Light Emitting Diodes
A light source that can be used within a circuit is the Light Emitting Diode (L.E.D.). This component gives out very little heat Energy, and so is more efficient than a Lamp.
The diagram below shows the symbol for an L.E.D.:-
Note - In circuit diagrams, two arrows denote light :-
Two arrows leaving symbol - light given out.
Two arrows entering symbol - light taken in.
LEDs also control the direction of current within a circuit. LEDs will only conduct when connected in a certain direction within the circuit. For an explanation of this, see Higher section below.
The large triangle of the symbol can be used as a pointer arrow:-
Towards the negative terminal - LED lights.
Away from the negative terminal - LED does not light.
The diagram below shows an LED connected correctly within a circuit:-
Protecting LEDs
In the above circuit, the LED is connected in series with a Resistor. LEDs require only very low Current to function, and are easily damaged, so the function of the Resistor is to reduce the current flow to a safe level.
The size of the Resistor depends on the supply Voltage and the requirements of the LED. The method below allows the calculation of the correct Resistor:-
The above diagram shows a simple LED circuit. If the LED functions at the given ratings, what is the required value of the Resistor?
Vs = VR + VLED
VR = Vs - VLED
VR = 12 - 2
VR = 10 V
IR = ILED = 10 mA = 0.01 A
R = VR / IR
R = 10 / 0.01
R = 1000 Ω
R = 1 kΩ
Diodes - Higher Level
A Diode is simply a P-N Junction connected to a power supply. Within the Higher Physics course there are two main functions of these P-N Junctions, depending on how they are used within a circuit.
There are two main ways that the P-N Junction are used:-
1. The L.E.D.
2. The Photo-Voltaic Cell.
The Light Emitting Diode
As can be seen above, an LED is a component used within a circuit to give light. At N5 level, the fact that an LED (and all Diodes) can only conduct in one direction was skipped over.
The reason that an LED can only conduct in one direction is due to the physical makeup of the LED itself.
An LED and all other diodes are simply a P-N Junction, placed within a circuit. The reason that they only conduct in one direction is every much tied to an understanding of the Band Gap.
Forward-Biased
The diagram below shows a P-N Junction within a series circuit:-
In the above diagram the P-type Semiconductor is connected to the positive terminal and the N-type Semiconductor is connected to the negative terminal. When connected in this way, the Semiconductor is said to be Forward-Biased. It is in this configuration that the Diode will conduct.
When a slowly increasing Voltage is placed across the Semiconductor, at first nothing happens. This is because at Voltages lower than ~0.7 V, the Depletion layer has not been overcome and the Semiconductor cannot conduct.
Once the "switch-on Voltage" has been reached, however, the Depletion Layer is overcome and the Semiconductor conducts.
The diagram below shows a typical graph of Current through a Diode, with the "switch-on Voltage" clearly seen :-
In the above graph, no current flows within this circuit until the "switch-on Voltage" (~2.55 V in this case) is reached.
Reverse-Biased
The diagram below shows a P-N Junction within a series circuit:-
In the above diagram the P-type Semiconductor is connected to the negative terminal and the N-type Semiconductor is connected to the positive terminal. When connected in this way, the Semiconductor is said to be Reverse-Biased. It is in this configuration that the Diode will not conduct.
Diode Band Theory
As stated above, all Diodes are simply P-N Junctions placed within a Circuit. Below is a full explanation as to how a Diode works based on Band Theory:-
When a Depletion Layer is formed in a P-N Junction, Electrons and Holes for an equilibrium position and there is no further movement of charges.
The diagram below is an Energy level diagram showing a P-N Junction in equilibrium:-
In the above diagram, the Y-axis represents increasing Electron Energy. That means that you would have to supply Energy to get an Electron (black dots) to go up on the diagram and, as such, without a power supply attached, there is no movement of charges.
Reverse Biased Diode Band Theory
If a power supply is attached to a P-N Junction in Reverse Bias, then the movement of charge causes the Band Gap to widen. This can be seen in the above diagram by a "steepening" of the slope between the P-Type and N-Type Semiconductors. This means that even more energy would be required to move a charge across the Band Gap and as such the P-N Junction (and therefore the diode) does not conduct.
Note (non examinable) - If a large enough Voltage is applied in Reverse Bias, a Current can be forced to flow in spite of the Band Gap. The Voltage required to do this is known as the "Breakdown Voltage" and once exceeded the component is irreparably damaged.
fORWARD Biased Diode Band Theory
If a power supply is attached to a P-N Junction in Forward Bias, then the P-Type Semiconductor is made more positive. This can be seen in the above diagram by the slope between the P-Type and N-Type Semiconductors becoming a downhill slope. This means that the Electrons now have more than enough Energy to move into the P-Type Semiconductor, and as such the P-N Junction (and therefore the diode) conducts.
When Electrons drop from the Conduction Band to the Valence Band of the P-Type Semiconductor, Energy is released due to the change in Energy level which leaves the Diode in the form of heat.
If the Depletion Layer is very close to the surface of the Diode, this Energy can be emitted as a Photon of light, creating a Light Emitting Diode. The Energy of the emitted Photon is equal to the "Recombination Energy" of the Electrons as they move between the levels.
The Photovoltaic Effect
Diodes can also be made so that the junction will absorb Photons of light. A Photon of light will cause an Electron from the Valence Band of the P-Type Semiconductor to gain enough energy to move across the junction into the N-Type Semiconductor's Conduction Band. By separating out an Electron and a Hole in this way, a Voltage is generated. This is known as the Photovoltaic Effect and if used in this way, the Diode is known as a Solar Cell.
The embedded website below allows a simulation of this experiment to be run :-
