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Chapter 18 Electronics Physics 12th Class Punjab Short Questions

Question No. 1 - What is photovoltaic cell? Discuss its working?

Answer No. 1 - It consists of a thick n type region covered by a thin p-layer type. When exposed to light, it absorbs photon which generated electron-hole pairs. The electric field at the junction moves electron and holes and a current flow through the external circuit. And this current is directly proportional to the intensity of light.

Question No. 2 - What is photodiode? Write down its two main applications.

Answer No. 2 - Photodiode is used for the detection of light. It is used in reverse biased state. Reverse current increase with the intensity of incident light. When no light incidents then reverse current will be negligible. It is used as:
1. Detection of both visible and invisible radiations.
2. Automatic switching

Question No. 3 - Why the base current is transistor is very small?

Answer No. 3 - Base is very thin so a number of electrons attracted by collector and very few enter into the base and base current is very small.

Question No. 4 - How is the XOR gate so called?

Answer No. 4 - One of its most commonly used applications is as a basic logic comparator which produce a logic "1" output when its two inputs bits are not equal. Because of the, XOR gate has an inequality status being known as an odd functions.

Question No. 5 - What is OR-GATE?

Answer No. 5 - OR-GATE implements the logic of OR operation. It has two or more inputs and a single output. Its output will be zero only when all the inputs are zero.

Question No. 6 - What do LED and LASER stand for?

Answer No. 6 - LED stands for Light Emitting Diode.
Laser stands for Light Amplification Stimulated Emission Radiation.

Question No. 7 - The anode of a diode is 0.2V positive with respect to the cathode, Is it forward biased?

Answer No. 7 - Yes when anode is 0.2V positive with respect to cathode, it is forward biased. But the value of potential barrier for germanium is 0.3 V and for silicon is 0.7 V. Therefore there will be no conduction of current.

Question No. 8 - Give two applications of gated in control system.

Answer No. 8 - Gates are widely used in control system. They control the function of the system by monitoring some physical parameters such as temperature, pressure or some other physical quantity of the system. Sensors are required to operate gates.

Question No. 9 - Why is the photodiode operated in reverse biased state?

Answer No. 9 - Photodiode is used for the detection of light. So it is used in reverse biased state. Reverse current increase with the intensity of incident light. When no light incidents, then reverse current will be negligible.

Question No. 10 - Define logic system and logic gates.

Answer No. 10 - A digital system deals with the quantities or variable which have only two discrete values or state.
The electronic circuits which implements the various logic operation are known as logic gates.

Question No. 11 - What is principle of vital ground?

Answer No. 11 - In an operational amplifier, if the non inverting terminal is grounded by the concept of virtual ground the inverting terminal is also at ground potential though there is no physical connection between the inverting terminal and the ground. This is the principle of virtual ground.

Question No. 12 - Give any two characteristics of operational amplifier.

Answer No. 12 - Input Resistance: It is the resistance between the + and - input of the amplifier. Whose value is of the order of several mega ohms.
Output resistance: It is the resistance between the output terminal and ground. Its value is only a few ohms.

Question No. 13 - Define current of gain of a transistor?

Answer No. 13 - The ratio of output current to input current is called current gain of a transistor. The ratio of collector current to base current is called current gain of a transistor.

Question No. 14 - What is semiconductor diode?

Answer No. 14 - P-N-Junction is called semiconductor diode.

Question No. 15 - What is meant by rectification?

Answer No. 15 - The conversing of alternating current signal into pulsating direct current signal is called rectification. The current for this purpose is called rectifier circuit.

Question No. 16 - How is p-n junction formed?

Answer No. 16 - A p-n junction is formed when a crystal of germanium or silicon is grown in such a way that is its one half is doped with a trivalent impurity and the other half with a pentavalent impurity.

Question No. 17 - Define Open Loop gain of operational amplifier.

Answer No. 17 - The ratio of output voltage to voltage difference between non-inverting and inverting inputs, when there is no external connection between the input and output is called open loop gain of operational amplifier.

Question No. 18 - Define Depletion region?

