EDC Lab Viva Questions with Answers

Electronic Devices and Circuits Lab Viva Questions with Answers

                                                          

1. Differentiate Electronics and Electrical Engg.?

We can say that electronics is the branch of science which deals with vacuum tubes, semi conductor materials like diodes, transistor etc. The maximum allowed voltage range of electronics engg is about 0 to 30 V. Electrical is the branch of science which deals with the currents in conductors, in other words, we can say that electrical engg deals with 0 to 230 V, 50 Hz operating devices. Operation of devices like motors, generators etc are included in electrical engg.  

2. Classify materials in terms of conductivity?

By the property of conduction of electricity, materials can be mainly classified as conductors, semiconductors and insulators. In conductors, the valance band (VB) and conduction band (CB) overlaps each other (i.e., energy gap is zero), so the electrons can easily move from valance band to the conduction band. Examples are Gold, Copper, Silver etc. In insulators, the energy gap is very wide (about 3 to 5 eV) and hence the electrons cannot flow from VB to CB. In semiconductors, the energy gap is narrower so the electrons from VB can move to CB by absorbing energy. The energy gap of Si is 1.1 eV and for Ge it is 0.7 eV.

3. Differentiate Semiconductor and conductors?

Semiconductor has four electrons in its VB. The energy gap of semiconductors is about 1 eV. So at normal condition the electrons cannot move from VB to CB. By giving some external energy to the electrons, electrons can move from VB to CB. The best semiconductors are Si, Ge, etc. The conductors have more than four electrons in its VB. The energy gap of conductors is 0 eV. So the electrons can easily move from VB to CB. Best conductors are Ag, Al, Cu.

4. Compare intrinsic and extrinsic Semiconductors?

Intrinsic semiconductors are the pure form of semiconductors. At room temperature, the conductivity of intrinsic semiconductor is zero. At room temperature, its VB is totally filled and the CB is empty. When some heat energy is supplied (as temperature increases), electrons can jump to CB and can move randomly. As temperature increases, the conductivity increases and the resistivity decreases. Extrinsic semiconductors are formed by intentionally adding impurities on it. At room temperature, the extrinsic semiconductor is very little conductive. The impurities added can be trivalent (resulting P type materials) or pentavalent (resulting N type materials).

5. Explain different types of extrinsic semiconductors and discuss their formation?

The extrinsic semiconductors are made of by adding impurities on fourth group elements of periodic table (Si, Ge etc). There are mainly two types of extrinsic semiconductors- P Type and N Type. P types are formed by adding trivalent impurities (third group elements in Periodic table such as arsenic, antimony, phosphorus) to the fourth group elements. N type semiconductors are formed by adding pentavalent impurities (fifth group elements like aluminum, boron etc) to the fourth group elements.

6. Discuss the process of doping?

Simply we can say that doping is the process of adding impurities. Extrinsic semiconductors are made by doping process. The two types of doped semiconductor materials are P type and N type materials.

7. Discuss the process of formation of PN Junction?

A P-N junction (technically termed as diode) is a piece of semiconductor material in which it’s one half is doped by P type impurity and the other half is doped by N type impurity. P-N junction was developed in the year 1936. The main advantage of P-N junction is it passes current in only one direction. So we can say that PN junction act as a switch in electronic circuits. The main application of PN junction is rectifier circuits.

8. Explain about cut in voltage (Barrier Potential)?

The cut in voltage is the minimum voltage in which a PN junction starts its conduction. For Silicon diode, it is 0.7 Volts and for Ge diode is 0.3 V. That is when the external applied voltage crosses the barrier potential, the diode conducts electrons.

9. Discuss about Forward bias and Reverse bias conditions?

If the externally applied voltage of the anode (P) is greater than the cathode (N) voltage in a PN junction diode, the diode is said to be in forward biased condition. If the anode voltage is less than the cathode voltage, then the diode is in reverse biased condition.

10. Compare between Diffusion and Drift currents?

The current formed by the diffusion of holes or electrons is termed as the diffusion current. In other words we can say that due to the non uniform concentration of charged particles, the transportation of charge occurs and this current is termed as diffusion current. Diffusion current doesn’t need an external applied electric field. Drift current is the current occurred due to the effect of an externally applied electric field. That is the motion of charged particles due to the effect of an externally applied electric field. If there is no electric field, the drift current becomes zero.

