Electronics can best be described as the branch of applied physics concerning the generation, control and amplification of electric currents and voltages by influencing the movement of electrons in a vacuum (vacuum tubes) or in semiconducting materials (transistors and diodes).


Semiconductors are a group of materials that are in the center of the resistance spectrum. These materials acquire special characteristics that are created by alterations in their crystal structure.

Pure semiconductor materials such as silicon have a rather high resistance to the flow of electric current. When impurities are added (a process called “doping”) to the pure semiconductor material (the amounts and types of these impurities are carefully controlled) the material will have a much lower resistance to electric current.

When an impurity such as phosphorus or arsenic (which are referred to as donor atoms since they donate a free electron to the crystal) is added, it causes the semiconductor material to have many extra and loosely held electrons. This type of semiconductor is called “N” or negative type. Doping elements such as arsenic and phosphorus are called pentavalent elements because their atoms have five electrons in their outer orbits. Also, electrons that are located in the outer orbit are referred to as valence electrons.

If impurities such as boron or gallium are added (which are referred to as acceptor atoms since they can accept an additional electron) to the semiconductor material, it creates a shortage of electrons. Boron and gallium are trivalent atoms that have three electrons in their outer orbits. This type of semiconductor material is called “P” or positive type. The area in the crystal where there is a vacancy caused by the absence of an electron is called a “Hole”.


When N and P type semiconductor materials are joined together such as in a diode, an unusual but very important phenomenon occurs at the junction where the two materials are joined. In an unbiased condition, free electrons from the N-type material drift across the junction into the P-type material. Holes in the P-type material drift across the junction and combine with free electrons in the N-type material.

This movement of free electrons and holes creates a narrow negative charge zone on the P side of the junction and a narrow positive charge zone on the N side. This narrow positive and negative charge region is called the junction barrier or the depletion zone and exhibits a potential voltage difference that must be overcome in order for current to flow through the junction.

This potential voltage cannot be measured directly with a voltmeter since the net charge across the entire PN device is zero. However, this potential can be measured as a voltage drop across the diode when the diode has a voltage applied to it in a circuit with a load. The junction barrier voltage is approximately 0.3 volts for a germanium diode and approximately 0.7 volts for a silicon diode.


If a battery is connected to a diode, with the negative terminal connected to the cathode of the diode and the positive terminal connected to the anode, current will flow easily and the diode will appear to have a very low resistance – like a conductor. The battery voltage must be greater than the junction barrier voltage, approximately 0.7 volts for a silicon diode.

A diode connected in this manner is said to have a forward bias applied to it. Current flow through the diode is due to free electrons in the N-type material being attracted to the positive charge of the battery. Any increase in the applied forward voltage will result in a corresponding increase in the amount of current flow. Also, the increased number of electrons in the area near the junction has the effect of reducing the size of the depletion zone. It is important to note that in normal operation, there is some resistance elsewhere in the circuit that the diode is part of in order to prevent excessive current flow.

Forward Biased Diode Drawing PDF


If the positive terminal of a battery is connected to the N type material or cathode of the diode (the banded end) and the negative battery terminal is connected to the P type material or anode of the diode, the junction acts like a very high resistance and only a very small amount of electrical current is able to flow.  When a diode is connected in this manner it is said to have a reverse bias applied to it.

This reverse bias causes free electrons in the N-type material to move toward the positive charge on the cathode of the diode. In a similar manner, holes in the P-type material move toward the negative charge on the anode. This movement of electrons and holes expands the depletion zone and the junction barrier voltage increases to equal the external applied voltage.

If the reverse voltage across the diode is allowed to increase past the peak inverse voltage (PIV) rating of the diode, the reverse current will increase rapidly and may destroy the diode. There is special class of diodes called zener diodes that are designed to be used in this breakdown voltage region – provided there is some in circuit resistance to prevent excessive current flow.


Any reversed biased diode will have a small amount of leakage current. The amount of leakage current depends on the material the diode is constructed of and also the temperature. An increase in operating temperature will result in a corresponding increase in leakage current. In a general purpose type silicon diode (1N4004) at normal operating temperatures, this leakage current is quite small, typically 50 nanoamperes (nA) at a reverse bias voltage of 400VDC. Germanium diodes generally exhibit much greater leakage currents.


