p-n junction electrical and optical properties

Question 1

What are the steps to make a p-n junction?

Answer 1

Step 1 : diffusion

Step 2: drive in - more diffusion

Question 2

What is the first step of a p-n junction? (4)

Answer 2

Diffusion
- Begins with just the n-type silicon, as the silicon has grown u put in an dopant.
- Clean silicon surface (very clean)
- Deposit small amount of Boron (very small) on surface
- Heat to about 1173K (very hot) for about 30min to increase to
diffusion coefficient

Question 3

What is the second step of a p-n junction? (2)

Answer 3

Drive in more diffusion
• More heat is applied at 1350C to drive in the Boron (acceptor dopant)
• The acceptor (p-type) doping is large enough that it dominates the donor concentration and we have a region – the p-n junction where the p type material is next to the n-type material

Question 4

How do you make electrical contacts for p-n junction diode?

Answer 4

• Metal is deposited on the p-type material and the n-type material creating a diode.
• Bonding wires are attached

Question 5

What is the junction of the p-n junction?

Answer 5

A region where the electrons and holes meet each other, electrons and holes “destroy” each other. Left with a region where there is no conductivity

Question 6

What happens to the carriers when there is no voltage applied under thermal equilibrium?

Answer 6

The atoms are spread equally on each side, with protons on the p-side and electrons on the n-side.

Question 7

What happens to the carriers when the diode is under forward bias, positive voltage is applied to p-type material?

Answer 7

Holes repel the positive charge and the electrons repel off the negative charge from the battery

Question 8

What happens to the carriers when the diode is under reverse bias, positive voltage applied to n-type material? (2)

Answer 8

Holes are attracted to the positive charge and away from the junction. Similarly for electrons.
The junction depletes

Question 9

What happens if there is no charge carriers in a p-n junction?

Answer 9

If there are no charge carriers the conductivity drops to 0 and it becomes an insulator

Question 10

When does a p-n junction conduct more efficiently?

Answer 10

when the p region is positive with respect to the n region that’s called forward bias

Question 11

What is the reverse bias?

Answer 11

When the p region is negative with respect to the n region that’s called reverse bias and the diode is a poor conductor

Question 12

What happens if enough reverse bias voltage is applied to the p-n junction diode?

Answer 12

If enough reverse bias voltage is applied the depleted (insulator layer) breaks down and the diode will conduct

Question 13

What is happening in the reverse bias?

Answer 13

No carriers and so has a high resistance and no current flowing

Question 14

What happens during the forward bias?

Answer 14

When the forward bias case (positive voltage on the p region), a large current flows

Question 15

What occurs in the pnp bipolar transistor?

Answer 15

The current that is injected into the base region creates carriers in the depletion region and increases the current flowing between the emitter and collector

So a signal applied to the base is amplified

Question 16

What is a pnp bipolar transistor made up of? (9)

Answer 16

input voltage
forward biasing voltage
p - emitter
junction 1 
n - base
junction 2
p- collector 
reverse-biasing voltage
load (output voltage)

Question 17

What happens if you inject a small signal in a pnp bipolar transistor? (2)

Answer 17

If the signal put in here is small then a much larger signal between the emitter and collector
The small signal you’ve injected causes a big change in the conductivity

Question 18

What is the depletion region?

Answer 18

Electrons and holes diffuse from the p and n regions at the p-n junction they meet and recombine – in the depletion layer,

Question 19

What happens in the depletion layer?

Answer 19

In the depletion layer there are no carriers but there are positive and negatively charged ions so associated with the charges on the ions, creating an unbalanced change giving rise to a electric field and thus a voltage

Question 20

What is the voltage called in the depletion region?

Answer 20

built-in voltage

Question 21

What does the built-in voltage do?

Answer 21

Can stop diffusion from occurring any further

Question 22

Where can light and matter exchange energy?

Answer 22

Light and matter can only exchange energy in energy quanta determined by the photon energy and available energy levels

Question 23

How does absorption occur in metals (optical properties)?

Answer 23

• Absorption of photons by electron transisiton
• Metals have a fine succession of energy states.
• Near-surface electrons absorb visible light.

Question 24

How does selected absorption occur in semi-conductors (optical properties)?

Answer 24

Absorption by electron transition occurs if hν > Egap

• If Egap < 1.8 eV, full absorption; color is black (Si, GaAs)
• If Egap > 3.1 eV, no absorption; colorless (diamond)
• If Egap in between, partial absorption; material has a color.

