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Chapter 25: Semiconductors Please remember to photocopy 4 pages onto one sheet by going A3→A4 and using back to back on the photocopier.
A Semiconductor is a material whose resistivity is between that of a good conductor and a good insulator. Examples of materials which are semiconductors are Silicon and Germanium.
The resistance of a semiconductor decreases as its temperature increases*.
Intrinsic Conduction is the movement of charges through a pure semiconductor.
A Thermistor is an electrical component whose resistance decreases rapidly with increasing temperature.
Symbol for a thermistor:
A Light Dependant Resistor (LDR) is an electrical component whose resistance decreases rapidly when light shines on it.
Doping Doping is the addition of a small amount of atoms of another element to a pure semiconductor to increase its conductivity*.
Extrinsic Conduction is the movement of charges through a doped semiconductor.
An n-type semiconductor is a semiconductor in which electrons are the majority charge carriers.
- In this case there is one ‘extra’ electron due to the addition of the Phosphoros atom.
A p-type semiconductor is a semiconductor in which holes are the majority charge carriers.
- In this case there is one ‘missing’ electron due to the addition of the Boron atom. This is equivalent to a ‘Positive Hole’ which moves in the opposite direction to an electron.
Component Symbol
The operation of semiconductor devices depends on the effects that occur when p-type and n-type semiconductor material are in close contact. This is achieved by taking a single crystal of silicon and doping separate but adjacent layers of it with suitable impurities. The junction between the p-type and the n-type layers is referred to as the p–n junction and this is the key to some very important aspects of semiconductor theory.
Devices such as diodes, transistors, silicon-controlled rectifiers, etc., all contain one or more p–n junctions.
The p-n junction* When a piece of p-type semiconductor is joined to a piece of n-type semiconductor, the junction between the two is known as a p-n junction.
The Depletion Region The depletion region is so named because it is formed from a conducting region of the semiconductor which has been depleted of all free charge carriers, leaving none to carry a current. Understanding the depletion region is key to explaining modern semiconductor electronics in action.
- Due to thermal agitation, some free electrons in the n-type material diffuse over to the p-type material, where they combine with nearby positive holes, with the result that the region is depleted of two of its charge carriers.
- Similarly on the p-type side some positive holes diffuse over to the n-type material, where they too combine with nearby electrons, with the result that the region gets depleted of two more of its charge carriers.
- The end result is that a depletion region is formed at the junction of the p-type and n-type materials, where there are no free charge carriers. This region therefore acts as an insulator.
- Work now needs to be done to bring charge from one side of this depletion region to the other; therefore a potential difference (voltage) exists across the region.
- This voltage is typically about 0.1 volts for a germanium diode, and 0.6 volts for a silicon diode.
- This voltage is known as the junction voltage.
Current flow across a p-n junction Forward-biased p-n junction
- Here the positive terminal of the junction is connected to the p-type and the negative terminal is connected to the n-type.
- Electrons in the N-type area above are repelled from the negative terminal and drive in to the depletion region.
- Similarly positive holes in the P-type are repelled from the positive terminal and drive into the depletion region from the other side.
- The end result is that the width of the depletion layer is reduced.
- In fact if the voltage across the terminals is sufficiently great (greater than what is known as the junction voltage) the depletion layer is completely broken down and the region now becomes a conductor.
Graph of Current against Voltage for a Semiconductor diode.
- The left hand side of the graph represents the p-n junction in reverse bias and little or no current flows.
- At –100 volts, the junction gets destroyed.
- In forward bias (the right hand side) very little current flows until the voltage reaches 0.6 Volts (junction voltage).
- At this stage the depletion layer gets completely broken down and the region now becomes a conductor, and therefore the current rises suddenly.
A diode is a device that allows current to flow in one direction only.
The rectifier is a device that uses diodes to convert a.c. to d.c Examples of appliances that contain a rectifier: Radio, television, computer, battery charger, mobile phone charger.
Demonstrating the action of a diode Use a light bulb in series with the semiconductor and the battery. Observation: Bulb lights when switch is closed (circuit is forward biased), but not when the diode is put in the other way around (reverse biased).
The Light-Emitting Diode (LED)
An LED consists of a junction diode made from semiconducting material. Light is emitted from the junction when it is forward biased and the brightness is approximately proportional to the forward current.
Mandatory Experiments
- To investigate how Resistance changes with Temperature for a Thermistor
- To investigate the variation of Current with Potential Difference for a Semiconductor Diode
Leaving Cert Physics Syllabus
Content Depth of Treatment Activities STS
Conduction in materials
Conduction in semiconductors. References to charge carriers.
