12th Class Physics CHAPTER 7 ELECTRONICS Short Question Answers

As we know that electrons are majority charge carries and holes are minority charge carrier in the n- type substance, whereas holes are majority charge carriers and electrons are minority charge carries in the p – type substances. Current produced by the flow of electrons is called electronic current in n- type substances, while the current produced by the flow of holes is called conventional current in p- type substance. If a battery is connected in the current, both the currents flow in the opposite directions because holes moves from the higher potential and electrons move from lower potential and electrons move from lower potential to the height potential point in an electric field.

Net charge on n – type or p – type substance is zero, because both these substances remain electrically neutral in the absence of a battery (or electric Field). Each and every atom in n – type or p –type is neutral. Thus, both of them will be electrically neutral as a whole.

As the anode (p –type) of the diode is at high potential with respect to cathode (n –type), so it is said to be forward biased. In this case, potential o anode is greater by 0.2 V than that of cathode. Therefore, this diode (or p- n junction) is forward biased. Moreover, in the case of forward biased junction, the electrons always flow from lower potential to higher potential against the electric field, so flow the electrons takes place from cathode to anode. Hence, the diode is forward biased.

In the case of P- n junction, n – region contains free electrons as majority charge carriers and p – region contains holes as majority charge carriers. Just after the formation of junction, the free electrons in the n – region due to their random motion, diffuse into the p – region. As a result of their diffusion, the recombination n of electrons and holes take place within in depletion region. Since the charge carriers are removed, so a charge less region is formed around the junction in which charge carriers are not present. Thus, we can say that depletion region has to net charge.

When diode is forward biased the width of the depletion region is narrowed down (i.e decreased) and when the diode is reverse biased the width of the depletion region is widened (i. e increased)

Height of potential barrier is between p and n side of silicon diode. When electron recombines with a hole, 0.7 eV energy is released in the form of a photon. No 0. 7 eV = 0.7 * 1.6 * 10¹⁹j
So,    E = 0.7 * 1.6 * 1019^j
But E = hf = 6. 63 * 10^-34 * f
Or f = E/6.63 * 10^-34     =   0.7 * 1.6 * 10¹⁹
                                                6.63 * 10^-34
or f = 1.6 * 10¹⁴ Hz
Again
λ = c/f =      3*10⁸              = 1.77 * 10^-6 m
1.6 * 10¹⁴

Or  λ = 1770 * 10^-9 m
= 1770 nm
Hence, photon emitted from Si diodes have wavelength much greater than visible light (wave length of visible light, 750  _ 400 nm). It means that wave length of the photons emitted from Si lies in infrared region which is not visible.

In reverse – biased state, the reverse current is almost negligible is the absence of light. But a reverse – current starts when its p –n junction is exposed to light. When light enters the junction, it creates electron – hole pair which causes the flow of small current in the reverse direction. As the in the tensity of light increases. So the number of electrons and holes also increase. As a result, reverse current increase as shown in the fig 18.12 (c) such a photo – diode can turn its current ON and OFF in house circuits in nano – seconds.

The base is very thin, of the order of 10^-6 m and the base region has very small doping level as compared to emitter and collector. Thus, it has only a small number of electrons and holes to recombine with the charges already present in the base. In other words, majority of the electrons coming from emitter do not combine with some holes and escape out of the base. As a result, lc  l_E and hence l_{B is} very small.

For normal operation of a transistor connected in a circuit, emitter – base junction (input) i is forward and base – collector junction (out put) is reverse – biased. In common – emitter amplifier, input is applied between the base and emitter. The output is obtained between collector and emitter. Thus the emitter – base junction (E- B) is forward – Based and collector – base (C- B) junction is reverse biased. Voltage applied across E – C junction is always greater than the voltage applied across E – B junction.

In an inverting amplifier the input signal voltage V_l to be amplified is applied at inverting terminal (-) through a resistor R_l and V_o is the input voltage. The non- inverting terminal (+) is grounded and its potential is zero. As open loop gain AoL is very high ( 10⁵) so by knowing the relation
AoL = V₀/ V₊ – V_–   (output/input)
For any value of Vo, V₊ – V .  0
Or = V₊  V
Since V₊  is zero is ground potential so V- is virtually at ground potential i. e V o
This is called principle of virtual ground
(B). Calculation of voltage gain. Current through R₁ = l₁ =     V_l  – V/R_l

Or  l_l = V_l/R_l…………………………………..(1)
Current through R₂  = l₂ = V – V₀/R₂
l₂  =  -V_0/R₂ …………………………..(2)
Using Kirchhoff  s current rule
l₁  = l₂
Putting the values of  l₁ and l₂  in equation (3) we get
V_l/ R_l = -V₀/ R₂
or V₀/V_i = -R₂/R₁
Gain of inverting amplifier = output/ Input

Or  G= V₀/ V_i
G = _–R₂/R₁
Hence, this is voltage gain of inverting amplifier.

