9 2 Semiconductor Doc

Sekolah Menengah Sains Muzaffar Syah

9.2

Comparison Between Insulator, Semiconductor and Conductor

Metals o Good conductors of electricity because they have free electrons that can move easily between atoms o The resistance of metals is generally very low.

Insulators o Poor conductors of electricity because they have too few free electrons to move about. o The resistance of insulators is very high.

Semiconductors A material that has an electrical conductivity that is between that of a conductor and an insulator.   

The resistance of semiconductors is between that of conductors and insulators. Semiconductors can be pure element such as silicon or germanium. At 0 Kelvin it behaves as an insulator. When the temperature increases, the conductivity of the electricity will increase because its resistance will be lowered.

Electricity conductivity in semiconductors occurs because there is two type of charge carriers:  Electron which is negatively charge  Hole which is positively charge

Characteristics of a silicon atom

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There are four electrons in the outermost shell of a silicon atom and they are shared between four other neighbouring atoms to form four covalent bonds. At very low temperature, pure silicon crystal is an insulator and has a high resistance to current flow. As the temperature of pure silicon crystal increases, the energy of the vibrating atoms in the silicon crystal causes some electrons to break free. Each free electron leaves behind a hole in the bonding structure. The hole behaves as if it has a positive charge. Free electrons and holes are known as charge carriers. Conduction in a semiconductor is by means of a movement of free electrons and holes in opposite direction.

Conductivity of semiconductors can be increased by:(a) increasing the ……………… of the semiconductors. The heat energy supplied by the temperature increase is converted to the kinetic energy of the electrons. (b) adding …………….. to the semiconductors (doping)

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What is doping process?

Doping is a process of adding a certain amount of specific impurities called dopants to a pure semiconductor to increase its electricity conductivity. o

Atoms of the impurities added should have almost the same size as the atoms of the semiconductors.

o

By adding different kinds of impurities, two types of semiconductors can be obtained; the n-type semiconductor and the p-type semiconductor.

 Describe n-type semiconductor

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 Describe p-type semiconductor

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A n-type semiconductor is produced when pentavalent atoms are added to intrinsic semiconductor atoms. Each pentavalent atom donate a free electron ,because there will be one extra electron. The electrons are now the majority charge carriers in the n-type semiconductor since there are more free electrons than holes. Examples of pentavalent atoms are Arsenic,Phosporus and Antimony and called donor atoms.

A p-type semiconductor is produced when trivalent atoms are added to pure semiconductor atoms. Only three of the four bonds formed by the trivalent atoms are complete. The vacancy is a hole with positive charge. The holes are now the majority charge carriers in the p-type semiconductor since there are more holes than free electrons. Examples of trivalent atoms are Indium, Boron and Gallium and called acceptor atoms.

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Sekolah Menengah Sains Muzaffar Syah Comparison between the n-type and p-type semiconductor Aspect

n-type Semiconductor

p-type Semiconductor

Pure semiconductor Dopants material Function of the dopants material Valens electrons of the dopant material Majority charge carriers Minority charge carriers

Describe semiconductor diodes

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 What is the p-n junction?       What is the depletion layer? 

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The simplest semiconductor device is a diode. A diode is made by joining a p-type and n-type semiconductors A diode is a device that allows current to flow in one direction only but blocks it in the opposite directions.

A p-n junction is formed when a n-type and p-type semiconductors are joined together. The boundary between the p-type and n-type regions is called the junction. A layer called the depletion layer is formed in the middle. At the p-n junction, electrons from the n-side move to the pside and recombine with the holes. Holes from the p-side similarly move into the n-side, where they recombine with electrons. As a result a depletion layer is a very narrow region which has lost all its available free electrons and holes and thus behaves almost like pure silicon,i.e with high resistivity. Any further movement of charges across the boundry in the depletion layer will be repelled by the charges in the layer. The region around the junction is left with neither holes nor free electrons. This neutral region which has no charge carriers is called the depletion layer. This layer which has no charge carrier is a poor conductor of electricity.

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Sekolah Menengah Sains Muzaffar Syah Extra Notes : What is junction voltages?  The layer of the negative charge in the p-type region will prevent the majority charge carriers from the n-type region(the electrons) from crossing the boundary.  Similarly , the positive charge layer in the n-type region will prevent the majority charge carriers from the p-type region(the holes) from crossing the boundry in the opposite direction.  Thus, a potential difference ,known as the junction voltage is produced. In its normal state a p-n junction delivers no current since the charges are in equilibrium.  The effect of this junction voltage is to prevent charge carriers from drifting across the junction.  The junction voltages for germanium and silicon are approximately 0.1 V and 0.6 V respectively. In order for electric current to flow through the diode, the voltage applied across the diode must exceed the junction voltage.

Describe the function of diodes

What is forward bias?

What is reversed bias?

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The p-type of the diode is connected to the positive terminal and the n-type is connected to the negative terminal of a battery. the cell voltage greater than the junction voltage. The depletion layer is narrow , and the resistance of diode decreases. Hence a large current flows through the diode. The diode conducts current because the holes from the p-type material and electrons from the n-type material are able to cross over the junction. A light bulb will light up.

