9 2 semiconductor doc

Sekolah Menengah Sains Muzaffar Syah

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Metalso Good conductors of electricity because they

have free electrons that can move easilybetween atoms

o The resistance of metals is generally verylow.

Insulatorso Poor conductors of electricity because they

have too few free electrons to move about.o The resistance of insulators is very high.

SemiconductorsA 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

There are four electrons in the outermost shell of a silicon atom andthey are shared between four other neighbouring atoms to form fourcovalent bonds.

At very low temperature, pure silicon crystal is an insulator and has ahigh resistance to current flow.

As the temperature of pure silicon crystal increases, the energy of thevibrating atoms in the silicon crystal causes some electrons to breakfree.

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 kineticenergy of the electrons.

(b) adding …………….. to the semiconductors (doping)

9.2

Comparison Between Insulator, Semiconductor and Conductor

Characteristics of a silicon atom


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A n-type semiconductor is produced whenpentavalent atoms are added to intrinsicsemiconductor atoms.

Each pentavalent atom donate a free electron,because there will be one extra electron.

The electrons are now the majority chargecarriers in the n-type semiconductor sincethere are more free electrons than holes.

Examples of pentavalent atoms areArsenic,Phosporus and Antimony and calleddonor atoms.

A p-type semiconductor is produced whentrivalent atoms are added to puresemiconductor atoms.

Only three of the four bonds formed by thetrivalent atoms are complete. The vacancy is ahole with positive charge.

The holes are now the majority charge carriersin the p-type semiconductor since there aremore holes than free electrons.

Examples of trivalent atoms are Indium, Boronand Gallium and called acceptor atoms.

What isdopingprocess?

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

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

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

Describen-typesemi-conductor

Describep-typesemi-conductor


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Comparison between the n-type and p-type semiconductor

Aspect n-typeSemiconductor

p-typeSemiconductor

Pure semiconductor

Dopants material

Function of the dopantsmaterialValens electrons of thedopant materialMajority charge carriers

Minority charge carriers

Describe semiconductor diodes 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.

What is the p-n junction? 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 isformed in the middle.

At the p-n junction, electrons from the n-side move to the p-side and recombine with the holes.

Holes from the p-side similarly move into the n-side, wherethey recombine with electrons.

As a result a depletion layer is a very narrow region whichhas lost all its available free electrons and holes and thusbehaves almost like pure silicon,i.e with high resistivity.

Any further movement of charges across the boundry in thedepletion layer will be repelled by the charges in the layer.

What is the depletion 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 conductorof electricity.


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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 mustexceed the junction voltage.

What is forward bias?

The p-type of the diode is connected to thepositive terminal and the n-type isconnected to the negative terminal of abattery.

the cell voltage greater than the junctionvoltage. The depletion layer is narrow , andthe resistance of diode decreases. Hence alarge current flows through the diode.

The diode conducts current because theholes from the p-type material andelectrons from the n-type material are ableto cross over the junction.

A light bulb will light up.

What is reversed bias?

The n-type is connected to the positiveterminal and the p-type is connected to thenegative terminal of the battery.

The cell voltage is lower than the junctionvoltage. The depletion layer is wide , andthe resistance of diode increases. Henceonly a very small current (leakage current)flows through the diode.

The reversed polarity causes a very smallcurrent to flow as both electrons and holesare pulled away from the junction.

When the potential difference due to thewiden depletion region equals the voltageof the battery, the current will cease.Therefore the bulb does not light up.

Describe the function of diodes


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Activity 1: To observe current flow through a semiconductor diode in forward bias or reverse bias

Electrical circuit Draw a circuit diagram Brightness of thebulb

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

The cathode of the diode is connected tothe positive terminal of the d.c. powersupply

Connected the diode in a forward biasedcircuit to an ac voltage supply

1. The current ________ in a forward biased circuit and _________ flow in a reverse biasedcircuit.

2. Diodes have a ______ resistance when they are forward-biased and a very _______ resistancewhen 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.

A rectifier is an electrical device that converts alternating current todirect current.

Rectification is a process to convert an alternating current into a directcurrent by using a diode.

Two type of rectification:(a) Half-wave rectification(b) Full-wave rectification

Describethe use ofdiodes asrectifiers


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Half-waverectification

Norectification

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

Half-waverectificationby usingone diode

When a diode is connected in series with the resistor, any current that passesthrough the resistor must also pass through the diode.

Since diode can only allow current to flow in one direction, therefore thecurrent 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 reversebias.

Full-waverectification

A process where both halves of every cycle of an alternating current is made toflow 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.

Full-waverectificationby usingfour diodes

Inputac current

Describethe use of acapacitor tosmooth outoutputcurrent andoutputvoltage in arectifiercircuit

When the current pass throughthe resistor and capacitor, thecapacitor is charged andstores energy.

When there is no current passthrough the resistor andcapacitor, the capacitordischarge and the energy fromit is used to produce voltageacross the resistor. As a resultit produces a smooth dc output.


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Activity 2:The figure shows a half-wave rectifier circuit that is connected to CRO. Sketch waveform of thevoltages 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.

To CROa

To CRO


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TUTORIAL 9.2

1. The adding of impurity into puresemiconductor material is known as ____(2006)A. dopingB. rectificationC. amplificationD. thermionic emission

2. Which of the following is correct about n-type semiconductor and p-typesemiconductor?

n-typesemicon-ductor

p-typesemicon-ductor

A Dopant Antimoni Gallium

B Fuction ofdopant

Acceptanceatom

Donoratom

C Valentce ofdopant

Trivalents Penta-valents

D Majoritychargecarrier

holes electron

3. Which of the following diagrams is an n-type semiconductor? (2006)

4. What is the function of a diode? (2007)A. Acts as a current amplifierB. Acts as an automatic switchC. Stores electric chargeD. Allows the current to pass in one

direction only

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

6. Which circuit will light up only one lightbulb?

7. A CRO is connected to a circuit as shownin diagram 1. The input terminal, PQ, isconnected to an ac power supply.

Diagram 1

Which of the following is the output waveform displayed on the screen?


