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?

Kritika Singh was enjoying TV programme at her home with her younger brother Surya at night. Suddenly, the light went off causing darkness all over. Surya asked her to bring candle along with matchstick to put the TV switch OFF. Kritika at once picked the mobile phone and pressed the button lighting up the surrounding. Her younger brother was surprised and asked, where the light was coming. Kritika proudly showed her mobile.
Read the above passage and answer the following questions:
(i) Which value is displayed by Kritika?
(ii) Name the material used in the formation of LED.
(iii) LED works, in which biasing?

Shorya usually enjoyed loud music. One day his music system was not producing very loud sound. Shorya gets disappointed. He discussed his problem with his big brother Kamal. Kamal advised him to connect an amplifier in series with the amplifier of the stereo system. This increased the sound of Shroya's music system considerably. Read the above passage and answer the following questions:
(i) Which value is displayed by Kamal?
(ii) Why the loudness of music system get increased on connecting amplifier?
(iii) Give the equation for current gain in the transistor as an amplifier.

Sanjay was preparing an electronic project for science exhibition. He required to light the LED using a 6 V supply. LEDs need only a very small current to make them light and they do not heat up in use. So he put a resistor in series to limit the current. Then there would be p.d. of 4 V across the resistor as there is always 2.0 V across the LED itself when it is conducting. The current should be 10 mA through both LED and the resistor. He could use the resistance by equation, R = V/I to calculate the value of R.
\(R=\frac { V }{ I } =\frac { 4V }{ 10mA } =\frac { 4V }{ 0.01A } =400V\)
Thus the protecting resistor should be around 400 \(\Omega \)
(a) What are the values exhibited by Sanjay?
(b) A semiconductor has equal electron and hole concentration of \(6\times 10^{ 8 }/m^{ 3 }\) . On doping with certain impurity, electron concentration increases to \(9\times 10^{ 12 }/m^{ 3 }\) .
(i) Identify the new semiconductor obtained after doping.
(ii) Calculate the new hole concentration.
(iii) How does the energy gap vary with doping?

Anita was thinking that C, Si and Ge have same lattice structure, but C is insulator while Si and Ge are intrinsic semiconductors. For its answer she met her friend Sunita. Sunita explained her that the four bonding electrons of C, Si and Ge lie respectively in the second, third and fourth orbit. So, energy required to take out an electron from these atoms known as ionisation energy Ig will be the least for Ge, followed by Si and the highest for the least for Ge, followed by Si and the highest for C. Hence number of free electrons for conduction in Ge and Si are significant while negligibly small for C.
(a) What are the values shown by Sunita ?
(b) If a pure silicon crystal has \(5\times { 10 }^{ 28 }atoms/{ m }^{ 3 }\). It is doped by 1ppm concentration of pentavalent arsenic.
If n₁ = 1.5 x 10¹⁶/m³, then calculate the number of electrons and holes.