LESSON9.2 electronics

LESSON 2-Understanding semiconductor diodes.

      Semiconductor Materials Semiconductors are materials which conduct electricity better than insulator, but no so well as ordinary conductors.The following table shows the comparison between insulator, conductor and semiconductor: 

Insulator Semiconductor ConductorExample material Glass,

ceramic,polythene

Silicon,germanium,selenium

Copper,aluminium,iron

Charge carrier No free electrons Free electronsand holes

Free electrons

Resistance High Between insulator and conductor

Low

Conductivity Decreasewhen the temperature

Increase when the temperature increase.Also increase when light shines on it or with presence of impurities

Decrease when the temperature

 Charge carriers in semiconductors 

        In a pure crystal of a semiconductor( intrinsic semiconductor) such as silicon, each atom of silicon has four electrons in outermost orbit that are involved in covalent bonding.             The vibrations of atoms causes some electrons to break free the bonds.When an electron is removed from a covalent bond, it leaves behind “a vacancy” and is called “a hole” in the bonding . Free electrons( negatively charged) and holes (positively charged) are known as charge carriers .Conduction in a semiconductor is by means of a movement of free electrons and holes in opposite direction.Semiconductors cannot conduct electricity as well as metals because they have smaller numbers of free electrons and holes. The conductivity of the semiconductors can be increased by a process is called “doping” Doping of SemiconductorsDoping is a process of adding a small amount of impurities into the pure crystal of semiconductor (intrinsic semiconductor).Atoms of the impurities added should have almost the same size as the atoms of the intrinsic semiconductor. Type of Semiconductors Material Different kinds of impurities are added produce different types of semiconductor ; the p-type and the n-type. (i)              p-type semiconductor                          

A p-type semiconductor is produced when trivalent atoms are added to intrinsic semiconductor atoms.Only fhree 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.

 (ii)              n-type semiconductor                            

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.

 Comparison between p-type semiconductor and n-type semiconductor 

p-type semiconductor

n-type semiconductor

Pure Semiconduktor

Silicon,Germanium

Silicon,Germanium

Doping substance

Indium,Boron,Gallium

Phosporus,Antimony,Arsenic

Function of doping substance

Aceptor atom Donor atom

Valency of doping substance

Pentavalent Trivalent

Majority charge Hole Electron

carrierMinority charge carrier

Electron Hole

  Semiconductor Diode A diode is a component (device) that allows electric current to flow in one direction only.A diode acts like a one-way valve to electric current.

 The structure and the symbol of a semiconductor diode A semiconductor diode can be made by joining pieces of n-type and p-type semiconductor.The semiconductor diode is also called p-n junction diode.The following figure shows structure and the symbol of a semiconductor diode :

 How does the p-n junction diode work? When p-type semiconductor material in contactwith n-type semiconductor material , a layer called the depletion layer is formed in the middle.At this junction , electrons from n-type material drifts across the junction to fill in the holes in p-type.The holes from p-type material drift in the opposite direction to unite with free electrons in the n-type material. 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 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.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. 

 When the in forward-biased arrangement, 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.

When the in reverse-biased arrangement, the cell voltage 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. Graph of current, I against voltage V for p-n jenis silicon dan germanium diode

                         Experiment to show a semiconductor diode flows current in one direction only.  

              Figure(a)                                               Figure(b) The diode is connected to the cell in the forward-biased arrangement as shown in Figure(a).The bulb light up.The experiment is repeated with the reverse-biased arrangement as shown in Figure (b)The bulb does not light up.The experiment shows that a diode allow the current in one direction only when the diode in the forward-biased arrangement. 

                Diode as a Rectifier A diode can act as a rectifier because it can convert alternating current(a.c.) into direct current(d.c).The process of converting a.c. to d.c. is called rectification.                             

                  There are two types of rectification process :(1)              Half -wave rectification(2)              Full - wave rectification  Half- wave rectification 

 For half of the cycle, A is more positive than B ,the diode conducts.For the other half cycle, A is more negative than B , no current can flow. Full-wave rectification 

 For half of the cycle, A is more positive than B ,the diode conducts and the current flows through the resistance.For the other half cycle, A is more negative than B ,the current flows through the resistance in the same direction as before.    Capacitor A capasitor is device which can(1)              store electric charge(2)              smooth out waveform in the rectified output(3)              separate the a.c and d.c (as a filter) Smoothing output wave by a capacitor By connecting a capacitor parallel to the resistance , the half-wave and the full-wave rectified waveform could be partially smoothed out.

 For half of the cycle, the capacitor is charged up. Energy is stored in the capacitor.For the other half cycle, the capacitor releases its charge (discharges)So the capacitor can produced a steady output or output is stablised.     

   TUTORIAL 2

1              Which of the statements on semiconductor material is not true? A              not obey the Ohm’s law

B              the charge carriers are holesC              Silion and Germanium are two examples of pure semiconductor materialD              conduct electricity better than insulator but no so well as ordinary conductors.

 2              Which of the flowing graph resistance, R against temperature, T is correct for a pure semiconductor material?

3              The process of adding a trivalent atom to a pure silicon crystal is known as               A              doping              B              donating              C              excitation              D              polarization

 4              Which of the following is not a doping substance.

A              antimony                                    B              germaniumC              indium                                    D              gallium 

           5              Which of the following pairs characteristics are true?

n-typesemiconduktor

p-typesemiconduktor

A Doping substance

antimony gallium

B Funtion of doping substance

aceptor atom donor atom

C Valency of doping substance

trivalent pentavalent

D Majority charge carrier

hole electron

 6              An n-type semiconductor is produced when some of the germanium atoms are replaced with

 A              divalent atomsB              trivalent atomsC              tetravalent atomsD              pentavalent atoms

 7              Which of the following diagrams is an n-type semiconductor?         

 8              Which of the following pairs atoms can act as an               impurity to produce an n-type semiconductor               when doped into a pure semiconductor?               A              Boron and phosphorus              B              Arsenic and antimony              C              Antimony and gallium              D              Indium and gallium              9              Which of the following is true according to a

p-type semiconductor? 

