Electronic Devices

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Questions:

Generic 5

9 x 5 = 45

(i) Using the necessary circuit diagram, draw the transfer characteristics of a base-biased transistor in CE configuration. With the help of these characteristics, explain briefly how the transistor can be used as an amplifier?
(ii) Why are NAND gate called universal gates? Identify the logical operations carried out by the circuit given as below.

How is a transistors biased to be in active state ?

Shalini's mother wants to purchase a TV set. Shalini and her mother visited electronic shops to look for some branded TV. The dealer showed them so many TVs such as LCD and LED TVs. Now they were confused which set to buy. Finally after discussing with friends, reading revelent magazines and searching the internet shalini convinced her mother for purchasing LED TV.
(a) What are the values displayed by Shalini ?
(b) What is a LED ?
(c) Which semiconducting material can be used for constructing LED ?

A student of class 12 was trying to understand the concepts of semiconductors and insulators in terms of energy gaps.
(a) How would you explain these concepts to the students ?
(b) What values should we imbibe from this in our daily life ?

(i) Differentiate between three segments of a transistor on the basis of their size and level of doping
(ii) How is a transistor biased to be in active state?
(iii) With the help of necessary circuit diagram, describe briefly how n-p-n transistor in CE configuration amplifies a small sinusoidal input voltage. Write the expression for the AC current gain.

Meeta father was driving her to the school.At the traffic signal she noticed that each traffic light was made of many lights instead of a single bulb. When Meeta asked this question to her father's he explained the reason for this.
Answer the following questions based on above information:
(i) What were the values displayed by Meeta and her father?
(ii) What are the tiny lights in traffic signals called and how do these operate?

Manoj wanted to do social work during vacations.He visited a remote village where there was no electricity. He made up his mind to help the villagers for getting the solar panels. For this he educated the villagers about techonology and uses of solar panels.he requested the villagers to Villagers agreed and applied for the solar panels. They got the same from government at reduced price. When the solar panels started working,the villagers were happy.
Read the passage and answer the following questions:
(i) What is solar panel?
(ii) What is the basic principle of working of a solar cell?
(iii) What are the basic values you assess in Manoj?

Zener diode is a specially designed \(p-n\) junction diode,in which both \(p-side\) and \(n-side\) of \(p-n\)junction are heavily doped. The zener diode is designed to operate in the reverse break down voltage region continuously without being damaged? Zener diode is used to remove the fluctuations from the given voltage and thereby provides a voltage of constant magnitude (i.e., zener diode is used as voltage regulator).
Read the above paragraph and answer the following questions :
(i) What is the most important use of Zener diode?
(ii) What are the essential conditions for proper working of Zener diode?
(iii) What do you learn from the above study?

Suppose a 'n'-type wafer is created by doping Si crystal having \(5\times10^{ 28 }\)atoms/\({ m }^{ 3 }\)with 1 ppm concentration of As.On the surface 200 ppm Boron is added to create \('p'\)region in this wafer. Considering \({ n }_{ i }=1.5\times10^{ 16 }m^{ -3 }\)
(i) Calculate the densities of the charge carriers in the \(n\& p\) regions.
(ii) Comment which carriers would contribute largely for the reverse saturation current when diode is reverse biased.

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Answers Key:

Generic 5

9 x 5 = 45

The active region of a transfer characteristics curve can be used to explain the transistor as an amplifier.
The resistance of output circuit is large being in reverse bias and resistance of input circuit is low being in forward bias.
When input voltage, VBE comes in active region, Ie flows in output and Vo varies significantly as V_o = V_CE = V_CC - I_CR_L This change in output voltage.is obtained as amplified form.
(ii) NAND gates are termed as universal gates because all three basic gates namely AND, OR and NOT can be made using NAND gate.
The given circuit perform the logic operations of AND gate as Y = \((\overset { = }{ A.B } )\) = A.B

Transistor is said to be in active active state when its emitter-base junction is forward biased and base-collector junction is reverse biased.