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The n-type is connected to the positive terminal and the p-type is connected to the negative terminal of the battery. The cell voltage is lower than the junction voltage. The depletion layer is wide , and the resistance of diode increases. Hence only a very small current (leakage current) flows through the diode. The reversed polarity causes a very small current to flow as both electrons and holes are pulled away from the junction. When the potential difference due to the widen depletion region equals the voltage of the battery, the current will cease. Therefore the bulb does not light up.

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Sekolah Menengah Sains Muzaffar Syah Activity 1: To observe current flow through a semiconductor diode in forward bias or reverse bias Electrical circuit

Draw a circuit diagram

Brightness of the bulb

The cathode of the diode is connected to the –ve terminal of the d.c. power supply

The cathode of the diode is connected to the positive terminal of the d.c. power supply

Connected the diode in a forward biased circuit to an ac voltage supply

1. The current ________ in a forward biased circuit and _________ flow in a reverse biased circuit. 2. Diodes have a ______ resistance when they are forward-biased and a very _______ resistance when they are reverse-biased. 3. The function of a diode is to allow ________to pass through in one ___________ only. 4. The brightness of the bulb will __________when the DC current in a forward biased circuit is replaced with AC current of the same voltage.

 Describe the use of diodes as rectifiers

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A rectifier is an electrical device that converts alternating current to direct current. Rectification is a process to convert an alternating current into a direct current by using a diode. Two type of rectification: (a) Half-wave rectification (b) Full-wave rectification

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Sekolah Menengah Sains Muzaffar Syah Half-wave rectification

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A process where only half of every cycle of an alternating current is made to flow in one direction only.

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When a diode is connected in series with the resistor, any current that passes through the resistor must also pass through the diode. Since diode can only allow current to flow in one direction, therefore the current will only flow in the first half-cycle when the diode in forward bias. The current is blocked in the second half-cycle when the diode is in reverse bias. A process where both halves of every cycle of an alternating current is made to flow in the same direction. In the first half, the current flows from A to P to TU to R to B In the second half, the current flows from B to S to TU to Q to A. The direction of the ac current passing through the resistor for each half cycle is the same ie T to U.

No rectification

Half-wave rectification by using one diode

   Full-wave rectification

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Full-wave rectification by using four diodes Input ac current Describe the use of a capacitor to smooth out output current and output voltage in a rectifier circuit

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When the current pass through the resistor and capacitor, the capacitor is charged and stores energy. When there is no current pass through the resistor and capacitor, the capacitor discharge and the energy from it is used to produce voltage across the resistor. As a result it produces a smooth dc output.

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Sekolah Menengah Sains Muzaffar Syah Activity 2: The figure shows a half-wave rectifier circuit that is connected to CRO. Sketch waveform of the voltages observed on the CRO screen when the time-base is on. 1.

2. The figure shows a full-wave rectifier circuit that is connected to CRO. (i) Draw arrows to show the current flow in the first half cycle and to show the current flow in second half cycle in the diagram. (ii) Sketch the waveform of the voltages observed on the CRO screen when the time-base is on.

a

To CRO

To CRO

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Sekolah Menengah Sains Muzaffar Syah TUTORIAL 9.2 1. The adding of impurity into pure semiconductor material is known as ____ (2006) A. doping B. rectification C. amplification D. thermionic emission 2. Which of the following is correct about ntype semiconductor and p-type semiconductor? n-type p-type semiconsemiconductor ductor A Dopant Antimoni Gallium B Fuction of dopant C Valentce of dopant D Majority charge carrier

Acceptance atom Trivalents

Donor atom Pentavalents

holes

electron

5. Which of the following resistance, R, versus temperature, T, graphs is correct for a pure semiconductor?

6. Which circuit will light up only one light bulb?

3. Which of the following diagrams is an ntype semiconductor? (2006) 7. A CRO is connected to a circuit as shown in diagram 1. The input terminal, PQ, is connected to an ac power supply.

Diagram 1 Which of the following is the output wave form displayed on the screen? 4. What is the function of a diode? (2007) A. Acts as a current amplifier B. Acts as an automatic switch C. Stores electric charge D. Allows the current to pass in one direction only

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Sekolah Menengah Sains Muzaffar Syah 8. Which of the following traces displayed on a CRO is the rectification of an ac current?

11. Diagram 4 shows a full-wave rectifier.

9. Diagram 2 shows a rectification circuit for an ac current. . Diagram 4

Diagram 2 When an output resistor is connected to a CRO, the shape of the wave form on the screen is __

Which of the following is the correct path of the current through diodes K, L, M and N and resistor R? A. L  R  N B. M  R  N C. K  R  M D. N  R  L 12. Diagram 5 shows a circuit of four diodes which are connected to batteries and a resistor.