Input of the gate are given below
A = 1          B = 0
(a). If output of the gate = o, then gate is AND gate or NOR gate or XNOR gate.
(b). When output of the gate = 1 then the gate is OR gate or NAND gate or XOR gate.

Electronics is the branch of physics which deals with the emission behavior and effects of electrons in devices and with the utilization of these devices. Electronic devices and circuit have very important role in the development of computers, telecommunication apparatus, automatic machines, audio- sets, video sets, automatic washing machines and calculator etc.

The substance of resistivity of order of 10-4 ohm meter is good conductors of electricity and the resistivity of insulators are of order 108 ohm – meter. There are certain substances which have intermediate values of resistivity i. e of order of 104 ohm- meter. They are known as semiconductors. Examples: The example of such substance (semiconductors) is the crystals of Geranium and silicon which lie in the fourth group of periodic table.

N – types substance:
Definition
The germanium or silicon crystals formed after adding the pentavalent impurity (donor impurity) is known as N – type substance.
Explanation:-
When a germanium a silicon crystal is formed after adding an impurity of an element from fifth group of the periodic table (i.e antimony, arsenic or phosphorous) the impurity donates free electrons as current carriers to the crystal.
P- type substance           
Definition:-
A  crystal of germ anent or silicon formed after adding a trivalent impurity (accepter impurity ) is known as P – type substance.
Explanation:-
When a trivalent impurity such as boron, Gallium or Indium is added to a germanium or silicon crystal, the three valence electrons of each trivalent atom from covalent bonds with three of its neighbors and there exists a vacancy for an electron in the fourth covalent bond. This vacancy is known as a hole and provides the crystal as a positive.

Amplitude modulation is that process in which sound waves to be transmitted are superimposed in electromagnetic waves of the aerial only, then sound waves are carried by the e.m waves with the velocity of light. The sound waves are converted into electrical oscillations by a microphone and then superimposed on e.m waves. The amplitude of modulated wave’s beings to fluctuate with the frequency of sound waves. Electromagnetic waves are called carrier waves.

A pour semi – conductor behaves like as insulator at temperature near absolute zero become because the valence electrons of each atom are tightly held in covalent bonds with neighboring atoms. When the temperature is increased to about 270C or some high voltage across the material is applied, some electrons become free by breaking the covalent. When the bond is broken and an electron is freed, an electron vacancy is left there in the covalent bond. The vacant electrons side is called a hole represented by a small circle as shown in the above fig. Breaking of covalent bonds results in generation of electron – hole pair and conduction due to charges produced by pair conductivity are both low at room temperature due to the low value of thermal energy which breaks the valence bonds.

Conductors:-
In case of conductors, the highest energy bond with some electrons is unfilled as shown if fif.(a) these electrons can be exited from lower to higher energy level within the bond. Such excitation is possible on applicable of even weak electric field.
Insulation:-
In an insulator the highest occupied level is completely filled. Also the forbidden energy band above the occupied band is wide. The electron cannot jump over from lower to upper permissible energy level (fig b)
Semi- conductor
In semi- conductor the gap between the filled energy band and next higher permitted energy band is small. It means that the forbidden band is small. The thermal energy, even at room temperature is sufficient to let some electrons from lower permitted band to higher permitted band i.e. conduction band (fig c) the electrons is conduction band are free to transport (carry) charge.

P- N junction
If a signal germanium crystal contains donor impurities (n – type impurities) on             one side and accepter impurities (p – type impurities) on the other side, a p – junction is formed. Such a junction between p and a materials forms a semi- conductor diode as shown in fig (a)
Initially there is an excess of holes to the left of the junction (p- region) and excess of electrons to the right (n – region) . A diffusion of electrons and holes across the junction in opposite directions takes place as shown in the fig(b). This change takes place in a narrow region called as the depletion region. As a result, a layer of net positive charge (holes) appears on the n- side of the junction and layer of net negative charge (electrons) appears on the p Side of the junction. Due to these two layers having opposite charges in the depletion region and electric field or potential barrier is set up at the junction. This potential barrier further steps the diffusion (movement) of electrons and holes across the junction.

