9.2.2.2 P-type Semiconductor

  1. A p-type semiconductor can be produced by doped some trivalent atoms into a semiconductor.
  2. Trivalent atom is atom has only three valence electrons. Examples include aluminum, boron, and gallium.
  3. Figure above shows an aluminium atom (which is trivalent ) in the center, surrounded by four silicon atoms.
  4. We can see that, the trivalent atom form 4 covalent bonds with the silicon atoms around. Since the trivalent atom has only three valence electrons and each neighbour shares one electron, only seven electrons are in the valence orbit..
  5. This means a hole exists in the valence orbit of each trivalent atom. A trivalent tom is also called an acceptor atom because each hole it contributes can accept a free electron.
  6. The more trivalent impurity that is added, the more holes in the semiconductor, and hence the greater the conductivity of the semiconductor.
  7. Some free electrons will also formed in the semiconductor when some electrons are promoted to shell with higher energy level.
  8. The holes outnumber the free electrons, hence they are called the majority carrier and the free electrons are called the minority carriers.
  9. Since the positive charge carrier (the holes) outnumber the negative charge carrier (the free electrons), the semiconductor is called a p-type semiconductor, where the p stands for positive. 

 

p-type semiconductor n-type semiconductor
Doping Material Trivalent:
aluminum, boron, and gallium
Pentavalent:
antimony, and phosphorus
Role of doping material Atom receiver Atom donor
Majority Charge Carrier Holes Free electrons
Minority Charge Carrier Free electrons Holes

 

 

9.2.2.1 N-type Semiconductor

  1. n-type semiconductor can be produced by doped some pentavalent atoms into a semiconductor.
  2. Pentavalence atoms are atoms that have 5 electrons in the valence shell. Examples of pentavalent atoms include, antimony, and phosphorus.
  3. Figure above shows how the silicon crystal appears after doped with a phosphorous atom, which is pentavalence.
  4. We can see that, the pentavalence atom form 4 covalent bonds with the silicon atoms around. Since a pentavalence atom has 5 electrons, there is an extra electron left over and it is a free electron.
  5. Each pentavalent in the silicon crystal produces one free electron. Therefore, the pentavalence atom is called the donour.
  6. The more pentavalence impurity that is added, the more free electrons in the semiconductor, and hence the greater the conductivity of the semiconductor.
  7. Some holes will also formed in the semiconductor when some electrons are promoted to shell with higher energy level.
  8. The free electrons outnumber the holes, hence they are called the majority carrier and the holes are called the minority carriers.
  9. Since the negative charge carrier (the electrons) outnumber the positive charge carrier (the holes), the semiconductor is called an n-type semiconductor, where the n stands for negative. 

 

9.2.2 Doping of Semiconductors

  1. One way to increase the conductivity of a semiconductor is by doping
  2. Doping is a process of adding a small amount of impurities to a semiconductor.
  3. The impurities added to the semiconductor are called dopants.
  4. By adding impurity atoms to a conductor can increase its electrical conductivity
  5. There are two types of semiconductor depending on the type of impurities doped, namely
    1. the n-type semiconductor
    2. the p-type semiconductor

 

9.2.1 Semiconductors

  1. Semiconductor is a class of crystalline solid with conductivity between a conductor and an insulator.
  2. Example of semiconductors are:
    1. Silicon
    2. Germanium
    3. Boron
    4. Tellurium
    5. Selenium

The Silicon Crystal


  1. The typical example of semiconductor is silicon.
  2. Silicon has 4 valence electrons. Each of these 4 electrons are shared with another 4 silicon atoms to form 4 pairs of covalent bond, as shown in the diagram above.
  3. The bonded valence electrons are not free to move. Therefore silicon is not a good conductor at room temperature.
  4. At room temperature, a silicon crystal acts approximately like an insulator because only a few free electrons and holes are presence.

Free Electron and Hole


  1. If a bonded electron absorbs heat energy from the surrounding, it may be promoted to higher energy level.
  2. These electrons are free to move when they are at a higher energy level.
  3. If an electron is promoted to higher level, a vacancy is left in the valence shell, and it is called a hole.
  4. A hole has the tendency to pull electrons. Therefore a hole is assumed carries positive charge.
  5. Both of the free electrons and the holes can help to conduct electric current.
  6. Therefore, with the presence of the free electrons and holes, the conductivity of a semiconductor is higher than an insulator.

Resistance Change Due to Temperature Change

  1. As the temperature increases, more and more electrons are getting promoted to become free electrons and at the same time creating more and more holes. Therefore the conductivity of a semiconductor increases as the temperature increases.
  2. The graph below shows the resistivity change of a conductor and semiconductor against the temperature. The resistance of a semiconductor decreases as the temperature increases.

