Explain why a reverse biased p-n junction will not conduct.
- Energy is supplied to the p-type material by the battery.
- This increases the internal electric field strength.
- The electrons in the conduction band of the n-type do not have enough energy to overcome the larger potential difference of the electric field.
- Electrons cannot pass into the conduction band of the p-type so the p-n junction will not conduct.
Explain how an n-type semiconductor can be created.
The semiconductor has impurities added that have five electrons in its outer shell. Four of these are used to fill the valence band. The fifth electron is in the conduction band. This is free to move and so conduction increases.
What is the definition of 1 amp?
1 Coulomb of charge passing a point in 1 second.
How is a p-n junction connected in reverse bias?
Connect n-type to positive of supply Connect p-type to negative of supply
What is the terminal potential difference, Vtpd?
It is the potential difference which is able to supply the user when the circuit is complete and current flows.
What is potential difference?
The number of Joules of energy transferred per Coulomb of charge.
Describe the shape of a graph of current in a capacitance circuit against time for charging a capacitor.
Starts from a maximum value and decreases to zero.
In which mode is this operating?
Photoconductive
Explain why an LED may not produce light EVEN when it is forward biased and connected to a supply voltage.
- The electrons in the n-type conduction band are not provided with enough energy to move into the conduction band of the p-type.
What is capacitance?
The charge stored per volt.
Describe the shape of a graph of voltage across capacitor against time for charging a capacitor.
Starts from 0V. Increases to the supply voltage.
From the graph, how do you find the EMF ?
When the current = 0A. Find point on graph where current = 0A, read off corresponding voltage.
Explain how a p-type semiconductor can be created.
The semiconductor has impurities added that have three electrons in its outer shell. These three do not completely fill the band. So there are electrons free to move in this band so conduction increases.
Explain how an LED produces light.
- When the LED is forward biased. Energy supplied to the electrons in the n-type conduction band by the battery.
- The electrons now have enough energy to overcome the internal electric field and move through the electric field into the empty conduction band of the p-type semiconductor material.
- The electron then drops from the conduction band of the p-type to the valence band of the p-type.
- The electron gives out the energy as a photon of light as it drops into a lower energy band.
From the graph, how can you calculate the internal resistance of the cell?
internal resistance = - gradient
How is the internal electric field/layer created in a pn junction?
- Electrons from the conduction band of the n-type move into the conduction band of the p-type and drop into the valence band of the p-type material.
- This leaves the n-type material slightly positively charged and the p-type material slightly negatively charged around the junction.
- This creates a potential difference which gives an electric field.
- The electrons in the conduction band of the n-type, do not have enough energy to overcome the potential difference of the electric field, to pass into the conduction band of the p-type.
Describe the shape of a graph of voltage across capacitor against time for discharging a capacitor.
Starts from the supply voltage Decreases to 0V.
Explain how a photovoltaic cell produces an electric current.
- A photon is absorbed by an electron in the valence band of the p-type material.
- If this energy is high enough the electron can jump into the conduction band of the p-type.
- The electron experiences a force from the internal electric field.
- The electron is moved through the electric field to the conduction band of the n-type.
- This creates a current.
What are the lost volts, Vlost?
This is the potential difference unavailable to the user because of the internal resistance of the supply.
When calculating power should you use Vpeak or Vrms?
Vrms
What is the short circuit current?
The maximum current a supply can give - this is achieved when the terminals of the supply are joined with a short thick wire ( almost no external resistance)
Explain how a pure semiconductor conducts.
Conduction band is empty. Valence band is full. There is a small energy gap. If the temperature is increased the electrons in the valence band can gain enough energy to jump the gap into the conduction band. These electrons are free to move and so conduction increases.
Explain what happens to the reading on the voltmeter when the circuit is changed from figure 1 to figure 2.
Resistance in the circuit decreases.
Current in the circuit increases.
Lost volts will increase Vlost = Ir
E - Vlost = Vtpd
So the reading on the voltmeter will decrease as it measures the Vtpd.
In which mode is this operating?
Photovoltaic
