By MICHAEL JOHN UGLO
THE study of electronics is very important as it gives much needed information for all applications in science and technology today. Electrons are the use of one of the three main subatomic particles found in an atom. It is negatively charged or carries a negative charge. The electricity is a flow of electrons. The other two are protons which carry a positive charge and neutrons which carry no charge at all or are neutral. These three particles are found in all matter whether solid liquid, gas or plasma (ionised gasses)
Electrons are responsible for chemical reactions that take place in the world. In the study of electronics, the electrons are used to do any task like storing information in a computer or mobile phone, communicating in audio or sound, light or pictures or increasing the capacity of sound and pictures (amplification) and controlling devices (switching) and equipment to do useful work.
All matter including solid, liquid gas and plasma (ionised gasses) contains electrons as we have seen. Out of all these forms of matter, some matter can conduct electricity well and these are called conductors. Others cannot conduct electricity at all so these are called non-conductors or insulators. Some substances are in the middle between these two extremes which means they can conduct electricity but not as well as conductors.
These substances are called semi-conductors. It is the semi-conductors that become very important in the study of electronics.
The elements found on the group four of the periodic table of elements like carbon, silicon, germanium, gallium are automatically semi-conductors simply because they have four electrons called valence electrons in their outer shells. These four valence electrons cannot donate electrons like group one, group two or group three elements or accept electrons like group five, group six or group seven elements to involve in chemical reactions to form chemical compounds. What they do is to share their electrons with any of these other elements in these other groups to form covalent bonds and therefore covalent compounds.
In electronics, the most commonly studied group four element and also commonly used as a very good semi-conductor is silicon. The other most commonly studied and used elements are germanium and gallium arsenide. In silicon in particular, the atoms like any other group four elements, are covalently bonded together to form silicon crystals.
Semi-conductor crystals can conduct electricity because some electrons can be available on the conduction band. In increased temperatures, few more electrons can become available because high temperatures can displace electrons from their bonds after it displaces them as they become excited and enter the conduction bands. Thus, more electrons will become available and therefore more electricity can be conducted. Thus, pure semiconductor crystals are called intrinsic semi-conductors.
The other group of semi-conductors are called extrinsic semi-conductors. These are semi-conductors which are impure. That means some impurities have been added to them.
Elements from group five of the periodic table like phosphorus added to silicon crystals can use up four electrons to attain a full octet shell. One electron will be left unbonded and this electron carrying a negative charge will flow to conduct electricity when a voltage is applied. This process of adding an element like phosphorus as an impurity to a semiconductor crystal like that of silicon is called doping.
The semi-conductors formed in such a manner are called n-type semi-conductors as they involve passage of electrons. On the other hand, a group three element like boron added as the impurity to silicon crystal is also doping and this will fill up three of the four empty spaces on the silicon octet shell. This means it will be one electron in short supply so it creates a gap called the hole. This hole is now carrying a positive charge.
The semi-conductors formed using such a method are called p-type semiconductors as they involve movement of holes that carry positive charges. When a voltage is applied to this doped semi-conductor, the nearby electrons will travel to fill in this hole and in the process of doing that, they will leave behind a hole they previously occupied and so the chain continues.
This allows the holes to flow in the opposite direction while the electrons flow the other way. Hence, in this manner, the electrons flow from the cathode (negative) to the positive (anode) terminal while the holes flow in the reverse direction. That is, they flow from the anode to the cathode. This is called bipolar movement or passage and transmission.
When you combine an n-type and a p-type semiconductor interesting results show. That is, nearby electrons will flow to the holes and fill the holes while leaving behind a positive ion at their side. The holes will be filled while leaving behind a negative ion. Thus, that region is going to be depleted of any holes or electrons but will leave a region of positively charged and negatively charged region of ions on the opposite sides.
Any electron that travelS to the holes on the other side will be repelled by the negative ion.
Similarly, any holes that are going to be passed in the reverse direction towards the electrons will be stopped or repelled by the positive ions. That particular region is called a depletion region. However, there is still going to be a flow of small electric current due to the oppositely charged regions or polarised regions that is present allowing the leakage of electrons called the leakage current.
Therefore, it is advisable to remove or disconnect any voltage source like a wet cell from a car battery or even dry cells to avoid leakage currents.
Semi-conductors are used to manufacture electronic devices. This is termed as solid-state electronics. Some common devices made from semi-conductors are diodes and transistors. Diodes are made by combining an n-type and a p-type semiconductor.
The production and uses of diodes are varied. Diodes can allow the positive component (forward current) of the alternating current(ac) to pass through while stopping the negative (backward) flow of the alternating current. This means, diodes convert alternating current into direct (dc) current. This is called rectification. Therefore, diodes can be used as rectifiers.
The other semi-conductor product is called a transistor. A transistor is made by sandwiching two n-type semiconductors on each side and a p-type semi-conductor in the middle. Or in the other arrangement, an n-type semiconductor in the middle and two p-type semi-conductors on each side. In that arrangement two major effects are achieved. One is called amplification and the other is called switching.
