semiconductor diodes ch: 2

Question 1

Explain working of p-n junction diode

Answer 1

p-type has holes majority carriers & n-type has electrons majority carriers. At the junction the holes and electrons are close to each so electrons get attracted towards holes & vise versa.

When atom gains an extra electron from outside it’ll become -ion, so each electron that has crossed the junction from the n-side will become -charged. When atom loses an electron from outside it’ll become +ion, so each hole that has crossed the junction from the p-side will become +charged.

Question 2

what is P-N junction?

Answer 2

A region where p-type and n-type semiconductors are joined

Question 3

Explain working of forward bias

Answer 3

The holes are repelled by the +terminal & the electrons are repelled by the -terminal. So the electrons & holes move towards the junction and fill in the the depletion region as well as barrier potential. At a particular voltage the depletion region breaks & a large amount of current flows

Question 4

Explain working of reverse bias

Answer 4

The electrons are attracted towards +terminal & holes are attracted towards -terminal, this increases the depletion region & barrier potential, but a small amount of carriers manage to cross the junction & a small amount of current flows in reverse biased condition.

Question 5

What is Biasing?

Answer 5

The process of applying external energy to a diode.

Question 6

Reverse saturation current

Answer 6

current that flows due to minority carriers in reverse biased condition.

Question 7

V-I forward characteristics

Answer 7

+terminal to anode & - to cathode. When voltage is applied initially negligible or no current flows through the diode(becuz of barrier potential). When the external voltage increases due to barrier potential a large amount of current starts flowing & the diode is said to be conducting.

Question 8

Knee voltage

Answer 8

voltage at which diode conducts

Question 9

V-I reverse characteristics

Answer 9

-terminal to anode & + to cathode. When the voltage is applied initially the current is small and constant , as voltage is increased reverse saturated current increases & damages the diode.

Question 10

breakdown voltage

Answer 10

the voltage at which reverse saturated current increases & damages the diode.

Question 11

What is Avalanche breakdown?

Answer 11

When high electric field is applied to the reverse biased condition the electrons gain kinetic energy & break other covalent bonds & create more hole-electron pairs, these pairs cross the depletion region.

Question 12

What is Zener breakdown?

Answer 12

The p-n junction is is heavily doped so that the electrons in valence bond on p region can jump off to conduction band of n region.

Question 13

Characteristics of zener diode

Answer 13

*it’s forward characteristics are similar to that of a normal p-n junction diode.
*initially a small amount of current flows, if the voltage increases above the zener voltage there’s a sharp increase in the current, now dure to this there is only change in current & the voltage remains the same

Question 14

Applications of p-n junction & zener diode

Answer 14

p-n junction: *wave shaping circuits *used as switch *communication circuits *rectifier diode in power supply.
zener diode: *voltage regulator *wave shaping circuits *protection circuits *fixed reference voltage

Question 15

What is LED

Answer 15

p-n junction that emits light when in forward bias LED(light emitting diode)

Question 16

working of LED

Answer 16

*consists of 3 layers p-type semiconductor, n-type semiconductor & active region.
*works on the concept of electroluminescence
*When external voltage is applied the valence electrons break bonds with the parent atom and enter conduction band ,the conduction band has a higher energy level so the electrons are free to move, but eventually their energy level lowers and releases energy in form of photons.

Question 17

What is Varactor diode

Answer 17

A diode whose internal capacitance changes with change in voltage (reverse biased condition)

Question 18

Explain the working of varactor diode?

Answer 18

*when the p-n junction diode is connected in reverse biased condition then the depletion layer acts as dielectric of capacitor and the P and P region acts as capacitor plates.
*When the voltage increases the depletion region widens & the distance b/w 2 plates is increased & capacitance in reduced.
*When the voltage decreases thn the depletion layer narrows down & this reduces the distance b/w 2 plates & capacitance increases.

Question 19

Characteristics of Varactor diode?

Answer 19

*low cost
*small and light weight
*doesn’t work in forward biased condition
*produces less noise

Question 20

Applications of LED and Varactor diode?

Answer 20

LED: *traffic signal lights *remote controllers *decoration lights *alarms
Varactor diode: *low noise parametric amplifiers *automatic frequency control device *self balancing bridge circuits *radio &television

Question 21

What is a tunnel diode?

Answer 21

when the impurity atoms is increased in a normal diode it becomes tunnel diode.

Question 22

V-I characteristics of of tunnel diode

Answer 22

Initially if the forward voltage is increased current increases rapidly till it reaches its max value called peak current (Ip) and the corresponding voltage peak voltage(Vp).
If the voltage is increased even more current decreases, till it reaches its minimum value known as valley current (Iv).
If the voltage is increased further, then the current acquires the normal
PN junction diode value.
The region between the peak point (A) and the valley point (B) is negative resistance region because the current decreases with an increase in voltage. The current which increases rapidly when small increased in voltage is
known as Tunneling current.

Question 23

What is a Photo diode?

Answer 23

2 terminal P-N junction device which operates in reverse biased condition.
has a small transparent window which allows the light to strike p-n junction
reverse current increases with increase in light intensity.
no incident light, the reverse current is minimum, and is known as Dark current.
light intensity increases, the value of reverse current increases.

Question 24

Characteristics of Photo-diode

Answer 24

vertical line represents the
reverse current flowing through the device
and the horizontal line represents the
reverse-biased potential.
The first curve represents the dark current
that generates due to minority carriers in
the absence of light.
curve shows almost equal spacing in
between them. This is so because current
proportionally increases with the luminous
flux.