11. Describe the meaning of 1N4007?

IN4007 is actually represents a diode. The expansion of the diode is

IN means it is a single junction diode

400x indicates the voltage, current and power

4007 represents the reverse voltage from 50V to 1000V, the maximum forward current is 1A

12. Justify the Diode current Equation?                     

The diode equation can be derived as

I=I_o(e^(V/ηVT)-1),

Where, I_o is the Reverse saturation current, and the value of η=1 for germanium, and 2 for silicon.

13. What you mean by Peak inverse voltage (PIV)?

PIV is the maximum affordable reverse voltage of a diode. When the applied reverse voltage exceeds the PIV, the PN junction gets damaged (diode goes to breakdown condition). It is also known as Peak Reverse Voltage (PRV). The PIV is also termed as reverse breakdown voltage.

14 Discuss about Reverse saturation current?

The reverse saturation current (Ico) is produced due to the diffusive flow of minority charge carriers, when a diode is at reverse biased condition. At R-B condition, the holes will get attracted to the negative terminal of the applied cell and the electrons will get attracted to the positive terminal of the cell. Hence the depletion width increases and the flow of charges through the junction becomes zero. But there exists thermally produced some electron-hole pairs. These thermally generated electron-hole pairs are less in number and the electrons produced are repelled by the –ve terminal and the holes are repelled by the +ve terminal. This will cause a small current passes through the PN junction. This current is very small and is almost constant.      

15. Discuss about the testing of diode by a multimeter?

The working condition of diode can be easily tested by a multimeter. For testing a diode, the anode (P side) of the diode is connected to the positive terminal of multimeter and the cathode (N side) of the diode is connected to the negative terminal of the multimeter. If the readed voltage is in the range of 0.3 to 0.7 Volts, then the diode is in good condition.

16. Mention the capacitive effects formed in a P-N junction.

There are mainly two types of capacitance formed in a PN junction.

a. Diffusion Capacitance: The diffusion capacitance occurs in forward biased condition.

b. Transition capacitance: Transition capacitance occurs in the reverse biased condition.

17.Discuss about the Break down condition, compare avalanche Break down and Zener Break down?              

The breakdown occurs due to the impact of ionization of electron-hole pair in a PN junction diode is termed as the Avalanche breakdown.

At the reverse biased voltage condition, the electron hole pair breaks in a zener diode and this breakdown is termed as zener breakdown.

18. Discuss the effect of temperature on reverse saturation current and barrier voltage?

As the temperature increases, the reverse saturation current also increases. As the temperature raise by 10 degrees, the reverse saturation current doubles.

As the temperature increases, the barrier voltage decreases. For every temperature increase of 1 degree, the barrier voltage decreases by 2.5 mv

19. Differentiate between static and dynamic resistances?

The ratio of voltage and current (V-I ratio) at the Q-point(operating point) is the static resistance.

The inverse to the slop of the forward bias characteristic curve at operating (Q point) is termed as the dynamic resistance.

  

20. Mention the applications of  PN Diode and Zener diode?

The main applications of a PN junction diode are: Rectifier, clipper, clamper, switch etc.

Voltage regulator circuit is the main application of zener diode.

21.What is the major difference of PN junction and zener diode?

The main difference of PN junction diode and zener diode is the PN junction is lightly dopped and the zener diode is heavily dopped.

22. Zener diode is not used in forward biased condition. justify your answer?

The response of a zener diode in forward bias condition is same as that of a PN junction diode. So generally the zener diode is used in reverse bias state with zener breakdown.

23.Discuss the effect of temperature on Zener diode?

The effect of temperature on zener diode depends on the breakdown voltage. If the breakdown voltage (V_z) is less than 6V (breakdown voltage), V_z is inversely proportional to temperature.

The breakdown voltage V_z is directly proportional to temperature, if the the breakdown voltage is greater than 6V

24. Give the advantage of silicon over germanium?

The breakdown voltage of silicon is more than germanium. The reverse saturation current of silicon is less than that of germanium. Since the raw material is sand, silicon diode is cheaper than germanium diode. The cut-off voltage of silicon diode is 0.7 V and that of germanium is 0.3V.