(a) The values displayed by Shalini are :
(i) Creativity
(ii) Curiosity
(iii) Sharing Knowledge.
(b) LED means light emitting diode which emits light of chateristic wavelength when conducting.
(c) We can use that semiconducting material for which forbidden energy gap is 1.8 eV or more so that light photons emitted lie in visible light range.

(a) By drawing energy band gap.
(b) (i) Ready to change with a little push.
(ii) Redy/Eager to learn and move to a higher level.

(a) Emitter: It is of moderate size and heavily doped
Base: It is very thin and lightly doped Collector : It is moderately doped and larger in size.
(b) Transistor is said to be in active state when its emitter-base junction is (suitably) forward biased and base-collector junction is (suitably) reverse biased.

(c)When a small sinusoidal voltage is superposed on the dc base biased, the base current will have sinusoidal variation superimposed on the value of I_E. As a consequence, the collector current also will have sinusoidal variations, superimposed on the
Value of I_C, producing corresponding (amplified) changes in the value of V₀_.
ac current gain, \({ \beta }_{ ac }={ \left( \frac { \Delta { I }_{ C } }{ \Delta { I }_{ B } } \right) }_{ { V }_{ CB } }\)

(i) Meeta is curious and have a quality of learning. Meeta's father is a helping hand to her daughter and have good knowledge about the scientific facts.
(ii) Meeta's father told that these are LED nd their power consumption is very low
(iii) LED is a heavily doped \(p-n\) junction diode which under forward bias emits spontaneous radiation. The order of minimum band gap of an LED to emit light in the visible region is about \(1.8 eV\)

(i) Solar panel is a panel having large number of solar cells fitted side by side on it.
(ii) Solar cell is basically a solar energy concerter.It is a \(p-n\)junction device which converts solar energy into electrical energy .If this energy is stored in a capacitor,it can be used for glowing bulbs.
(iii) Manoj displayed his will to do something for society.Trough his shear in a capacitor,it can be used for glowing bulbs.
(iii) Manoj displayed his will to do something for soceity.Through his shear efforts,he got electricity within the reach of villagers, who were suffering otherwise.?The work of Manoj was highly appreciated by villagers.

(i) The most important use of zener diode is in marking the constant voltage power supply.
(ii) The essential conditions for proper working of Zener diode are as follows:
(a) The Zener diode must be reverse biased
(b) The Zener diode must have voltage greater than Zener break down voltage.
(c) The Zener diode is to be used in a circuit where current is less than the maximum Zener current limited by the power rating of the given Zener diode.
(iii) To be happy in life,one has to be like a Zener diode which evens out all fluctuations and displays a constant output. Ups and downs are part of life.One who evens them out is successful and happy.

(i)When As is implanted in Si-crystal, n-type wafer is created. The no of majority carrier electrons due to doping of As is
\(n_{ e }={ N }_{ D }=\frac { 1 }{ 10^{ 6 } } \times5\times10^{ 28 }=5\times10^{ 22 }/m^{ -3 }\)
No of minority carriers (holes) in \(n-\)type wafer is
\({ n }_{ h }=\frac { { n }_{ 1 }^{ 2 } }{ n_{ e } } =\frac { (1.5\times10^{ 16 })^{ 2 } }{ 5\times10^{ 22 } } =0.45\times10^{ 10 }/m^{ 3 }\)
when B is implanted in Si-crystal p-type wafer is created with no of holes
\({ n }_{ h }=N_{ A }=\frac { 200 }{ 10^{ 6 } } \times(5\times10^{ 28 })=1\times10^{ 25 }/m^{ 3 }\)
Minority carriers (electrons) created in the p-type wafer is
\({ n }_{ e }=\frac { { n }_{ 1 }^{ 2 } }{ n_{ h } } =\frac { (1.5\times10^{ 16 })^{ 2 } }{ 1\times10^{ 25 } } =2.25\times10^{ 10 }/m^{ 3 }\)
When \(p-n\)a junction is reverse biased, the minority carrier holes of \(n-\)region wafer \((n_{ h }=0.45\times10^{ 10 }/m^{ 3 })\) would contributte more to the reverse saturation current than minority carrier than minority carrier electrons \((n_{ e }=2.25\times10^{ 7 }/m^{ 3 })\)