1 Marks Question

Question 11 Mark

An incomplete sentence about transistors is given below:
The emitter ...... junction is ....... and the collector ..... junction is ..... The appropriate words for the dotted empty positions are, respectively.

'Collector' and 'base'.
'Base' and 'emitter'.
'Collector' and 'emitter'.
'Base' and 'base'.

Answer

'Base' and 'base'.

Explanation:

In transistors, the emitter−base junction is forward biassed and the collector−base junction is reverse biassed. This is done to make majority carriers cross the emitter−base junction; this helps the electric current to flow through the transistor.

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Question 21 Mark

When a semiconducting material is doped with an impurity, new acceptor levels are created. In a particular thermal collision, a valence electron receives an energy equal to 2kT and just reaches one of the acceptor levels. Assuming that the energy of the electron was at the top edge of the valence band and that the temperature T is equal to 300K, find the energy of the acceptor levels above the valence band.

Answer

2KT = Energy gap between acceptor band and valency band

$\Rightarrow2\times1.38\times10^{-23}\times300$

$\Rightarrow\text{E}=(2\times1.38\times3)\times10^{21}\text{J}$

$=\frac{6\times1.38}{1.6}\times\frac{10^{-21}}{10^{-19}}\text{eV}=\Big(\frac{6\times1.38}{1.6}\Big)\times10^{-2}\text{eV}$

$=5.175\times10^{-2}\text{eV}=51.75\text{meV}=50\text{meV}.$

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Question 31 Mark

When a p-type impurity is doped in a semiconductor, a large number of holes are created. This does not make the semiconductor charged. But when holes diffluse from the p-side to the n-side in a p-n junction, the n-side gets positively charged. Explain.

Answer

A p-type semiconductor is formed by doping a group 13 element with group 14 element (Si or Ge). As the group 13 element has only 3 electrons in its valence shell and the group 14 element has 4 electrons in its valence shell, when the group 13 element, say, Al, replaces one Si in the silicon crystal, only 3 covalent bonds are formed by it. And the fourth covalent bond is left in need of one electron. So, it creates a hole. Since the atom as a whole is electriclly neutral, the p-type semiconductor is also neutral.
In a p‒n junction, when the diffusion of holes takes place across the junction because of the difference in the concentration of charge carriers from p to n sides, these holes neutralise some of the electrons on the n side. So, the atom attached with that electron becomes one electron deficient and hence positively charged. This makes the n side of the p‒n junction positively charged and the p side of the p‒n junction negatively charged.

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Question 41 Mark

In a transistor:

The emitter has the least concentration of impurity.
The collector has the least concentration of impurity.
The base has the least concentration of impurity.
All the three regions have equal concentrations of impurity.

Answer

The base has the least concentration of impurity.

Explanation:

When the emitter of a transistor is forward biassed, the majority carriers move from the emitter to the collector through the base. As the base is thin and lightly doped, only small amount of a combination of electrons and holes takes place, leading to weak base current. This makes the collector current equal to the emitter current.

If we make any other choice for least concentration of impurity, we will have a low value of collector current. Thus, the purpose of a transistor will not be fulfilled.

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