Question 25

What is the Egap?

Answer 25

The space between unfilled states and filled states

Question 26

What happens to the photons in semiconductors during absorption?

Answer 26

hν > Eg
Photons with an energy greater than the band gap energy are strongly absorbed
The photon energy promotes an electron from the valence band to the conduction band and leaves a hole behind in the valence band

Question 27

What is the process of direct gap semiconductors?

Answer 27

In the depletion region of a forward biased diode the electrons and holes meet and recombine and emit photons

Question 28

What are LED energy bands?

Answer 28

In a forward biased pn junction in the depletion region electrons are at the bottom of the conduction band - they recombine with holes at the top of the valence band. The photons are emitted with an energy approximately the same as the band-gap energy.

Question 29

How does emission of light occur in a semiconductor?

Answer 29

Emission by electron transition occurs around hν = Egap

Question 30

How are solar cells operated? (4)

Answer 30

• • incident photon produces hole-elec. pair.
• • typically 0.5 V potential.
• • current increases w/light intensity
• The light that can be converted into useful electrical power by Si solar cells – it only absorbs light with a wavelengths smaller than 1.3μm

Question 31

What is polarisation?

Answer 31

Light propagates through a material by creating
a polarisation – which related to the number of
the dipoles

Question 32

What is the refractive index, n?

Answer 32

transmitted light distorts electron clouds

Question 33

In a material, why is light slowed down?

Answer 33

In a material, light is slowed down because of its interaction with the dipoles and the velocity of light in the material,

Question 34

What is dispersion?

Answer 34

The change in refractive index with photon energy is called dispersion – and it gives rise to the colour dispersion of a prism. The dispersion relates to how quickly the dipoles form when the electric field associated with light is applied

Question 35

What happens to red and blue light during refraction in two different mediums, glass and air?

Answer 35

So in glass, mostly SiO2,the blue light experiences a larger refractive index than red light and is refracted more than red light thus the prism disperses the colours

n1(Blue)>n1(Red) so
θr (Blue)>θr (Red)

Question 36

What can optical fibres do?

Answer 36

Optical fibres are used extensively to guide light that has been encoded with digital data that is the light is switched on and off to represent bits of information – in the long haul core of the internet.

Question 37

What is the optical fibre used in telecommunications?

Answer 37

Optical fibre cross-section - it has a core glass, n1, and a cladding glass, n2 crucially n1>n2
So there is total internal reflection at the boundary of the core and cladding glass and the guided light never leaves the core

Question 38

How is the optical fibre used in telecommunications? (3)

Answer 38

• The pulses of light are used to transport the information in the optical fibre
  • The pulses emerge from the fibre smaller and fatter – the pulsewidth is
  broadened due to dispersion – yes the same effect that causes the prism to
  disperse the colours
  • The optical fibres are most transparent for light with wavelengths in the near
  infrared around 1550nm – the loss is around 0.2dB/km – about every 100km the pulses need to Regenerated, Repeated and Retimed in what are called 3R repeaters

Question 39

What are the parts of the optical communications systems? (7)

Answer 39

Encoder is done using silicon microprocessors

The electrical to optical (E-O) conversion needs semiconductors with band-gap close to photon energies with minimum fibre transmission loss

fibre-optic cable
repeater
fibre-optic cable

The optical to electrical (O-E) conversion needs semiconductors with band-gap close to photon energies with minimum fibre transmission loss

Decoder is done using silicon microprocessors

Question 40

What are the materials used for light sources (E-O): first telecoms window?

Answer 40

Made from AlGaAs/GaAs materials.

A laser diode is very similar to the LEDs we discussed previously but with the end facets cleaved to a mirror finish- the light then bounces between the mirrors - Lasers are more efficient than a LEDs at converting electrical energy into optical energy

Question 41

What are the materials used for light sources (E-O): second and third telecoms windows?

Answer 41

Made from InGaAsP/InP materials.

The 4 element (quaternary) alloy InGaAsP is required for latticed matched alloys

The light is emitted from the region show and flashes on and off to represent bits (1 &0s) in what is known as On Off Keying (OOK)

Question 42

What are photodiodes (O-E)? (3)

Answer 42

The alloy InGaAs is extensively used for photo- detection in the second and third windows.

Often in a p-i-n structure as shown. It’s like the solar cell but with a larger depletion layer (i-region)

The light creates electron-hole pairs in the In0.47 Ga0.53As alloy (band-gap 1600nm) and the built-in field drives the current around an external circuit to give an electrical output.