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

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

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

Source: www.commons.wikimedia.org 

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.

P-N junction

Source: www.commons.wikimedia.org 

Diode circuit symbol

Source: www.commons.wikimedia.org 

Half wave rectifiers

Source: www.commons.wikimedia.org 

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

Source: www.en.wikipedia.org 

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.

Source: www.commons.wikimedia.org  

Making of BJT

Source: www.watelectronics.com 

Integrated circuits

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

1. Linear or analogue, containing amplifying and oscillating circuits.
2. 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.

V_o / V_i = 1+R_f / R₁  ( non inverting)

-R_f / R₁ = V_o / V_i (inverting)

Past questions

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

Answer: D

3. When impurities are added to semiconductors, the conductivity of the semiconductor. (Jamb 2015)

A. Decreases

B. Increases then decreases.

C. Increases

D. Remains constant

Answer: A

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

Answer: B

5. Distinguish between p-type and n-type semiconductors? (Wassce 2018)

Answer: 

6. List three advantages of p-n junction diode over diode valve. (Wassce 2017)

Answer: 

1. It requires low voltage for it to function
2. It can not be easily broken or damaged 
3. 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

Answer: B

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

Answer: D