p-n junction electrical and optical properties Flashcards
What are the steps to make a p-n junction?
Step 1 : diffusion
Step 2: drive in - more diffusion
What is the first step of a p-n junction? (4)
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
What is the second step of a p-n junction? (2)
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
How do you make electrical contacts for p-n junction diode?
- Metal is deposited on the p-type material and the n-type material creating a diode.
- Bonding wires are attached
What is the junction of the p-n junction?
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
What happens to the carriers when there is no voltage applied under thermal equilibrium?
The atoms are spread equally on each side, with protons on the p-side and electrons on the n-side.
What happens to the carriers when the diode is under forward bias, positive voltage is applied to p-type material?
Holes repel the positive charge and the electrons repel off the negative charge from the battery
What happens to the carriers when the diode is under reverse bias, positive voltage applied to n-type material? (2)
Holes are attracted to the positive charge and away from the junction. Similarly for electrons.
The junction depletes
What happens if there is no charge carriers in a p-n junction?
If there are no charge carriers the conductivity drops to 0 and it becomes an insulator
When does a p-n junction conduct more efficiently?
when the p region is positive with respect to the n region that’s called forward bias
What is the reverse bias?
When the p region is negative with respect to the n region that’s called reverse bias and the diode is a poor conductor
What happens if enough reverse bias voltage is applied to the p-n junction diode?
If enough reverse bias voltage is applied the depleted (insulator layer) breaks down and the diode will conduct
What is happening in the reverse bias?
No carriers and so has a high resistance and no current flowing
What happens during the forward bias?
When the forward bias case (positive voltage on the p region), a large current flows
What occurs in the pnp bipolar transistor?
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
What is a pnp bipolar transistor made up of? (9)
input voltage forward biasing voltage p - emitter junction 1 n - base junction 2 p- collector reverse-biasing voltage load (output voltage)
What happens if you inject a small signal in a pnp bipolar transistor? (2)
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
What is the depletion region?
Electrons and holes diffuse from the p and n regions at the p-n junction they meet and recombine – in the depletion layer,
What happens in the depletion layer?
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
What is the voltage called in the depletion region?
built-in voltage
What does the built-in voltage do?
Can stop diffusion from occurring any further
Where can light and matter exchange energy?
Light and matter can only exchange energy in energy quanta determined by the photon energy and available energy levels
How does absorption occur in metals (optical properties)?
- Absorption of photons by electron transisiton
- Metals have a fine succession of energy states.
- Near-surface electrons absorb visible light.
How does selected absorption occur in semi-conductors (optical properties)?
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.
What is the Egap?
The space between unfilled states and filled states
What happens to the photons in semiconductors during absorption?
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
What is the process of direct gap semiconductors?
In the depletion region of a forward biased diode the electrons and holes meet and recombine and emit photons
What are LED energy bands?
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.
How does emission of light occur in a semiconductor?
Emission by electron transition occurs around hν = Egap
How are solar cells operated? (4)
- 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
What is polarisation?
Light propagates through a material by creating
a polarisation – which related to the number of
the dipoles
What is the refractive index, n?
transmitted light distorts electron clouds
In a material, why is light slowed down?
In a material, light is slowed down because of its interaction with the dipoles and the velocity of light in the material,
What is dispersion?
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
What happens to red and blue light during refraction in two different mediums, glass and air?
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)
What can optical fibres do?
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.
What is the optical fibre used in telecommunications?
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
How is the optical fibre used in telecommunications? (3)
- 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
What are the parts of the optical communications systems? (7)
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
What are the materials used for light sources (E-O): first telecoms window?
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
What are the materials used for light sources (E-O): second and third telecoms windows?
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)
What are photodiodes (O-E)? (3)
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.