Flows of Free Electrons and Holes

  1. We have learned that, there are 2 types of charge carrier in a semiconductor, the free electrons and the holes.
  2. The free electrons carry negative charge whereas the holes carry positive charge.
  3. If a potential difference is applied to a semiconductor, the electrons and holes will start to flow.
  4. The electrons will flow to the negative terminal whereas the holes will flow to the positive terminal.
  5. Video below explain how the free electrons and holes flow in an electric field.

 

9.1.5 Uses of the Cathode Ray Oscilloscope

In a laboratory, a cathode ray oscilloscope can used to

  1. display different types of wave form. 
  2. measure short time interval
  3. measure potential difference (as a voltmeter)

Displaying Wave Forms

  1. A cathode ray oscilloscope can be used to display different types of waveform by connecting a power supply to the Y-input. 
  2. Figure below shows a few types of waveform displays on an oscilloscope.
Click on the links below for discussion about measuring short time interval and measuring potential difference
  1. Measuring Short Time Interval
  2. Measuring Potential Difference

Measuring Potential Difference

  1. In order to measure the potential difference, we need to move the bright spot to the centre before the Y-input is connected to any circuit.

  2. We also need to set the Y-gain. However, this can be adjusted later so that the signal can be fully displayed on the screen.
  3. The potential to be measured is then applied to the Y-plates via the Y-input terminals. 

Measuring Potential Difference of a Direct Current

  1. The time-base is switched off. When a potential difference is applied to the Y-input, an electric field is set up between the plates. This will deflect the cathode ray either up or down.
  2. The deflection of the electron beam by an electric field is proportional to the voltage applied. The reading of the voltage can be determined by referring to the Y-gain.
  3. For example, in the figure above, if the Y-gain is set to 2V per division (2V/div), then the reading of the potential difference is 4V.
  4. If the terminal of the direct current is inverted, the bright spot will be deflected to the opposite side, as shown in the diagram below. The reading of the potential difference will remain the same (4V).

Effect of the Y-gain

Figure below shows the display of the CRO when the Y-gain is set to 1V/div and 5V/div respectively for a potential difference of 4V.

Effect of the Time Base

  1. The time base move the bright spot across the screen at a constant speed.
  2. Usually, the speed is very high. As a result, we are not able to see the motion of the bright spot, but a straight line across the screen.
  3. Figure below shows the display of a CRO when the time base is ON and OFF.

Measuring Potential Difference of an Alternating Current

  1. If an alternating current is connected to the Y-input, a changing potential difference will be applied between the Y-plates.
  2. The changing potential difference will move the bright spot up and down continuously.
  3. As a result, vertical straight line will form on the screen of the CRO. The reading of the potential difference can be determined by referring to the Y-gain.
  4. For example, for the diagram above, if the Y-gain is set to 2V/div, then the maximum potential difference (peak voltage) is 4V.

Effect of the Time Base

  1. If the time base is switched on, it will move the bright spot across the screen horizontally.
  2. The result of the vertical motion caused by the Y-plate and the horizontal motion caused by the time base is a sinusoidal wave form.
  3. Diagram below shows the CRO display when the time base is on and off.

 

Measuring Short Time Interval

  1. A cathode ray oscilloscope can be used to determined the time interval between 2 pulses, even though the time interval is very small.
  2. Figure above shows 2 pulses on the screen of a cathode ray oscilloscope.
  3. If the time base is set to 2 ms/div, the time interval between the 2 pulses can be calculated as follow:
t = 6 x 2ms = 12 ms = 0.012s.

 

 

9.1.4 Working Principle of CRO

The cathode-ray oscilloscope (C.R.O.) consists of the following components:

  1. The electron gun.
  2. The deflecting plates.
  3. A fluorescent screen.

The Electron Gun

Parts of Electron Gun Function
 Filament  To heat the cathode.
 Cathode  Release electrons when heated by filament.
 Grid
  •  The grid is connected to a negative potential. The more negative this potential, the more electrons will be repelled from the grid and fewer electrons will reach the anode and the screen.
  • The number of electrons reaching the screen determines the brightness of the light. Hence, the negative potential of the grid can be used as a brightness control.
 Focusing Anode and 
  •  The other feature in the electron gun is the use of the anode.
  • The anode at positive potential accelerates the electrons and the electrons are focused into a fine beam as they pass through the anode.
 Accelerating anode


The Deflecting Plates

Part of the deflecting system Function 
 Y-plate The Y-plates will cause deflection in the vertical direction when a voltage is applied across them.
  X-plate  On the other hand, the X-plates will cause the electron beam to be deflected in the horizontal direction if a voltage is applied across them.