A              Thedoping substance is trivalent atomsB              The majority charge carrier is free electronsC              The conductivity decreases when temperature is increased

 10              Which of the following graph current,I against potential difference,V for a semiconductor diode?

 11              A function of a diode is as F A              Amplifier                                              B              RectifierC              Smoother                                     

       12              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?               A              Replace with a new bulb              B              Reverse the diode connection              C              Increase the number of dry cells              D              Connect a resistor parallel to the bulb 13              The diagram shows a circuit containing bulbs X, Y and Z.

                            Which bulb in the circuit will light up when the switch is on?               A              Z only              B              X and Y only              C              Y and Z only              D              X,Y and Z          14              Which of the following circuit causes only a bulb is lighted up? 

15              The diagram shows a circuit containing diode D,               resistor R and a.c. power supply is applied at terminal PQ. The waveform of the a.c. supply is shown in the diagram.

 Which of the following waveform output is formed at CRO screen?

 16              Which of the following waveform has been rectified?

17              The diagram shows a circuit containing two diodes , a resistance is connected to a.c. voltage.

                  Which of the following waveform formed on the         CRO screen? 

.18              The diagram shows a rectification circuit which uses four diodes K,L,M and N and resistor, R.

               Which of the following is the direction of the current flowing? 

A              K Þ N  Þ M Þ L                        B              L  Þ M  Þ N Þ RC              L   Þ R Þ ND              M Þ  R Þ N                 

19              The diagram shows a rectification circuit which uses four diode , a resistor and a battery.

              Which of the following is the direction of the current flowing?

 A              U Þ S ÞNÞOÞQÞ RÞT ÞM ÞLB              U Þ S ÞTÞMÞNÞO ÞQ ÞM ÞLC              L Þ M ÞNÞOÞQÞR ÞT ÞS ÞUD              L Þ T ÞSÞQÞRÞ OÞN ÞM ÞU    

20              The diagram shows a rectification circuit which uses four diodes and three resistors P ,

Q and R.

Which of the following resistor is only thedirect current  flow?

 A              P                                               B              QC              R                                               D              P ,Q and R             21

The function of the capacitor in fhe figure above as                         

A              a rectifier                                   B              an amplifierC                a current filter                        D              a voltage smoother         22

Based on the circuit above, which of the following waveform is formed on the CRO screen?

                    23              Which of the following circuit does the bulb not light up

                         24              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 the Boron atom?                                       .........................................................

(iii)              name the majority charge carrier in the semiconductor material. 

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

 .................................................................... ………………………………………………… ………………………………………………… ………………………………………………… ………………………………………………...

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

                      (i)              Name the line X.

 ....................................................................

(ii)              Give the name and the function of voltage at X . 

.................................................................... ………………………………………………… ………………………………………………… ………………………………………………… …………………………………………………

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

 ............................................................................. ............................................................................. ............................................................................. .............................................................................         25              (a)              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.

 Figure (a)

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

(ii)              State the characteristic of the diodecauses 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. ………………………………………….. …………………………………………. …………………………………………..

(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.

 Figure (b)

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

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

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

26              The diagram shows a rectification circuit which uses four diodes W ,X, Yand Z and resistor R.

 (a)              What does the word rectification mean?

............................................................................. 

(b)               Draw the waveform formed on the CRO screen.

                      (c)              By using the letters A,B,C,D,E,F,G and H in the figure above state the direction of the current flows when at half of the cycle, A is more positive than B.

 ....................................................................

(d)  What happen to the direction of the current flows in resistor R compared to the your answer in (c) when at the other half cycle, A is more negative than B.

 ....................................................................

 ....................................................................

(e)              What happen to the output waveform when(i)              the terminals of diodes X and Y are reversed

 ............................................................

   (ii)              only the terminals diode Y are reversed

                                     …………………………………………..

(iii)              the terminals of diodes W, X, Y and Z are reversed. 

............................................................(f)              Draw the waveform formed when a capacitor is connected parallel to resistor R in the

figure above.

                      27              Abu wants to power a ticker timer and a transistor radio. The ticker timer requires an input voltage of 12 V a.c. and the transistor radio requires an input voltage 12 V d.c. The power supply at Abu’s school is 240 V a.c.

 (a)              Abu  used a transformer as shown in Figure (a) to power the ticker timer.

                                              Figure (a)

 (i)              Name the type of the transformer is used.

                                        ............................................................

(ii)              Give one reason why the soft-iron core is used in the transformer? 

…………………………………………..(iii)              The number of turns in the primary coil is 2000. Calculate the number of turns in the secondary coil.[ Assume that the transformer is an ideal transformer ]

(b)              The transformer in Figure (a) is not suitable to power the radio. A modification is required before the radio is used.

(i)              Explain why the transformer is not suitable to use? 

                                    ............................................................                                     ............................................................

(ii)              Name two electronic components that are needed to use.                                     .................…..................………………                                     …………………………………………..

(c)              Figure (b) shows a circuit that can be modified to produce a full-wave rectification and smoothing output wave

                       Figure (b) 

Complete the circuit in the dotted box by adding the electronic components in b(ii)           28              (a)              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.

              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

   (b)              Figure (b)              shows a circuit consisting of a transformer, an ammeter and two light bulbs. The ammeter reading is 0.5 A and both bulbs light up with normal brightness.

 Figure (b)

(i)              What is the output voltage of the transformer?(ii)              Calculate the efficiency of the transformer.

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