Forward biasing:-
If we want to send current across the  p – n junction, we have to reduce the height of the potential barrier at the junction. This can be done by connecting the positive terminal of the battery to the  p-  side of the junction and negative terminal to the n – side of the junction as shown fig (a) the polarity of concentrations repels the reduces  the height of potential barrier and it gives easy way  to flow of electric current through the junction. Hence a p – n junction or a semi- conductor diode conducts when it is forward biased.
Reverse Biasing:-
On the order hand, if negative terminal of the battery is connected to the p- side of the junction and positive terminal to away from the junction, both the holes and the electrons move barrier at the junction due to attraction . Thus the height of potential barrier at the junction increases and depletion region becomes more wide as shown in fig (b)

In this state, no current flows through the junction and circuit. Hence semi – conductor diode does not conduct when it is reverse biased. We note that semi- conductor diode allows the current to flow only in one direction when it is forward biased. The flow of current is practically zero in the opposite direction when it is reverse biased. It acts as a tube diode.

Rectification:- The conversion of alternating current (A.C) into direct current (D.C) or pulsating current is called rectification. The device which is used to covert alternating current into direct current is known as rectifier. In most of the electronic devices like Radio and T.V etc, the diode serves as a rectifier. These are two types of rectifiers (i). Half – wave rectifier (ii). Full wave rectifier

LED stands for Light Emitting Diodes.
Definition:- Light emitting diodes is basically a p – n junction diode made from the semi – conductor gallium arsenide phosphate. When electric current passes through such a circuit, energy ids released in form of light (photons). It is called a light emitting diode (LED).

(ii).   LASER:- Laser stand for light amplification by stimulated Emission of Radiation
Definition:- A laser is device which produces very narrow intense beam of light having the following properties.
(i). It is monochromatic (of one frequency or one wavelength)
(ii). It is coherent (crest and troughs of beam are in phase)
(iii). It is unidirectional (radiations of beam travel in the same direction)
The light emitted by an ordinary light source is not only incoherent but also emitted in all directions. So laser light is different from the ordinary light.

Sami – conductor:-
The materials which are neither good conductors nor good incoherent but are partly (intermediately) conductor are called semi- conductors
Properties:-
(i). A_t ok, a semi conductor acts like an insulator
(ii). The resistivity of semi – conductor to the flow of electric current lies between those of conductors (10^-6 ohm_cm).
(iii) In the case of conductors the electronic resistance increases linearly with increase in temperature.
But in semi – conductors the resistance decreases experimentally with increase in temperature as shown in the graph.

Photo – Diode:-
A photo diode is a semi – conductor device usually made from silicon. It consists of a normal p – n junction with temperature window through which light can enter as shown in fig.
A p – type material is diffused on the n – type substance. The diffuse takes place through a window formed on the top of n – type material in silicon diode. This window is in contact with p – type region and serves as an electrode. The base is made of certain conducting metal on n – type material and it acts as another electrode. The window controls the area receiving the light.
Use: – Photo diodes are used as ‘fast’ counters which produce a current pulse, every time a been of light is light is interrupted.

Solar cell:-
A photo voltaic cell (solar cell) is a device which converts light energy into electrical energy.
When light is made to fail on this cell, the voltage across its terminals increase. The value of voltage increases with the increase of intensity of incident light. The photo – voltaic cell is basically a junction device.  The cells are made of semi – conductor materials. It is generally made from cither silicon or Selenium.
Uses:-
Photo cells are used in satellites are space vehicles to convert solar energy into electrical energy which can be used to operate the other electrons equipments.

Transistors      
Transistor is a semi – conductor device which which can amplify both threw current as well as voltage. There are two types of transistors:-
(i). P – n –p transistor
(ii). N –p – n transistor
(i). P – n- p Transistor
In p –n- p transistor type material is sandwiched between two p – type material as shown in the fig. (b)
 (ii). n- p –n Transistor:-
      In  n –p –n transistor p – type material is sandwiched between two n –type materials as shown in the fig (b).

Symbols of p –n – p and n – p-n transistors:-
The two types of transistors along with their circuit symbols are shown in the following figures. A transistor consists of three portions or regions. The three portions of the transistor are known as emitter, base and collector respectively.
(i). The middle portion of the transistor is called base.
(ii). The region on left hand side of transistor is called emitter.
(iii). The region on right hand side of transistor is called Collector.