A transistor is a triode which means it has three electrodes; a base, a collector and an emitter. All necessary corrections are made to the three electrodes. A base is where a little current as low as 0.05 milliamperes can be allowed to flow through. Once it saturates, a hundred-fold increase or surge in current can be initiated at the collector and exudes through the emitter. That is the collector current can be 0.95 milliamperes while the while the emitter combines 0.05milliampere base current together with the collector current totalling 1 ampere or totalling a 100-fold or 100 per cent increase.
This is how a load such as a loud speaker or a motor can be run with as little as 0.05 amperes. This is an example of a transistor’s use for amplification.
A transistor can be used for switching as well. When the base current is increased from a 0.05amps to anything above it that will result in the base emitter to be quickly reversed as emitter is highly doped than collector and forms positive ions. Thus, the polarity changes as emitter becomes more positive than the collector.
Thus, the proportional relationship of voltage of base and emitter to that of the base collector voltage is affected so the passage of the greater current from collector to emitter is reversed whereby the electron flow from the emitter to the collector via the base to run loads is stopped. This is called reverse biasing. Here it acts as a very effective switch.
Thus, decreasing or decreasing the base current will either cause the transistor to be in a state of saturation known as forward biasing or reverse biasing respectively.
This particular of increasing and decreasing the doping of semi-conductor devices gives the necessary required electronic devices.
Transistor applications in the world result in radio broadcasts, television mobile phones, computers and any latest electronic devices you could think of. In the olden days it was a device called a vacuum tube that was used to allow flow of electrons and ions through a vacuum for amplification and switching which was very crude.
Now we have transited from vacuum tubes to bipolar transistors and now the type of transistor that revolutionised the electronics industry is the Mosfet which stands for metal oxide semiconductor field effect transistor. Mosfet uses electric impulse to cause electrons to concentrate under the gate to allow electrons to flow from the source to the drain.
The bipolar transistor as described earlier works by a flow of electrons from the base to allow build-up of electrons at the collector for the current to flow towards the emitter from the collector.
The M invention allows the transistor to be miniaturised to fit so many tens of thousands and even millions and billions of Mosfet transistors in ICs (integrated circuits) on a chip like a silicon chip of a magnitude 5mm by 5mm to attain different functions. The ICs to build a transmitter and a receiver on a mobile phone, the ICs to produce all programs with a control unit and arithmetic and logic unit to work with an operating system of a computer or any microprocessors like that of a car’s ignition or a washing machine are all done through the use of miniaturized millions of Mosfet transistors.
Mosfet’s ability to use less power and can be made chiefly has allowed for its fabrication and IC constructions brought about massive explosion in the current technology age progresses. The other group of Mosfet transistors are called CMOSFETs or simply complementary metal oxide semi-conductor field effect transistors. They simply work by voltages rather that currents. When the gate is not applied with a voltage, the source remains negative and the drain remains positive and there is a blockade under the insulated layer.
When a voltage is applied at the gate, then it creates a positive charge under the insulating layer whereby electrons are attracted to the gate. In that way electrons are allowed to flow from the source to the drain. The more voltage as applied to the gate the more the electrons accumulated under the gate and the more current flows. The CMOSFET or simply CMOS is used to make products like computer memories.
An application is seen in the digital electronics system. Digital electronics involves basically one (1) and zero (0) digits. The voltage levels are arranged so a voltage of 6 volts is 1 and a voltage of 1 is 0. So with these arrangements, a place value system is used to denote the alphanumeric values. That is to write numbers and letters to write words and to do arithmetic using the digital or the logic system.
Binary numbering system
The numbering system used is called the binary numbering system that uses numbers 0 and 1 to base 2 while the decimal system we are most familiar with uses 0 to 9 that uses base 10. There are also other numbering systems like the octal numbering system that uses numbers 0 – 7 which is to base 8 and hexadecimal numbering system that uses numbers from 0 to 15 which is to base 16. To covert numbers from binary to octal and hexadecimal is very good because they shorten the longer binary versions into shorter octal and even much shorter version hexadecimal.
Digital electronics uses logic gates to convey information for consumption. The mathematics used in this technology is Boolean algebra to make inputs and produce predicted outputs. Software are programmed and are available to facilitate logic and IC designs to accomplish certain tasks such as computer aided designs (CAD) or Computer aided manufacturing (CAM).
The main logic gates used are namely And, Or and Not gates. Other gates used are derived from these three main gates such as Not And as Nand, Not Or as Nor and Exclusive Or as Xor. These gates form the basis for flipflops as bistable and counters, registers, monostable, oscillators from multivibrators and so on used in the logic circuits to accomplish different tasks in logics and digital systems.
Digital information is very effective and can be transmitted to very long distances with so much clarity than does frequency or amplitude modulation which are greatly affected by statics and electromagnetic interference (EMIs). As such space communications and communication satellites use digital electronics for communication purposes.
Next week: Part II of Science in Action series – Matter in the Universe and Technology
- Michael Uglo is the author of the science textbook “Science in PNG, Pacific, Asia & Caribbean and a lecturer in avionics, auto- piloting and aircraft engineering. Send comments to: [email protected]