Part VI — Electronics and Control Systems Basics

Chapter 6: Semiconductor Devices and Control Systems

6.1 Semiconductors

A semiconductor (e.g., silicon, germanium) has electrical conductivity between that of a conductor and an insulator, and its conductivity can be precisely controlled through doping (adding impurity atoms). N-type semiconductors are doped with pentavalent impurities (e.g., phosphorus), creating an excess of free electrons (majority charge carriers). P-type semiconductors are doped with trivalent impurities (e.g., boron), creating an excess of "holes" (majority charge carriers, representing the absence of an electron).

6.2 The PN Junction Diode

A PN junction diode is formed by joining P-type and N-type semiconductor material, and conducts current predominantly in one direction. In forward bias (P-side connected to positive terminal, N-side to negative), the diode conducts readily once the forward voltage exceeds a threshold (approximately 0.7V for silicon diodes, approximately 0.3V for germanium diodes). In reverse bias, the diode blocks current flow (except for a very small leakage current) until reverse breakdown voltage is reached.

6.3 Transistors

A Bipolar Junction Transistor (BJT) is a three-terminal semiconductor device (Emitter, Base, Collector) available in NPN and PNP configurations, used for amplification and switching applications. A small current at the base terminal controls a much larger current flow between emitter and collector, providing current amplification (current gain, β = Ic/Ib).

6.4 Basic Control Systems

A control system manages, commands, directs, or regulates the behaviour of other devices/systems. An open-loop control system does not use feedback (output has no influence on the control action, e.g., a simple washing machine timer), while a closed-loop (feedback) control system continuously compares actual output to the desired output and adjusts input accordingly to minimise error (e.g., a thermostat-controlled heating system), generally providing greater accuracy and disturbance rejection than open-loop systems, at the cost of greater complexity.

6.5 Practice Set — Electronics and Control Systems (14 MCQs)

1. A semiconductor's electrical conductivity lies between that of:
   (a) Two different insulators (b) A conductor and an insulator (c) Two different conductors (d) A superconductor and a conductor
2. N-type semiconductors are created by doping with:
   (a) Trivalent impurities (b) Pentavalent impurities (c) No impurities at all (d) Only metallic impurities
3. P-type semiconductors have an excess of majority charge carriers called:
   (a) Free electrons (b) Holes (c) Protons (d) Neutrons
4. A PN junction diode, in forward bias, conducts readily once forward voltage exceeds approximately (for silicon):
   (a) 0.1V (b) 0.7V (c) 5V (d) 12V
5. In reverse bias, a PN junction diode:
   (a) Conducts fully, identical to forward bias (b) Blocks current flow except for a small leakage current (c) Produces amplification (d) Generates AC output directly
6. A germanium diode's typical forward-bias threshold voltage is approximately:
   (a) 0.3V (b) 0.7V (c) 1.5V (d) 5V
7. A Bipolar Junction Transistor (BJT) has how many terminals?
   (a) Two (b) Three (c) Four (d) Five
8. The three terminals of a BJT are:
   (a) Anode, Cathode, Gate (b) Emitter, Base, Collector (c) Source, Drain, Gate (d) Positive, Negative, Neutral
9. In a BJT, a small base current controls:
   (a) A much larger collector-emitter current (b) Only the base voltage (c) Nothing; base current has no effect (d) Only the reverse leakage current
10. An open-loop control system is characterised by:
    (a) Continuous feedback comparing output to input (b) No feedback; output has no influence on control action (c) Being always more accurate than closed-loop systems (d) Requiring a comparator element
11. A closed-loop (feedback) control system:
    (a) Has no comparator element (b) Continuously compares actual output to desired output and adjusts accordingly (c) Cannot be used for temperature control (d) Is always simpler than open-loop systems
12. A thermostat-controlled heating system is an example of:
    (a) An open-loop control system (b) A closed-loop (feedback) control system (c) Neither type of control system (d) A purely mechanical system with no control action
13. Compared to open-loop systems, closed-loop systems generally provide:
    (a) Lower accuracy (b) Greater accuracy and disturbance rejection, at greater complexity (c) No difference in performance (d) Simpler construction with equal performance
14. Current gain (β) in a BJT is defined as:
    (a) Ib/Ic (b) Ic/Ib (c) Ic × Ib (d) Ic + Ib

Answer Key

1.(b)

2.(b)

3.(b)

4.(b)

5.(b)

6.(a)

7.(b)

8.(b)

9.(a)

10.(b)

11.(b)

12.(b)

13.(b) 14.(b)