Advantages. Following are the advantages of a transistor over a vacuum tube- (i). Transistors are very small in size as compared to the tubes. Thus, the instruments and equipments using transistors and of small size and handy for out – door entertainment and filed work. (ii). The operating life of the transistors is greater than that of the tubes. (iii). Transistors consume less power than the tubes because in tubes a large amount of power is needed to heat the cathode for thermion emission which is not at all needed by the transistors. (iii). Transistors require lower supply voltages from about 4.7 to 75 volts, compared with 75 to 350 volts generally required by the tubes. (iv). A transistor circuit is generally simpler than its equivalent tubes circuit. Disadvantages;- The only disadvantages of the transistor is that its characteristics are strongly temperature dependent.

When p –type and n type substances are joined together, the new arrangement is called a p –n junction. It is also called a semi –conductor diode.

When a crystal of germanium or silicon is grown in such a way that its one half is doped with a trivalent impurity and the other half with a pentavalent impurity, than p –n junction is formed.

The generation of a potential difference at the common boundary of a p – n junction due to movement of electrons from n- type substance is called potential barrier. Potential barriers are also called as junction barrier or potential hill.

(i). Depletion region:-
The charge less region around the p – n junction is called depletion region.
(Biasing). The application of some electric potential across the diode is known as biasing

The values of potential barrier for germanium and silicon are 0.3 V and 0.7 V respectively.

The ratio between forward bias voltage ΔV_f and current Δl_f is known as forward resistance of the p – n junction i.e.

R_f = ΔVf/Δl_f

In a half wave rectification, only one half of input cycle of an A.C. is converted into D. C. In a full wave rectification both positive and negative halves of an input A.C. is converted into D.C.

A semi conductor diode can be used to convert alternate current (A.C) into direct current (D.C). It means that semi- conductor diode can be used as a rectifier.

Following are the applications of photodiodes.
(i). They are used for automatic switching i.e. ‘ON’ or ‘OFF’ of a current.
(ii). They can be used to detect visible and invisible light.
(iii). They are used in logic circuits.
(iV). They are used in optical fibre communication equipment.

A p-n junction in which potential barrier between p and n regions is used to drive a current through external circuit when lights is incident on junction is called photovoltaic cell.

A photo diode is operated in reverse biased because in reverse bias it acts like a very low resistance and there is no need of extra energy to conduct the current which is supplied by incident lightr.

The working principle of a light emitting diode (LED) is based upon the emission of light energy through the surface of a diode in the form of photon.

It is an electronic instrument which is formed by the combined effect of n – type and p –type substance in such a way that the central substance is sand witched by its opposite substance at the other ends.

The ratio of input alternating amplified voltage ‘V_out’ to the low input alternating voltage ‘V_in’ is called voltage gain ‘B’ of a transistor. Mathematically, it is written as
B = V_out/V_in

Let l_c be the collector current and l_B be the base current than the ratio of collector current ‘l_c’ to the base current ‘l_B’ of a certain transistor is always constant. This constant ‘B’ is called as current gain of a transistor. Mathematically, it can be written as
B = l_c/l_B = Constant

An electronic device that converts low input alternating voltage into high output alternating voltage is called an amplifier. Amplification is done by the use of transistors.

Output resistance
It is resistance between output terminal and ground terminal of the operational amplifier. Its values are very low.
Input resistance:-
It is resistance between positive and negative inputs of the op- amplifier. Its values are very high – of the order of several mega ohms.

An operational amplifier (op – amp) consists of many discrete components made on a very small silicon chip which is further closed in capsule. This capsule provides the input and output connections for its external use.

Following are the main uses of operational amplifier. (i). If can be used inverting and non- inverting amplifier. (ii). It can be used as a comparator. (iii). It can be used an automatic night switch.

A position in an operating amplifier where inverting and non – inverting input voltage becomes equal, is called saturation.

It is the ratio of output voltage ‘V₀’ to the voltage difference ‘V_i’ between the inverting and non – inverting inputs of an operational amplifier.
That is,
A_Lo  = V_o/V_i

The equation of a gain ‘G’ of a non – inverting amplifier is given by
G = 1 + R_l/R₂
Where R_l and R₂ are the external resistances of an amplifier.
The power Gain ‘G’ is intendment of the internal circuit of an amplifier but depends upon externally connected resistance R₁ and R₂.

The op – amp acts as a comparator when it compares a certain voltage with respect to some reference voltage. This ability makes an op – amp as a comparator of voltages.

The circuit symbol of an operational amplifier is shown in the fig below. Input inverting out pot