semiconductors and p-n junctions

Question 1

describe layout of bands in a conductor

Answer 1

bands are close or overlap so the conduction band contains electrons that are free to move.

valence band completely filled, conduction partially

Question 2

describe layout of bands in an insulator

Answer 2

valence band is full but no electrons in conduction band

energy gap too big for electrons to move

Question 3

describe layout of bands in a semiconductor

Answer 3

valence band is full but no electrons in the conduction band

small energy gap between bands so electrons can move to conduction band when they have sufficient energy

Question 4

when atoms bond, what happens to electrons

Answer 4

they interact causing energy levels of each atom to change to energy bands

Question 5

what are the two top bands called

Answer 5

the valence and conduction bands

Question 6

what is the most commonly used semiconductor

Answer 6

silicon

Question 7

valency of silicon

Answer 7

4

Question 8

what does it mean by a ‘valency of 4’

Answer 8

4 outer electrons which need to be paired up with electrons from another atom

Question 9

what is doping

Answer 9

adding an impurity to a material to increase its conductivity and decrease its resistance.

Question 10

charge carrier in n-type doping

Answer 10

electron

Question 11

charge carrier in p-type doping

Answer 11

holes

Question 12

how is depletion layer formed in a p-n junction diode

Answer 12

• initially both sides neutral
• electrons from n-type drift to p-type to fill holes, leaving holes where they left
• n-type becomes + near junction and p-type -
• potential barrier, voltage across junction
• depletion layer*

Question 13

what is meant by biasing the diode?

Answer 13

applying a voltage

Question 14

two ways to bias a diode

Answer 14

1. forward bias

2. reverse bias

Question 15

forward bias

Answer 15

cell connected positive end to p-type and negative to n-type

Question 16

reverse bias

Answer 16

cell connected negative end to p-type and positive end to n-type

Question 17

how does a forward bias diode work?

Answer 17

electrons from n-type given enough energy to overcome depletion layer

repelled from negative end of battery and move anti-clockwise

Question 18

how does a reverse bias diode work?

Answer 18

electrons in n-type attracted to positive terminal of the cell causing depletion layer to increase in depth

LEDs etc. will only work if connected correct way

Question 19

LED

Answer 19

forward biased p-n junction that emits photons

electrons fall to lower energy level and release photon: colour depends on energy jump

Question 20

photovoltaic mode

Answer 20

no bias voltage

photons with sufficient energy incident on junction and their energy absorbed, exciting electrons to conduction band.

extra electrons in p-type move to n-type causing a potential difference in the junction

solar cells/solar panels