The Fluorescent Screen

  1. The screen is coated with a fluorescent salt, for example, zinc sulphide.
  2. When the electrons hit the screen, it will cause the salt to produce a flash of light and hence a bright spot on the screen.

Using CRO


Function
 1. Power switch  To switch on and off of the oscilloscope
2. Focus control To control the focus of the spot on the screen.
3. Intensity control To control the brightness of the spot on the screen.
4. X-offset
5. Y-offset
Y-offset moves the whole trace vertically up and down on the screen, while X-offset moves the whole trace from side to side on the screen.
6. Time base control Whenever we switch on the time-base, we are actually applying a sawtooth voltage to the X-plates (Figure below).




* This make the electron beam sweep across the screen at a constant speed.
* By knowing the period of each cycle, T, we can then know how fast the beam is sweeping across the screen. The time-base is thus a measure of time for the oscilloscope.
7. Y gain control * the "Volts/Div." wheels amplify an input signal so that for a division a given voltage level is in valid. A "division" is a segment, a square on the screen of the oscilloscope.
* A setting of ".5" i.e. means, that the height of a single square equals a voltage of 0.5 V. An amplitude of 1 V would have a size of two divisions vertical to the abscissa.
8. d.c./a.c. switch d.c. – d.c. and a.c. voltage displayed.
a.c. – only a.c. voltage displayed.
9. X-input and Y-input Electric input connect to the X-plate and Y-plate.


Example
Table below shows the sample display of direct current and alternating current when the time base is switched ON and OFF.

 Direct Current (Time Base Switched Off)  Direct Current (Time Base Switched On)
 Alternating Current (Time Base Switched Off)  Alternating Current (Time Base Switched On)

 

9.1.3 Cathode Ray Oscilloscope

(Cathode Ray Tube Television)
  1. Cathode-ray tubes have become part of everyday life.
  2. They can be found in the screens of television sets and computer monitors.
  3. In the Physics laboratory, we use the cathode-ray tube in the oscilloscope to study waveforms.
  4. In SPM, you need to know
    1. How cathode ray is produced (Thermionic emission and electron gun)
    2. The characteristics of cathode ray (Study by using the Maltese Cross Tube and Deflection Tube).
    3. the structure of a Cathode Ray Oscilloscope
    4. how to operate a Cathode Ray Oscilloscope
    5. the uses of a Cathode Ray Oscilloscope

(Cathode Ray Oscilloscope)

 

9.1.2 Cathode Ray

Electron Gun

(The electrons released by the cathode are accelerated by the accelerating anode and form a beam of electrons)
  1. If a high positive potential (anode) is placed in front of the heated metal, the emitted electrons will be accelerated and form a beam of electrons.
  2. The device is called an electron gun.
  3. The beam produced is called the cathode ray.

Properties of the cathode ray

The properties of cathode ray can be studied by using the Maltese Cross tube and the Deflection Tube.

Maltese Cross Tube

  1. We can use a Maltese cross tube to investigate the characteristics of cathode ray.
  2. Figure below shows the illustration of a Maltese Cross tube.

Experiment

6V DC Supply: ON

3kV EHT: OFF
The filament glows and emits light.
The light is blocked by the cross and hence a shadow is formed on the screen.

6V DC Supply: ON

3kV EHT: ON
The electrons are accelerated by the anode and hence produces a cathode ray.
The cathode ray hit on the screen cause a fluorescent effect.
The cathode ray is also blcok by the cross. Therefore, a shadow form on the screen.

6V DC Supply: ON

3kV EHT: OFF

with Magnet
The cathode ray can be deflected by the magnetic field.
The direction of deflection can be determined by Fleming's Left Hand Rule. (Note: The direction of current is in the opposite direction of electrons movement).

6V DC Supply: ON

3kV EHT: OFF

with Electric Plate
The cathode can be deflected by an electric field.

Deflection Tube


The characteristics of cathode ray can also be determined by using a refflection tube.

Conclusion

  1. Both the experiment of Maltese Cross Tube and the Deflection Tube show that
    1. Cathode ray can produce fluorescent effect.
    2. Cathode ray can be deflected by the electric field.
    3. Cathode ray can be deflected by the magnetic field. The direction of deflection can be determined by using Fleming’s Left hand Rule.

