set up a strong electric field (about 108 V/M) across the depletion layer. This electric field is strong enough to break or rupture the covalent bonds thereby generating electron-holes pairs, even a small increase in reverse voltage will result in large current flow, therefore after the breakdown voltage is reached the resistance drops markedly. It occurs at a voltage less than 6ev.
iii) Avalanche breakdown :this form of breakdown occurs in lightly doped junction thereby having a wide depletion layer where the electric field is not strong enough for zener breakdown to occur instead the minority carriers (accelerated by the field) break the covalent bond.A relatively large current flow once the avalanche point is reached. For avalanche this voltage always occur at a voltage above 6ev.

4a) Discuss intensively extrinsic and intrinsic semiconductors.

Answer
INTRINSIC(PURE)SEMICONDUCTOR: This is the semiconductor material in a pure form silicon or germanium with no doping atoms added are called intrinsic semiconductors. At room temperature, some of the electrons acquire sufficient energy for them in o break the covalent bond between atoms and become free mobile electrons. This is called thermal generation of electron-hole pairs. Electrons generated thermally create a gap in the crystal structure called hole, the atom associated with the hole positively charged, since it has lost an electron.This positive charge may attract another electron released from another atom, creating a hole elsewhere.
When a potential is applied across the semiconductor material, holes drift towards the negative terminal (unlike charges attract), and electrons towards the positive terminal, and hence a small current flows.

Extrinsic (impure) semiconductor: when a suitable impurity is added to a semiconductor;its current conducting property changes appreciably and this process of adding an impurity is doping and the added impurity is called a dopant. Adding extremely small amounts of impurities to pure semiconductors in a controlled manner is called doping.
N type semiconductor
Antimony, arsenic and phosphorus are called n-type impurities because they donate electrons. A material is said to be n type semiconductor when an impurity like antimony, arsenic and phosphorus are added to a pure semiconductor such as silicon.

Arsenic, antimony and phosphorus have five valence electrons (pentavalent) and when a semiconductor such as silicon is doped with one of these substance, some impurity atoms are incorporated in the tetrahedral structure resulting in N- type material. The fifth valence electrons is not rigidly bonded and is free to conduct, the impurity Atom donates a charge carrier. The resulting material is called n-type material.
It has an excess conduction: This means that the resulting n-type material contain free electro
P type semiconductor
indium, aluminum and boron have three valence electrons and when a semiconductor such as silicon is doped with one of these substance, some if the semiconductor atoms are replaced by semiconductor impurity resulting into P-type semiconductor.One of the four bond associated with the semiconductor material is deficient by one electron and this deficiency is called a Hole.
It has a deficit conduction: This is when holes give rise to conduction when it capture free electron from neighboring atom.

b) Explain what is meant by linkage current in a bipolar junction transistor and why this can be usually ignored.

Answer
Linkage current is the current which is produce by the result of minority charge carriers. And minority charge carriers are caused by few charge(electrons and holes) in a reversed current.
It is usually ignored because it is very small as a result of the minority charge carriers and hereby it is ignored.

C) state four application of the transistor.

. answer
A. It is used as detector
B. It is used as amplifier
C. It is used as motor rectifier
D. It is used as oscillator

5) in a tabular form differentiate between the three transfiguration terms of input resistance, output resistance, current gain and voltage gain.
Answer

5b) Draw the circuit symbol and give one(1) application of SCR, varactor diode, schottky diode and light emitting diode.
Answer
Application of SCR
(1) Used for switching an A.C power control.

Application of varactor diode
(1)

Application of schottky diode
(1)used for the construction of an integrated circuit design for high speed digital logic application.

Application of light emitting diode
(1)it is used as an indicator

6a) using well labelled diagram show the crystal lattice of intrinsic semiconductor and N type semiconductor and P type semiconductor.

Answer
When additional mobile electrons are introduce by doping a semiconductor with pentavalent atom i.e N type semiconductor these mobile electrons are called majority charge carriers while the relatively few holes produce as a result of intrinsic action in the semiconductor.
For P type semiconductor, the additional holes are introduced by doping a trivalent atom. The holes are called majority carriers while the relatively few electrons in the p type material as a result of intrinsic action are called minority charge carriers.
IN SUMMARY,
majority carriers in N type semiconductor are mobile electrons while minority charge carriers are holes.
IN P TYPE majority charge carriers are holes while minority charge carriers are electrons.
NOTE:Majority charge carriers are charge carriers that are dominating or well sufficient in that element why minority charge carriers are charge carriers that are few i.e not sufficient, very few.

6b) Explain majority and minority charge carriers as regard to extrinsic semiconductor.