Semi Conductors

Question 0

Intrinsic conduction

Answer 0

When a semiconductor material starts to conducted due to heating the material

Question 1

Define semiconductor

Answer 1

A material with a conductivity or resistivity of the that between of a conductor and an insulator

Question 2

How is silicon an insulator.

Answer 2

• At very low temperatures the outer electrons of the silicon atoms are involved in bonding the atoms together. Electrons are not available to act as conductors.
• Almost no conduction takes place . The silicon is essentially an insulator.

Question 3

How is silicon a conductor at room temperature?

Answer 3

• At room temperature the silicon atoms vibrate more vigorously. Some of the bonds are broken.
• There are now free electrons and positive holes available to act as conductors.
• The silicon is now a conductor.
• The higher the temperature the greater the conduction.

Question 4

What is conduction due to in intrinsic conduction?

Answer 4

In intrinsic conduction the number of free electrons and positive wholes are equal and conduction is due to both.

Question 5

What is an LDR?

Answer 5

A light dependent resistor (LDR) is a semiconductor material that responds to light instead of heat. As intensity of the light falling on the L.D.R. increases the level of conduction increases. A brighter light means less resistance or more conductivity.

Question 6

What is extrinsic conduction?

Answer 6

Extrinsic conduction (doping) is when the addition of a controlled quantity of an impurity increases conduction.

Question 7

Explain n-type doping.

Answer 7

• A controlled quantity of a group V element such as Phosphorous is added to the silicon.
• Phosphorous has five outer electrons. Four of the outer electrons are involved in bonding and the fifth is available to act as a conductor.
• Adding phosphorous to silicon makes it a conductor.
• Due to heat in the surrounding air some of the bonds will be broken making electron-hole pairs available to conduct also.
• However the fifth outer electron of the Phosphorous is the majority charge carrier.
• Since the majority charge carrier is the negative electron this is called n-type doping.

Question 8

Explain p-type doping

Answer 8

• A controlled quantity of a group 111 element such as Boron is added to the silicon.
• Boron has three outer electrons . This results in a positive hole. This positive hole is available to act as a conductor.
• Adding boron to the silicon makes it a conductor.
• Due to heat in the surrounding air some of the bonds will be broken making electron-hole pairs available to conduct also.
• However the positive whole due to the boron is the majority charge carrier.
• Since the majority charge carrier is the positive hole this is called P type doping

Question 9

Describe a p-n junction

Answer 9

As single crystal of silicon is p-type doped on the left and n-type doped on the right.

• some free electrons diffuse across the junction from the n-type region to the p-type region
• some positive holes diffuse across the junction from the p-type region to the n-type region
• near the junction the n-type region loses electrons and become positively charged
• Near the junction the p-type region gains electrons and becomes negatively charged
• Eventually these charged regions are sufficient to prevent further diffusion of charges across the junction. A potential barrier has been set up.
• This potential barrier is called the junction voltage. it is 0.6 V for silicone and 0.2 V for germanium
• There are no charge carriers on either side of the junction and this area is called the depletion layer.

Question 10

Describe reverse bias p-n junction:

Answer 10

A voltage is applied across the p-n junction as shown above. Negative terminal to the p- type and positive terminal to the n-type .

• This external voltage reinforces the potential barrier.
• The width of the depletion layer is increased.
• The resistance of the p-n junction is very high. The p-n junction does not conduct.

Question 11

Describe forward bias p-n junction.

Answer 11

• A voltage is applied across the p-n junction as shown above. Positive terminal to the p-type and negative terminal to the n-type.
• the width of the depletion layer is greatly reduced.
• A significant current will flow if the external voltage exceeds the junction voltage which is 0.6 V for silicon.
• the resistance of the p-n junction is very low. The p-n junction is now a conductor.