5 Marks Questions

Question 15 Marks

What are N-type semiconductors? Their conductivity is higher than that of pure semiconductors. Why? Explain with figure.

Answer

N-type semiconductor : The semiconductor obtained by adding fifth group elements like $As , Sb , P$ etc. in the form of impurity atoms to a intrinsic semiconductor is called N-type semiconductor.
Phosphorus, arsenic or antimony have five electrons in their valence orbit. Therefore, when an atom of a pentavalent impurity replaces tetravalent Ge or Si , its four valence electrons form covalent bonds with the four adjacent atoms, but one valence electron remains. Due to the very weak bond with the impurity atom on it, it remains almost free. Therefore, at normal temperature itself this free electron transitions into the conduction band. In this way, each atom of the fifth group impurity element provides a free electron as a charge carrier. For this reason, conductivity also increases as a result of increase in the number of charge carriers.

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Question 25 Marks

On the basis of band theory, differentiate between conductors, insulators and conductors.

Answer

Distinguish between Conductor, Insulator and Semi-conductor

S.No.	Property	Conductor	Insulator	Semi-conductor
1.	Electrical conduc tivity and its value	Very high $10^{-7} mho / m$	Negligible $10^{-13}$ $mho / m$	In between conductor and insulator from $10^{-7} mho / m$ to $10^{-13} mho / m$
2.	Resistivity and its value	Negligible less than $10^{-5} \Omega-m$	Very high $10^5 \Omega- m$	In between $10^{-5} \Omega- m$ to $10^5 \Omega- m$
3.	Energy gap and its value	Zero or minimal	Very much high as in diamond $\Delta E_{g}=7 eV$	More than conductors but less than insulators. For ex., in Ge , $\Delta E _{ g }=0.72 eV$ and in Si , $\Delta E _{ g }=1.1 eV$
4.	Flow of current temperature	Due to free electrons and very high	Due to free electrons and negligible	Due to free electrons and hole but more than insulators
5.	Temperature-cofficient of resistance	Positive	Negative	Negative
6.	Effect on conducting on increasing tempe- rature	Conductivity decreases	Conductivity increases (very small increment)	Conductivity increases
7.	On adding impurities	Resistance increases and conductivity decreases	Resistance remains unchanged	Resistance decreases
8.	Flow of current	Occurs easily	Does not occur	Very rarely
9.	Example	Cu, Ag, Au, Na, Pt and Hg etc.	Wood, plastic, mica, diamond and glass etc.	Ge , Si , Ga, As etc.

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Question 35 Marks

What is meant by rectification? Draw a diagram of a half wave rectifier electric circuit. Write a brief description of its working. Draw the waveforms of input voltage and output voltage for this rectifier circuit.

Answer

Rectification is the process by which alternating current (voltage) is converted into direct current (voltage). The device used for this is called rectifier as mentioned earlier. A P-N junction diode forward biased current flows in forward bias and current flows in reverse bias does not (negligible current). Use of this uni-directional property of diode is done in rectification. There are two types of rectifiers.
Working process: In the first half cycle in alternating potential, the diode (D) is in the state of forward bias, hence current flows, but in the second half cycle, due to the reverse bias of the diode (D), current does not flow. Thus the load resistance $\left(R_L\right)$ in one complete cycle the current flows in the same direction (for half the time period). Hence the alternating current/potential gets rectified.

(i) In the above figure, in the positive half cycle of alternating potential, the diode (D) is in forward bias state due to the positive potential at the upper end of the secondary coil (S) of the transformer (T). Hence, current flows in the load resistance $R_L$, due to which the output voltage $V_o=i R_L$ is obtained between A and B .

(ii) In the above figure, due to negative potential at the upper end of the secondary coil (S), the diode (D) is in reverse bias state. Therefore, the current in $R _{ L }$ is zero and the output voltage $V_o=0$ is obtained between $A$ and $B$.
This process can be displayed in wave pattern :

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Question 45 Marks

What is meant by forward bias and reverse bias of p-n junction diode? Draw experimental electrical circuit diagram to study V-I characteristic in forward bias and reverse bias of p-n junction diode. Draw forward bias and reverse bias V-I characteristic curves of p-n junction diode and briefly explain the following :
(i) Threshold or cut-in voltage
(ii) Breakdown voltage.

Answer

Forward bias : When the P end of the P-N junction is connected to the positive end of the external battery and the N end is connected to the negative end of the battery, then the P end receives higher potential than the N end. This position of the joint is said to be in forward bias junction state.
Reverse bias: When the p end of the $p - N$ junction is connected to the negative end of the battery and the N end is connected to the positive end of the battery, then the junction is called reverse biased.
Forward bias characteristic curve : In forward bias system, the curve drawn between voltage $\left( V _{ f }\right)$ and forward current $\left( I _{ f }\right)$ is called forward bias characteristic curve. To obtain the forward bias characteristic curve, a circuit is arranged as shown in the figure.

Characteristic curve in Reverse Biasing : In the reverse bias system, the curve drawn in reverse voltage $\left( V _{ r }\right)$ and reverse current $\left( I _{ r }\right)$ is called reverse bias characteristic curve. To obtain the reverse bias characteristic curve, the circuit is arranged as shown.