 

9.1.1 Thermionic Emission

  1. Thermionic emission is a process of emission of charge particle (known as thermion) from the surface of a heated metal.
  2. The charge particles normally are electrons.
  3. The rate of emission (number of electrons emitted in 1 second) is affected by 4 factors, namely
    1. the temperature of the heated metal,
      When the temperature of the metal increase, the emission rate of electron will increase.
    2. the surface area of the heated metal,
      When the surface area of the metal increase, the emission rate of electron will increase.
    3. the types of metal
      The rates of thermionic emission are different with regard to different types of metals.
    4. the coated material on the surface of the metal.
      If the surface is coated by a layer of barium oxide or strontium oxide, the rate of emission will become higher.

Thermionic Diode

  1. Thermionic emission is applied in thermionic diode.
  2. A diode is an electrical component that only allowed current flows in one direction.
  3. Figure below shows the illustration of a thermionic diode.
  4. Electrons can only released from the tungsten filament (when it is hot) and move toward the anode which is connected to the positive terminal.
  5. Electrons are not allowed to move in the opposite direction because no electrons will be released from the anode.
  6. As such, the electrons can only move from left to right but not the other way round.

 

8.5.3 Renewable and Non-renewable Energy

The frequently asked questions related to this topic are: what is renewable energy? the examples of Renewable and non-renewable energy and the advantages of using renewable energy.

Renewable energy sources are the energy sources which can be continuously replaced. Examples of renewable energy are:

  1. Hydro energy
  2. Solar energy
  3. Wind energy
  4. Geothermal energy
  5. Biomass
  6. Tidal energy


Advantages of Using Renewable Energy
1. Clean and do not pollute the environment.
2. Easily obtain.
3. Can be replenished once it has been used.
4. Can prevent energy crisis.

Hydroelectric


Advantages

  1. It is a renewable energy.
  2. Building a dam does not pollute the environment.
  3. In a lot of country, water can be easily obtained and is free.
  4. Building a hydroelectric plant does not involve very high technology as nuclear power plant.

Disadvantages

  1. Building a dam will cause a large area flooded with water, and hence seriously destroys the ecosystem nearby.
  2. The flooded area causes the loss of wild life habitat and agriculture land.
  3. Dam failure happens will cause a disaster to the lower reaches area of the river.
  4. The cost to build a dam is very high.

Fossil Fuel.

Advantages

  1. It is relatively easy to be collected as they are present in large amount in one place.
  2. It contains high energy capacity.

Disadvantages

  1. It releases a lot carbon dioxide when it is burned. Large amount of carbon dioxide presence in the atmosphere will cause the increase of the temperature of the atmosphere, known as global warming. Global warming is the most serious environmental problem that we are facing now.
  2. Fossil fuels are non-renewable energy.
  3. Pollute the air. Burning of fossil fuel will release particles and some hazardous gases such as sulphur dioxide and nitrogen dioxide which will pollute the air.

Solar Power

Advantages

  1. It is a renewable energy.
  2. Sunlight is free and can be obtained easily.
  3. It does not pollute the air.

Disadvantages

  1. A large area is needed to install the solar plate.
  2. The amount of sunlight at non-tropical area depends on the season.
  3. The energy collected need to be store in a cell so that it can be used at the time sunlight is not present.
  4. Its capacity is limited by the capacity of the cell. Normally a cell has relatively low capacity.

Nuclear Power

Advantages:

  1. The nuclear fuel such as uranium and plutonium has high energy capacity. Small amount of uranium can produces huge amount of energy.
  2. Nuclear energy does not pollute the air.

Disadvantages

  1. Hazardous radioactive waste is produced during nuclear reaction.
  2. The radioactive waste will cause pollution in a very large area if leakage happens in the nuclear power plan.
  3. The cost to maintain a nuclear plant is very high.
  4. Building a nuclear plant involving very high technology. Only a few countries own such technology.

Biomass

  1. Biomass is biological material derived from living organisms such as wood and animal waste.
  2. As a renewable energy source, biomass can either be used directly via combustion to produce heat, or indirectly after converting it to various forms of biofuel.
  3. Heat energy produced from the burning of biomass can be used to generate electricity.

Advantages

  1. It is renewable energy.
  2. Using biomass to generate electricity can reduce the disposal of organic waste.
  3. Liquid biofuels will not pollute the air because they are lead-free and sulphur-free.

Disadvantages

  1. Large storage space is required to store the biomass.
  2. Land utilization can be considerable. Can lead to deforestation.
  3. Overall process can be expensive

Wind Power


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Advantages

  1. It is a renewable energy.
  2. It does not pollute the environment.
  3. It is suitable for mountain area because the wind speed in mountain area is relatively high.

Disadvantages

  1. A large area is needed to build the windmills.
  2. The windmills will produce high level of noise.
  3. The power generated is inconsistent due to the inconsistent of the wind speed and wind direction.