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. 

 

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

 

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.

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.

 

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)

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

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:

 

 

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)

 

(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)

 

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

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.

 

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