Fundamentals of electronics
Conduction in solids: every solid conductor of electricity can be classified as either a metal, a semiconductor, or a non-conductor termed insulator.
Metals: metals are characterized by low values of electrical resistivity of about 10-8 to 10-6 ohm meter, e.g. silver, gold, copper and aluminum. A metal exhibits charge neutrality because the negative charges on the free electrons are balanced by the positive charges associated with the nuclei. As temperature increases the resistance of metallic conductors increase due to higher collision between atoms.
Semiconductors belong to group IV in the periodic table with four electrons in their outermost shells. When all the valence electrons are constrained in covalent bonds, no charge carriers are free to move and hence there is no conduction.
Insulators have very high resistivity of between 107 to 1018ohm-meter. Examples are quartz, polyethylene and diamond. Here, all the electrons are bound to the crystals and none is free to participate in conduction.
Energy band model
Types of semiconductors
Impurities in semiconductors: pure semiconductor materials do not conduct current very well because of the limited number of free electrons in the conduction band. In addition they require that the holes and electrons be present in exactly equal numbers. These impurities are donor impurities and acceptor impurities.
P-N junction diode
A p-n junction is made by growing in a single semiconductor crystal, part of which is n-type and the other p-type. The boundary inside the crystal between p-and n-regions is termed p-n junction; p-n junction occurs whenever the impurity concentration changes from a predominance of donors to a predominance of acceptors over a sufficiently small distance.
Diode circuit symbol
Half wave rectifiers
Bipolar junction transistor (BJT)
Junction transistors belong to a group of two families of transistor types, one is the bipolar junction transistor (BJT), the second being the field effect transistor (FET). Amplifiers designed using BJTs are currently controlled while those designed using FETs are voltage controlled. The BJT is however, more widely used than the FET.
Schematic diagram of BJT
Construction of BJT
A bipolar junction transistor is a three-terminal device with two p-n junctions formed back to back. There are two types of BJT: the n-p-n and the p-n-p. In the n-p-n type, a p-type region is sandwiched between two n-type regions. On the other hand, the n-type region is sandwiched between two p-type regions.
Making of BJT
In the early days of solid state electronics, computing and electronic circuits were constructed from discrete and individual components such as diodes, transistors, resistors and capacitors. In the 1950s, it was made known that the possibility of forming all the components for such circuits in a single chip (wafer) of semiconductor material and this became known as integrated circuit (IC).
Types of IC
- Linear or analogue, containing amplifying and oscillating circuits.
- Digital, involving switch circuits. The digital are of two types TTL (transistor-transistor logic) and CMOS (complementary metal-oxide semiconductor logic)
Note: the equation for solving operational amplifiers which includes non-inverting and inverting respectively.
Vo / Vi = 1+Rf / R1 ( non inverting)
-Rf / R1 = Vo / Vi (inverting)
1.Which of the following diagrams represents correctly an n-p-n transistor? (Jamb 2018)Answer: D
2. A transistor functions mainly as a? (Jamb 2017)
A. Switch and an amplifier
B. Rectifier and an amplifier
C. Charge storer and a switch
D. Charge storer and an amplifier
3. When impurities are added to semiconductors, the conductivity of the semiconductor. (Jamb 2015)
B. Increases then decreases.
D. Remains constant
Solution: this shows that impurities slow or reduce current in a semiconductor
4. The p-n junction diodes can act as rectifiers because they ? (Jamb 2012)
A. Conduct current when forward-biased
B. Conduct current when reverse-biased
C. Block current when forward-biased
D. Conduct current in both directions
5. Distinguish between p-type and n-type semiconductors? (Wassce 2018)
|I) P-type semiconductor is doped with trivalent atoms||N-type semiconductors is doped with pentavalent atoms|
|II) P- type semiconductor has holes as majority charge carriers and electron as minority charge carriers||N-type semiconductor has electrons as majority charge carriers and holes as minority carriers|
|III) Examples of doping agent or dopant/trivalent atomic impurities are boron, aluminum etc.||Examples of doping agent or dopant/pentavalent atomic impurities are antimony, arsenic and phosphorus.|
6. List three advantages of p-n junction diode over diode valve. (Wassce 2017)
- It requires low voltage for it to function
- It can not be easily broken or damaged
- It is cheaper and quite easy to manufacture
7. When light falls on a metallic surface, the number of electrons that may be emitted would depend solely on the ? (Wassce 2002)
A. Area of the metallic surface
B. Frequency of the light
C. Intensity of the light
D. Time of exposure of the metallic to light
8. Charges on a conductor are? (Wassce 2004)
A. Evenly distributed over a surface of any shape
B. Concentrated on the inner surface of the conductor
C. Concentrated at the outer flat surface of the conductor
D. Generally concentrated at sharply curved surface