Important Short Questions and Answers: PN Junction Devices

PN JUNCTION DEVICES

1.What is a PN Junction ?How is it formed?

In a piece of semiconductor material if one half is doped by P-type impurity and other half is doped by N-type impurity, a PN Junction diode is formed. The plane dividing the two halves (or) zones is called PN Junction.

2. What is meant by diffusion capacitance (CD)?

The capacitance that exists in a forward bias junction is called a diffusion (or) storage capacitance (Cp) whose value is usually much larger than Cr, which exists in reverse based junction. This also defined as the rate of change of injected charge with applied voltage

Cp = (dQ/dW),

where

dQ -» represents the change in the number of minority carriers stored outside the depletion region when a change in voltage across the diode,

dv is applied.

3.What is Zener diode?

Zener diode is a specially designed PN junction diode. A reverse biased heavily doped PN junction diode. A reverse biased heavily doped PN junction diode which is operated in the breakdown region is known as Zener diode. It is also called as voltage regulator diode or breakdown diode.

4. List out the applications of LED.

·                                LEDs are more popularly used in display clocks , audio and video equipments ,traffic lights.

·                                It is also used as light source in optical fibre communication.

5. Define transition capacitance of P-N diode.

When a diode is reverse biased , the holes in the p- side and the electrons in the n-side drift away from the junction , thereby uncovering more immobile charges. As a result the thickness of depletion increases . this leads to capacitance effect across the region called transition capacitance.

6. Distinguish between shunt and series voltage regulator.

·                                      Series regulator

In a series regulator the regulating element is in series with the load and the regulation is done by varying the voltage across the series element.

·                                      Shunt regulator

In a shunt regulator the regulating element is in shunt with the load and the regulation is done by varying the current across the shunt element.

7. Draw the VI Characteristics of Zener diode.

8. Derive the ripple factor of full wave rectifier.

The ripple factor is a measure of how successfully a rectifier converts ac to dc. That is it is the ratio of rms value of ac component to the dc value.

Ripple factor= V_r(rms)/V_dc

9. Define peak inverse voltage in a diode.

Peak inverse voltage is the maximum negative voltage which appears across a non conducting reverse biased voltage.

10. What is Drift Current?

Under the influence of the externally applied electric field , the electrons are accelerated in one particular direction. They travel at a speed equal to drift speed. This movement of electrons will give rise to a current which is defined as the drift current.

11. What is barrier potential at the junction?

Due to the presence of immobile positive and negative ions on opposite sides of the junction an electric field is created across the junction . The electric field is known as the barrier potential.

12.What is a Rectifier?

A rectifier is a device which converts a.c. voltage to pulsating d.c. voltage, using one or more

Pn junction diodes.  Its types

i)half wave rectifier

ii)full wave rectifier

13. What is meant by drift current?

When an electric field is applied across the semiconductor material, the charge carriers attain a certain drift velocity which is equal to the product of the mobility of the charge carriers and the applied electric field intensity E. The holes move towards the negative terminal of the battery and electron move towards the positive terminal. This combined effect of movement of the charge carriers constitute a current known as drift current.

14. Define Hall effect?

If a metal or semiconductor carrying current I is placed in a transverse magnetic field B , an electric field E is induced in the direction perpendicular to both I and B , This phenomenon is known as Hall effect.

15.Give some application of Hall Effect.

i. hall effect can be used to measure the strength of a magnetic field in terms of electrical voltage.

ii. it is used to determine whether the semiconductor is p – type or n- type material

iii. it is used to determine the carrier concentration

iv.it is used to determine the mobility.