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.