10. Two diodes are connected in a circuit as shown in Diagram 3.

Diagram 3

Diagram 5 Which of the following is the correct path for the current? A. L  M  T  S  U  L  M  N O B. L  M  T  R  Q  O  N ML C. L  M  T  R  Q  O  N SU D. U  S  T  R  Q  O  N ML

Which of the following is the output wave form from resistor P?

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Sekolah Menengah Sains Muzaffar Syah 13. Diagram 6 shows a circuit which has three identical resistors P, Q and R, four diodes and an ac power supply.

Diagram 6 Which resistor permitted only direct current through it? A. P C. Q B. R D. P ,Q dan R 14. The function of capacitor in diagram below is ____

A. B. C. D.

Rectifier Flows alternating current AC and DC filter Voltage smoother

15. In which circuit the bulb does not light up?

16. The diagram shows a circuit containing bulbs X, Y and Z.

Which bulb in the circuit will light up when the switch is on? (2003) A. Z only B. X and Y only C. Y and Z only D. X, Y and Z only 17. Which of the following pairs of atoms can act as an impurity to produce an n-type semiconductor when doped into a pure semiconductor? (2004) A. Boron and phosphorus B. Arsenic and antimony C. Indium and gallium D. Antimony and gallium 18. The diagram shows a circuit consisting of a diode and a bulb. When the switch is on, the bulb does not light up.

What needs to be done to light up the bulb? (2005) A. Replace with a new bulb B. Increase the number of dry cells C. Reverse the diode connection D. Connect a resistor parallel to the bulb 19. Diagram below shows a circuit connected to the cathode ray oscilloscope (CRO).

Which of the following traces is the correct one shown on the CRO? (2007)

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Sekolah Menengah Sains Muzaffar Syah 20. The diagram shows the structure of a semiconductor material.

(i)

Name the line X. __________________________

(ii) Give the name and the function of voltage at X . ___________________________ ___________________________ ___________________________ ___________________________ (a) Based on the diagram above, (i) name the type of the semiconductor material. _________________________ (ii) what is the function of the Boron atom?

___________________________ (d) What happens to the bulb when the battery connection is reversed. Explain why? _______________________________ _______________________________

_________________________ (iii) name the majority charge carrier in the semiconductor material. ___________________________ (b) At 0 K , a pure semiconductor material has very high resistance. Explain why?

_______________________________ _______________________________

21. Figure (a) shows an arrangement of apparatus to study the conductivity of semiconductor diode by a student. When the switch is closed by the student the bulb does not light up.

_______________________________ _______________________________ _______________________________ _______________________________ _______________________________ (c) The diagram shows a p-type semiconductor is joined to an ntype semiconductor. Later the combination of the semiconductor materials are connected to a bulb and a battery.

Figure (a) (i) In the space below draw a circuit diagram for Figure (a)

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Sekolah Menengah Sains Muzaffar Syah (ii) State the characteristic of the diode causes the bulb does not light up? _______________________________ (iii) State the correction should be done to the connection of the circuit in Figure (a) to make the bulb is lighted up.

SPM 2007 22. Diagram 7 shows a simple transformer. (a) (i) Name the type of the transformer. _____________________________ (ii) (b)

_______________________________ (b) Figure (b) shows circuit X is connected to a transformer, resistor R and a cathode-ray oscilloscope. When the switch is opened the waveform as shown in Figure (b) is formed on the CRO screen.

(c)

(d)

Figure (b) (i) In the space below draw a circuit diagram for circuit X.

The number of turns on the primary coil in Diagram 7 is 1000. Calculate the number of turns on the secondary coil. The transformer in Diagram 7 is used to switch on an electrical appliances. The current in the primary coil is 0.1 A and the efficiency is 75 %. A student connects a television which uses direct current to the output of the transformer in Diagram 7. When the television is switched on, it does not function.

SPM 2004 Section C 23. Figure (a) shows circuits J, K, L, M and N each containing an ideal transformer. Diodes in the circuits are used for the purpose of rectification.

(ii) In the space below draw a waveform is produced in the CRO screen when a capacitor is connected parallel to resistor R.

(iii) Give the reason for your answer in b(ii). _______________________________ _______________________________

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Figure (a) Key : Np = Number of turns in primary coil Ns = Number of turns in secondary coil (i) What is meant by rectification ? (ii) Explain the working principle of a transformer. (iii) You are asked to make a 12 V battery charger . Study the circuits, J, K ,L, M and N in Figure (a) and consider the following aspects: type of transformer ratio of the number of turns in primary coil to secondary coil type of rectification characteristic of output current

SPM 2007 Section B 24. Diagram10.1 and Diagram 10.2 show two electrical circuits containing semiconductor diodes.

(e) What is meant by semiconductor? (f) Using Diagram 10.1 and Diagram 10.2, relate the lighting of the bulbs, the current and the way the diode is connected to the terminals of the battery to deduce a relevant physics concept.

(g) Diagram 10.3 shows a full wave rectifier circuit.

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(i) Draw the wave form of a full wave rectification. (ii) A capacitor is placed across the output to smooth the current. Draw the wave form produced. Explain how a capacitor is used to smooth the current.

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