(i) Threshold or cut-in voltage : In forward bias the current initially increases very slowly to almost negligible until the voltage across the diode exceeds a certain value. After this characteristic voltage, a very small increase in the diode bias voltage results in a significant (exponential) increase in the diode current. This voltage is called threshold voltage or cut-in voltage.
(ii) Breakdown voltage: When a P-N junction diode is in the reverse biased state, a sudden increase in the value of current to a higher value is shown at a certain value of voltage, this potential is called breakdown voltage or Zener voltage. This high value current can destroy a simple P-N junction.

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Question 55 Marks

Draw the complete I-V characteristic curve for an ideal P-N junction diode. Define the dynamic resistance in the forward bias state.

Answer

The experimental circuit arrangement to obtain the V-I characteristics of a forward biased P-N junction diode is shown in the figure.

Here, the potential difference V applied on the diode is changed through the potentiometer system, which can be read by a voltmeter connected in parallel to the diode. The current flowing in the diode corresponding to different values of potential was found to be I, noted by milliammeter. In this way the curve drawn for the values of V and I is obtained as shown in the figure.
Forward Dynamic Resistance : By definition of forward dynamic resistance,
$\begin{aligned}
r_f & =\frac{\text { Small change in forward bias potential }}{\text { Corresponding change in forward current }} \\
& =\frac{\Delta V}{\Delta I}
\end{aligned}$

$\Delta V$ and $\Delta I$ are shown in the figure to calculate the forward resistance near point $C$. If $\Delta V$ and $\Delta I$ are minimal then the dynamic resistance at point C will be the inverse of the slope of the tangent drawn to the curve at this point. The value of forward dynamic resistance is generally small (1 to 100 ohms).

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Question 65 Marks

Explain the concept of energy bands with a figure.

Answer

Energy band : When two atoms of a solid are close to each other as a result of interaction, each energy level gets divided into two energy levels of different energy, out of which the energy of one level is higher than the main energy

level and the energy of the other energy level is lower than the main level. The energy difference between them is very less. Similarly, as a result of interaction in a system of N atoms, the energy levels of each atom get divided into energy levels of N -different energies. Differentiation is not possible in these energy levels due to their very low energy levels. Hence an energy band is formed, in this way an energy band corresponding to each energy level is formed.

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Question 75 Marks

Define rectification.
Draw the circuit diagram of full wave rectifier. The semiconductor related to the given energy band diagram is: n-type semiconductor, p-type semiconductor or intrinsic semiconductor.

Answer

Rectification : Rectification is the process by which alternating current (voltage) is converted into direct current (voltage). The device used for this is called rectifier. A (P-N) junction diode conducts current in forward bias and does not conduct current in reverse bias (negligible current). This unidirectional property of diode is used in rectification.
Circuit diagram of full wave rectifier :

The semiconductor related to the energy band diagram given is an n-type semiconductor.

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Question 85 Marks

Describe the procedure for obtaining V-I characteristic curve in forward bias of P-N junction diode. Make a circuit diagram of the experimental arrangement.

Answer

The diagram to study the V-I characteristic (variation of current as a function of applied voltage) of a diode is shown in fig. (a). The voltage applied to the diode is connected through a potentiometer (or rheostat) so that the voltage applied to the diode can be changed. The value of current is noted for different values of voltage. A graph between V and I is shown in fig. (b). Here we use milliammeter for forward bias measurement because the required current is high. As can be seen in the figure, in forward bias the current initially increases very slowly and almost negligible until the voltage on the diode exceeds a certain value. After this characteristic voltage, a very small increase in the diode bias voltage results in a significant (exponential) increase in the diode current. This voltage is called threshold voltage or cut-in voltage. The value of this voltage is $\sim 0.2$ volts for germanium diode and $\sim 0.7$ volts for silicon diode.

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Question 95 Marks

In a semiconductor, electrons and holes move in opposite directions to each other. How? Explain with figure.

Answer

In a semiconductor, some electrons excited by receiving thermal energy at room temperature and transition from the valence band to the conduction band. The empty space created due to the departure of the electron is called 'hole'. The hole behaves like a positively charged particle.
(i) When the electron transitions from the covalent bond postion A, the electron located at another covalent bond position B makes the transition towards the hole created by absorbing thermal energy. With this the hole reaches from A to B.

(ii) The hole reaches C when the electron at B position transitions to another position C. Hence, in this way the electron transitions in the CBA direction and the hole in the ABC direction. In this way the hole and electron move in opposite directions.

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Question 105 Marks

Explain graphically the flow of current in semiconductors in the presence of an external electric field.

Answer

At room temperature, some electrons of semiconductors break covalent bonds and transition from valence band to conduction band. Due to breaking of covalent bond, a vacant space is formed, it is called hole. This hole generated due to electron transition behaves like a positively charged particle. In this way the hole works as a carrier of positively charged current. The electron from another covalent bond position transitions towards the hole generated in place of the free electron from the covalent bond position and the hole moves towards the second covalent bond position. In this way, holes and electrons move randomly in opposite directions to each other. External electrical field when applied, their motion becomes regular in which it is in the direction of electrical field and hole move in the opposite direction of electrical field and carry the current.

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Question 115 Marks

With the help of circuit diagram, explain the working of junction diode as a full wave rectifier. Draw its input and output waveforms. Which characteristic property of junction diode makes it suitable for rectification?

Answer

Characteristic property of junction diode : In the V-I characteristic of a junction diode, we see that it allows electric current to flow only when it is forward biased.

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