Electronics

SEMICONDUCTOR DEVICES–CLASS 12 PHYSICS QUESTION BANK

1. Draw a circuit diagram of OR gate.

2. What is a logic gate?

3. What types of charge-carriers are there in a n-type semi conductor?

4. Give the logical symbol of an AND gate. Mark the inputs and outputs.

5. Draw the logic symbol for a NAND gate.

6. Which biasing will make the resistance of p-n junction high?

7. Write the truth table for OR gate.

8. What is the change in the collector current, in a transistor of a.c. current gain 150, for a 100µA change in its base current?

9. When the voltage drop across a p-n junction diode is increased from 0.65V to 0.70V, the change in the diode current is 5mA. What is the dynamic resistance of the diode?

10. How does conductivity of a semiconductor change with rise in its temperature? Read more…

Solution to HC Verma Problem

“Referring to HC Verma Part 2, chapter 29, Electric field and potential, Q 69 I think that Force is inversely proportional to square of the distance between the charged particles, so acceleration is not constant and hence v*v  =  u*u  + 2*a*s   is not valid in this case. [...]

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Transistor as a Switch

(This is one of the Expected Question for AISSCE Physics 2011)

Explain the action of transistor as a switch.

To answer this question (with proper understanding) you should have an idea of the three regions in the VI characteristics of a transistor. viz; Cutoff region, Active region and the saturation region.

When the input bias of a CE transistor (Vbe) is very low, the transistor does not conduct. So the output voltage (Vce) will be almost equal to Vcc, the output bias voltage.

By increasing the input voltage, the transistor can be brought to saturation region when the transistor will be conducting well. Now the pd across the transistor (Vce) will be very low.

This is the working principle of transistor as a switch.

Law of mass action

When an intrinsic semiconductor is doped with pentavalent impurity, the number of free lectrons increases. Will it result in any change in the number of holes already present?

The answer is YES.

When the semiconductor is doped with pentavalent impurity, the n_e increases and will result in a decrease in n_h according to the relation.

n_i² = n_in_h

This relationship is called the mass action law.

When the no. of free electrons increases, greater no. of holes already present get neutralised.

The difference between electricity and electronics

Electricity is the branch of Physics (?) which deals with the flow of electrons and its effect. The electric bulb, electric iron etc are electric devices as they are based on the direct effects like heating effect of current.

Electronics is the branch of Physics (?) which deals with the control of the flow of electrons (by allowing or disallowing or regulating the percentage of flow) through a device.

The television, calculator, computer are all electronic devices. Evidently they are not limited to the direct effect of flow of electrons through a conductor or a resistor ar any other transducer.

Now, try to classify the following appliances into electric devices and electronic devices.

1. transistor radio
2. sodium vapour lamp
3. ups
4. television
5. electromagnet
6. geyser
7. calling bell
8. voting machine
9. soldering iron
10. quartz clock
11. desoldering machine
12. electric iron

Nowadays, many electric appliances are also fitted with some electronic component. Can you site some examples?

Doping

Doping is the deliberate addition of desirable impurities to an intrinsic semicounductor to modify its conducting propertires.

The impurity atoms added are called dopants and the semi conductor so formed is called extrinsic semiconductor.

Depending on the type of impurities added, extrinsic semiconduictors are divided into two : n -type and p-type.

Note:

The extrememly pure form of semiconductor is called intrinsic semiconductor.

The conductivity of an intrinsic semiconductor is extremely low (but greater than insulators), and therefore as such there is no much practical use of intrinsic semiconductor. But when suitably doped to make extrinsic semiconductor, it works wonders. We get greater control over the flow of electrons.