Semiconductor Electronics: Materials, Devices and Simple Circuits Test 31

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Semiconductor Electronics: Materials, Devices and Simple Circuits Test 31

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Question 1
1 / -0

The truth tables of logic gates A, B, C, D are given here. Identify them correctly.

A
A: OR, B: AND, C: NOR, D: NAND

B
A: OR, B: NOR, C: AND, D: NAND

C
A: AND, B: OR, C: NAND, D: NOR

D
A: OR, B: NOR, C: NAND, D: AND

Solution

(A) : Truth table shows that the output is low when both the inputs are low and the output is high when either of the input is high. Thus this truth table represents the $$OR$$ gate.

(B) : Truth table shows that the output is high when both the inputs are high and the output is low when either of the input is low. Thus this truth table represents the $$AND$$ gate.

(C) : Truth table shows that the output is high when both the inputs are low and the output is low when either of the input is high. Thus this truth table represents the $$NOR$$ gate. Also this truth table is the inverse to that of (A).

(D) : Truth table shows that the output is low when both the inputs are high and the output is high when either of the input is low. Thus this truth table represents the $$NAND$$ gate. Also this truth table is the inverse to that of (B).
Question 2
1 / -0

LCD stands for:

A
Light Carrying Diode

B
Liquid Crystal Display

C
Long Crystal Display

D
Light Crystal Display

Solution

$$LCD$$ stands for "Liquid Crystal Display".

$$LCD$$ is a special thin flat panel that can let light go through it, or can block the light. (Unlike an $$LED$$ it does not produce its own light). The panel is made up of several blocks, and each block can be in any shape. Each block is filled with liquid crystals that can be made clear or solid, by changing the electric current to that block.
Question 3
1 / -0

The rating of a zener diode is $$1 W, 15 V$$. Find the zener maximum current.

A
$$66.7 A$$

B
$$1 A$$

C
$$15 A$$

D
$$66.7 mA$$

Solution

Here the maximum power, $$P_{max}=1 W$$ and $$V_Z=15 V$$
Using, $$P=VI, $$ the zener maximum current is $$I_Z(max)=\dfrac{P_{max}}{V_Z}=\dfrac{1}{15}=66.7 mA$$
Question 4
1 / -0

Which of the following logic expressions represents the logic diagram shown ?

A
$$X = A\overline B + \overline A B$$

B
$$X = \overline {AB} + A B$$

C
$$X = \overline {AB} + \overline A \overline B$$

D
$$X = \overline A\overline B + A B$$

Solution

Trying to understand the logic of the given circuit, the end signal X is a summation of 2 signals.
These 2 signals are the outputs of two different AND gates:
One AND gate having A and B as its inputs and the other AND gate having negative A and negative B as its inputs.
So, we have:
$$X = (A. B ) + (\overline A. \overline B)$$
This is given as Option D.
Question 5
1 / -0

Find the maximum zener current for the zener diode as shown in figure. Given, $$V_Z=6 V, R_Z=1.5 \Omega, R=400 \Omega $$

A
$$35.00 mA$$

B
$$30.87 mA$$

C
$$34.87 mA$$

D
$$34.87 A$$

Solution

Here, $$I_T=I_Z+I_L$$

Thus, the zener current will be maximum when $$I_L=0$$ for infinite load resistance. So, $$I_T=I_Z(max)$$

By Kirchhoff's voltage law, $$V_{in}-V_Z=(R+R_Z)I_T$$

Thus, $$I_Z(max)=I_T= \dfrac{V_{in}-V_Z}{R+R_z}=\dfrac{20-6}{400+1.5}=34.87 mA$$
Question 6
1 / -0

A zener diode can be used as

A
oscillator .

B
voltage regulator.

C
rectifier.

D
transformer.

Solution
Question 7
1 / -0

A $$7.2 V$$ zener diode $$(7.2 V$$ at $$ 20^oC) $$ has a positive temperature coefficient of $$0.0007 /^oC$$. What is the zener voltage at $$45^oC$$ ?

A
$$7.20 V$$

B
$$7.33 V$$

C
$$7.13 V$$

D
$$6.36 V$$

Solution

Due to the positive temperature coefficient of zener diode, the increase in voltage of zener diode due to increase in temperature $$=V_0\alpha\Delta \theta$$
$$=7.2\times 0.0007\times 25V=0.13V$$
Hence, the zener voltage at temperature $$45^{\circ}$$ is $$7.2V+0.13V=7.33V$$
Question 8
1 / -0

Which of the following is/are true regarding breakdown voltage of Zener diode?

A
If the reverse bias voltage across a p-n junction diode is increased, at a particular voltage the reverse current suddenly decreases to a large value

B
The holes in the n-side and the conduction electrons in the p-side are accelerated due to the reverse bias voltage.

C
The voltage at which the rate of creation of hole-electron pairs is increased leading to the increased current is called avalanche breakdown

D
None of the above

Solution

Consider applying an external voltage to the junction with the the negative terminal connected to the P type material and the positive terminal connected to the N type material. For the P-N junction this would require injecting electrons into the P type material. These electrons would recombine with holes and therefore further deplete the majority charge carriers in the P type semiconductor.

Also removing electrons from the N type material would obviously cause further depletion the free electrons (the majority charge carriers in the N type semiconductor). This causes a widening of the depletion layer and an increase in the charge at the junction which reinforces the barrier voltage to oppose the applied voltage. The net result is that when the P-N junction is reversed biased it blocks current flow.
Question 9
1 / -0

Resistance of a wire is r ohm. The wire taken is double its length, then its intial resistance in ohm will be

A
$$\dfrac{r}{2}$$

B
$$4r$$

C
$$2r$$

D
$$\dfrac{r}{4}$$

Solution

$$R=\rho\dfrac{l}{A}$$

$$\implies R\propto l$$

Hence, on doubling length, resistance becomes twice that is $$2r$$.

Answer-(C)
Question 10
1 / -0

The attached figure shows the characteristics of:

A
Zener diode

B
Avalanche diode

C
Photodiode

D
LED

Solution

$$Answer:-$$ A
Zener diode is a form of semiconductor diode in which at a critical reverse voltage a large reverse current can flow.