Answer No. 18 - A region in a semiconductor device, usually at the junction of p-type and n-type materials in which there is neither an excess of electrons nor of holes is called depletion region. It charge less region.

Question No. 19 - Why the biasing requirement of the junction of a transistor for its normal operation?

Answer No. 19 - For the normal operation the base emitter junction of transistor is forward biased and collector base junction is reverse biased.

Question No. 20 - Give four application or uses of photodiode?

Answer No. 20 - It is used as:
1. Detection of both visible and invisible radiations.
2. Logic circuits
3. Automatic switching
4. Optical communication equipment

Question No. 21 - Write a note on LED.

Answer No. 21 - Light emitting diode are made from the special semiconductor such as gallium arsenide and gallium arsenide phosphide in which the potential barrier between p and n side is such that when electrons combines with a hole during forward biased conduction, a photon of visible light is emitted.
LED are used in 7-segment display small light sources etc.

Question No. 22 - Differentiate between the forward biased and the reserve biase.

Answer No. 22 - When p-side is positive and n-side is negative, the diode is forward biased. The width of depletion region reduces to which more current is allowed to flow across the junction. The forward resistance is few ohms.

Question No. 23 - Explain why an ordinary Silicon diode does not emit light.

Answer No. 23 - Ordinary silicon is opaque to light, So it does not emit visible light. It emits infrared light. To emit light, gallium arsenide or gallium arsenide photodiode are used as semiconductor.

Question No. 24 - What is the net charge on a n-type or p-type substance?

Answer No. 24 - Since they are electrically neutral substance. So the net charge on them is zero.

Question No. 25 - How the normal operation of transistor is achieved?

Answer No. 25 - For the normal operation the base emitter junction of transistor is forward biased and collector base junction is reverse.
In a common emitter amplifier, input signal is applied between base and emitter and output signal is taken across collector and emitter. Similarly emitter base junction is forward biased and collector base junction is reverse biased.

Question No. 26 - What is the role of potential barrier in a diode? How it is formed in diode?

Answer No. 26 - At the formation of pn-junction, the free electrons in n-region because of their random motion diffuse into the p-region. As a result of this diffusion a region is formed around the junction consisting of positive and negative ions. Due to charge on these ions a potential difference develops across the depletion region. This potential difference is called potential barrier, stops further diffusion of electron into the p-region.

Question No. 27 - How does the motion of an electron in n-type substance differ from the motion of holes in a p-type substance?

Answer No. 27 - More mobile and rapid than holes in p-type substance because electrons do not require holes for their motions but p-type substance because electrons do not require holes for their motion but hole depend upon electrons for their motion. Both move in opposite direction.

Question No. 28 - What is the potential barrier? What is the value of potential barrier of SI and Ge?

Answer No. 28 - The potential difference across the depletion region which acts as a barrier to the flow of charge carriers is called potential barrier. The value of potential barrier for germanium is 0.3V and for silicon is 0.7 V.

Question No. 29 - What is AND Gate?

Answer No. 29 - It is a logic gate that implements the truth table of AND operations. It has two or more input and single output.

Question No. 30 - What is the effect of forward biased and reverse biasing of a diode on the width of depletion region?

Answer No. 30 - When forward biased then the width of depletion region decreases and when reverse biased then the width of depletion region increase.

Question No. 31 - Why charge carriers are not present in the depletion region?

Answer No. 31 - This is due to the fact when an electron from an n region diffuses into the p-region, it leaves behind a positive ion. When this electron recombines with the hole in the p-region, a negative ion is formed, So no charge carrier are available in this region though it contains immobile positive and negative ions.

Question No. 32 - How the current flows in forward and reverse biased diode?

Answer No. 32 - In forward biasing, the external potential difference supplies energy to free electrons in n-region and holes in p-region to overcome the potential barrier, a current of the order of a few milli-amperes begins to flow across the p-n junction.
In reverse biasing, no current flow due to the majority charge carrier. However a very small current of the order of of few micro amperes flow across the junction due to minority charge carriers. It is also known as reverse current or leakage current.

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