Interpretation of I–V graph.
Conduction in semiconductors: the distinction between intrinsic and extrinsic conduction; p-type and n-type semiconductors.
Electronic devices. LED, computers, integrated circuits.
The p-n junction: basic principles underlying current flow across a p-n junction.
Demonstration of current flow across a p-n junction in forward and reverse bias, e.g. using a bulb.
Rectification of a.c.
LDR – light-dependent resistor. Thermistor.
Demonstration of LDR and thermistor.
Image Symbol
TO INVESTIGATE THE VARIATION OF CURRENT (I) WITH P.D. (V) FOR A SEMICONDUCTOR
DIODE IN FORWARD BIAS
APPARATUS:
Low voltage power supply, rheostat, voltmeter, ammeter, 330 Ω resistor, semiconductor diode
DIAGRAM:
PROCEDURE:
- Set up the circuit with the semiconductor diode in forward bias as shown.
- Adjust the potential divider to obtain different values for the voltage V and hence for the current I.
- Obtain at least ten values for V and for I using the voltmeter and the ammeter.
- Plot a graph of I against V and join the points in a smooth, continuous curve.
RESULTS:
V
(volts) I (milli- amps)
CONCLUSION:
We can see from the graph that almost no current flows until the applied voltage exceeds 0.6 V for a silicon diode but then the current rises rapidly.
NOTE:
A protective resistor, e.g. 330 Ω, should always be used in series with a diode in forward bias. Therefore even if the diode offers no resistance there will still be some other resistance in the circuit preventing the current getting too high.
A germanium diode, e.g. OA91, gives very little current between 0 and 0.2 V but the current then increases rapidly above this voltage. A light emitting diode gives very little current up to 1.6 V but then the current rises rapidly accompanied by the emission of light.
TO INVESTIGATE THE VARIATION OF THE RESISTANCE OF A THERMISTOR WITH
TEMPERATURE
APPARATUS:
Thermistor, glycerol, beaker, heat source, thermometer, ohmmeter, boiling tube
DIAGRAM:
PROCEDURE:
- Set up the apparatus as shown in the diagram.
- Use the thermometer to note the temperature of the glycerol, which we assume to be the same as the temperature of the thermistor.
- Record the resistance of the thermistor using the ohmmeter.
- Heat the beaker and for each 10 °C rise in temperature record the resistance and temperature using the ohmmeter and the thermometer.
- Plot a graph of resistance against temperature.
RESULTS: R (Ω) θ (^0 C)
CONCLUSION:
- From the graph we can see that as temperature increases the resistance decreases.
- We believe our data to be reliable because it resulted in a smooth curve, which the theory predicted.
PRECAUTIONS / SOURCES OF ERROR:
- Check for the resistance of the connecting leads and contacts on the ohmmeter. Subtract from later readings.
- Heat very slowly to try to maintain thermal equilibrium between the water and glycerol and thermistor. When the bunsen is removed wait until the temperature is steady before taking the resistance readings.
- Use glycerol in the test tube as it is a better heat conductor than water.
*Extra Credit The resistance of a semiconductor decreases as its temperature increases. This is because as the temperature of the material increases it heats up, releasing many electrons from their atoms. These electrons are now available for conduction and so resistance decreases. This ‘liberation’ of electrons can also be caused by light shining on the material.
*Doping is the addition of a small amount of atoms of another element to a pure semiconductor to increase its conductivity. You don’t have to know how this works but an explanation is given on page 287. Note that the overall charge of the material is still zero.
*It is also responsible for rectification of a.c. A forward-based diode conducts current. A reversed-biased diode does not conduct current. The semiconductor diode can therefore be used as a rectifier. A rectifier is an electrical component which converts alternating current (a.c.) to direct current (d.c.). Take any appliance that you have at home which seems to operate both on mains electricity and by battery. The appliance will only work by direct current, so if there are no batteries and the appliance is plugged into the ‘mains’, which is a.c. there must be a rectifier in your appliance which converts the a.c. to d.c. It also needs to have a transformer which drops the voltage from 240 volts down to whatever voltage the combination of batteries would supply. Next time you are about to throw one of these out, take off the back and see if you identify these two components (the transformer is usually positioned just at the point where the voltage goes in). We will look at these in more detail at the end of the chapter on Electromagnetic Induction.
Test yourself Set up the following circuit.
- Switch on the power and note that the lamp lights.
- Close the switch and note that the lamp glows more brightly.
Why? (hint: note the supply voltage).