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8. Which of the following traces displayedon a CRO is the rectification of an accurrent?

9. Diagram 2 shows a rectification circuitfor an ac current.

Diagram 2

When an output resistor is connected toa CRO, the shape of the wave form on thescreen is __

10. Two diodes are connected in a circuit asshown in Diagram 3.

Diagram 3

Which of the following is the output waveform from resistor P?

11. Diagram 4 shows a full-wave rectifier.

.Diagram 4

Which of the following is the correct pathof the current through diodes K, L, M andN and resistor R?A. L R NB. M R NC. K R MD. N R L

12. Diagram 5 shows a circuit of four diodeswhich are connected to batteries and aresistor.

Diagram 5

Which of the following is the correct pathfor the current?A. L M T S U L M

N OB. L M T R Q O N

M LC. L M T R Q O N

S UD. U S T R Q O N

M L


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13. Diagram 6 shows a circuit which hasthree identical resistors P, Q and R, fourdiodes and an ac power supply.

Diagram 6

Which resistor permitted only directcurrent through it?A. P C. QB. R D. P ,Q dan R

14. The function of capacitor in diagrambelow is ____

A. RectifierB. Flows alternating currentC. AC and DC filterD. Voltage smoother

15. In which circuit the bulb does not lightup?

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

Which bulb in the circuit will light upwhen the switch is on? (2003)A. Z onlyB. X and Y onlyC. Y and Z onlyD. X, Y and Z only

17. Which of the following pairs of atoms canact as an impurity to produce an n-typesemiconductor when doped into a puresemiconductor? (2004)A. Boron and phosphorusB. Arsenic and antimonyC. Indium and galliumD. Antimony and gallium

18. The diagram shows a circuit consistingof a diode and a bulb. When the switchis on, the bulb does not light up.

What needs to be done to light up thebulb? (2005)A. Replace with a new bulbB. Increase the number of dry cellsC. Reverse the diode connectionD. Connect a resistor parallel to the bulb

19. Diagram below shows a circuitconnected to the cathode rayoscilloscope (CRO).

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


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20. The diagram shows the structure of asemiconductor material.

(a) Based on the diagram above,(i) name the type of the

semiconductor material.

_________________________

(ii) what is the function of theBoron atom?

_________________________

(iii) name the majority chargecarrier in the semiconductormaterial.

___________________________

(b) At 0 K , a pure semiconductormaterial has very high resistance.Explain why?

_______________________________

_______________________________

_______________________________

_______________________________

_______________________________

(c) The diagram shows a p-typesemiconductor is joined to an n-type semiconductor. Later thecombination of the semiconductormaterials are connected to a bulband a battery.

(i) Name the line X.

__________________________

(ii) Give the name and the functionof voltage at X .

___________________________

___________________________

___________________________

___________________________

___________________________

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

_______________________________

_______________________________

_______________________________

_______________________________

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

Figure (a)

(i) In the space below draw a circuitdiagram for Figure (a)


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(ii) State the characteristic of the diodecauses the bulb does not light up?

_______________________________

(iii) State the correction should bedone to the connection of thecircuit in Figure (a) to make thebulb is lighted up.

_______________________________

(b) Figure (b) shows circuit X isconnected to a transformer, resistorR and a cathode-ray oscilloscope.When the switch is opened thewaveform as shown in Figure (b) isformed on the CRO screen.

Figure (b)

(i) In the space below draw a circuitdiagram for circuit X.

(ii) In the space below draw a waveformis produced in the CRO screen whena capacitor is connected parallel toresistor R.

(iii) Give the reason for your answer inb(ii).

_______________________________

_______________________________

SPM 200722. Diagram 7 shows a simple transformer.

(a) (i) Name the type of the transformer.

_____________________________

(ii)(b) The number of turns on

the primary coil inDiagram 7 is 1000.Calculate the number ofturns on the secondarycoil.

(c) The transformer inDiagram 7 is used toswitch on an electricalappliances. The currentin the primary coil is 0.1 Aand the efficiency is 75 %.

(d) A student connects atelevision which usesdirect current to theoutput of the transformerin Diagram 7. When thetelevision is switched on,it does not function.

SPM 2004 Section C23. Figure (a) shows circuits J, K, L, M and N

each containing an ideal transformer.Diodes in the circuits are used for thepurpose of rectification.


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Figure (a)Key :Np = Number of turns in primary coilNs = 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, Mand N in Figure (a) and consider thefollowing aspects:

- type of transformer- ratio of the number of

turns in primary coil tosecondary coil

- type of rectification- characteristic of output

current

SPM 2007 Section B24. Diagram10.1 and Diagram 10.2 show two

electrical circuits containingsemiconductor diodes.

(e) What is meant by semiconductor?

(f) Using Diagram 10.1 and Diagram 10.2,relate the lighting of the bulbs, thecurrent and the way the diode isconnected to the terminals of thebattery to deduce a relevant physicsconcept.

(g) Diagram 10.3 shows a full wave rectifiercircuit.


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(i) Draw the wave form of a full waverectification.

(ii) A capacitor is placed across theoutput to smooth the current. Drawthe wave form produced. Explainhow a capacitor is used to smooththe current.

9 2 semiconductor doc

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