Electronics Questions and Answers - Physics Form 4 Topical Revision

Questions

1. Sketch curves to show the variation of current and time as displayed on the CRO in each of the diagrams below
   
2. State the majority carriers for a p-type semi conductor.
3.  
   1. Using examples explain the difference between a semiconductor and a good conductor.
   2. A radio repairer wishes to use an ammeter to detect a faulty diode. With the aid of a circuit diagram describe how he will go about this task.
4. p- type and n-type semiconductors are made from a pure semiconductor by a process known as “doping”.
   1. What is doping?
   2. Explain how the doping produces an n-type semiconductor.
5. Sketch a current-voltage characteristic of a junction diode with a forward bias.
6. Using the components symbols shown in the figure below, sketch a series circuit diagram for a forward biased diode.
   
7.  
   1. Distinguish between semiconductors and conductors
   2. Give an example of a semiconductor and one for a conductor.
8. In the circuit below, when the switch s is closed, the voltmeter shows a reading.
   
   When the cell terminals are reversed and the switch is closed, the voltmeter reading is zero. Explain these observations.
9. What is meant by Donor Impurity in semiconductors.
10. You are provided with a diode, a resisitor R, an a.c source of low voltage and connecting wires. In the space provided, sketch the circuit diagram for a half-wave rectifier and indicate the terminals where the output voltage v₀ may be connected. (2mks)
11. Distinguish between intrinsic and extrinsic semi-conductors.
12. The diagram below shows a rectifier circuit for an alternating current (a.c) input.
    
    
    1. Describe the rectification process.
    2. Draw the traces of the signal obtained on CRO connected across QS and PR.

Answers

1.  
   
2. Positive holes
3.  
   1. Semi- conductor- conducting is by holes 
      Conductors- Conducting is by electrons.
      
      Examples
      Semi conductors - silicon
      Conductor – copper, tin, iron
   2.  
      
      Connect diode and check whether current flows reverse terminals of diode and check again.
      If it doesn’t conduct at all or it gives equal deflections when reversed, then it is spoiled. It should give deflection when connected in one way and not the other.
4.  
   1. Introducing group III or V ( impurities to group 4 elements to improve their conduction
   2. The group V elements form covalent bonds with the semiconductor atoms using 4 electrons each. Thus an electron is left free for conduction.
5.  
   
6.  
   
7.  
   1. Semiconductors are from group 4 while conductors are from group 1,2 or 3. Semiconductors poor conductors at low temp and improves conduction as temp increases. The conductors are good conductors at low temp and increases resistance with increase in temp.
   2. Semi- conductors- Germanium, silicon 
      Conductor – copper, silver, aluminum, etc. 
8. - Diode is forward – biased: current flows
   - Diode- reverse- biased; no current flows.
9. Is the atom introduced into the semiconductor (doping) to provide an extra electron for conduction
10.  
    
11. Intrinsic semiconductors are pure materials which conduct electricity under suitable conditions like increase in temperature.
    Extrinsic semiconductors are materials that have been fitted with impurities.
12.   
    1.  
       
       During the first half-cycle, D₁, and D₄, are forward biased while D₂, and D₃, are reverse biased. The path of the current is thus AQPRSB. During the second half-cycle, diodes D₂, and D₃, become forward biased while D₁ and D₄, are reverse biased. The path of the conventional current is then BSPRQA.

       Notice that during both half-cycles, current flows through the load resistor R, in the same directions. 

    2.