A semiconductor PN junction (diode) as designed has a very large resistance to current flow in one direction and a very low resistance to current flow in the other direction. This is what makes semiconductor materials different from ordinary conductors or insulators.

These PN junctions form the basis for the semiconductor components that ultimately control the many different electronic devices we use today. This includes two layer components such as diodes, three layer components such as bipolar transistors and even four layer components like silicon controlled rectifiers and triacs. Integrated circuits can contain millions of transistors made from PN junctions that are grown on a single silicon substrate.

It is the miniaturization of these PN junctions over time that has led to a revolution in sophisticated hand held electronics that are smaller, lighter, less power hungry and lower in cost than just a short time ago. A trend that continues today and will continue into the future.


The basis for electricity begins with the atom. An atom consists of a nucleus of protons and neutrons surrounded by orbiting electrons. Atoms in their normal state are electrically neutral with the number of electrons equaling the number of protons. The electrons, freed from its parent atom and set into motion is electricity. Generally only the electrons in the outer orbits of the atom are involved.


Another basic fact about electricity is the attraction and repulsion that exists between electrons and protons and is primarily due to the charge associated with each particle. The charge on the electron is called a negative charge and the charge associated with the proton is called a positive charge. An atom that has less than its normal number of electrons will have a positive charge. If there are more than the normal number of electrons, the atom will have a negative charge.

A fundamental law of electricity is that like charges will repel one another and opposite charges will attract one another. Two electrons will repel each other as will two protons. However, an electron and a proton will attract each other. Since most atoms except hydrogen have more than one proton, you might wonder why the nucleus of an atom remains together since the protons would be affected by the positive charges of the other nearby protons. The reason is the strong nuclear force (one of the four basic forces in nature) which keeps the protons and neutrons in the atoms nucleus together.

The first man made electrical phenomenon to be observed was static electricity. Certain substances such as amber will become negatively charged when rubbed with a piece of woolen cloth. This is due to an excess of electrons on that object. In a similar manner, a glass rod, when rubbed with silk, will gain a positive charge. In these changes, nothing but a rearrangement of electrons has taken place. Electric charges are being moved from one substance to another. However, none are destroyed – a principle that is known as the “conservation of charge”.

Charles Coulomb

In the years 1785 and 1786 a French physicist named Charles Coulomb described in precise mathematical terms how these positive and negative charges repel and attract each other. Coulomb’s law reveals that attraction and repulsion weaken very rapidly with distance and increase just as rapidly as the charged particles get closer together.

The ease in which electrons can move in a substance is determined by how tightly held or how free the electrons are in the outer orbits of its atoms. Certain materials such as glass, mica and most plastics have their electrons and protons closely associated with each other in a manner that inhibits the movement of electrons through that substance. These substances are known as insulators or dielectrics. Other materials, mostly metals, permit the movement of electrons and are known as conductors.


Voltage is the electrical pressure that drives electric current through a circuit and is measured in volts. A higher number of volts means more electrical pressure and a lower number of volts means less electrical pressure. In general, as the voltage in a circuit is lowered, the current flow will also decrease.


If two equally and oppositely charged objects are connected by a metallic conductor such as a wire, the charges will neutralize each other. This occurs when the electrons move through the conductor from the negatively charged object to the positively charged object. This is commonly referred to as electron current flow.

In some cases electrical current can be thought of as moving from positive to negative and is referred to as conventional current flow or when describing semiconductors, as “hole current”.

In an electric circuit, there is a continuous movement of electrons from negative to positive. If this electron movement is in one direction only, the flow is referred to as direct current (dc). If the flow alternates in either direction, the movement is referred to as alternating current (ac).

There are three interdependent quantities that affect the flow of electricity in a circuit. The first is the potential difference in the circuit referred to as electromotive force (emf) or voltage. The second is the rate or quantity of current flow and is referred to as the ampere. One ampere is equal to 6,280,000,000,000,000,000 electrons per second past any point in the circuit. The third quantity is the resistance of the circuit. Under ordinary conditions conductors will offer some opposition to the flow of electrical current and this resistance will limit current flow. The unit used to define the quantity of resistance is called the ohm.

When an electrical current flows through a conductor, two important effects can be observed. The first effect is the temperature of the wire is raised. This is the result of electrons moving from a higher potential to a lower potential, giving up heat in the process.

Hans Christian Oersted

The second effect is the creation of a magnetic field. In the year 1807, a Dane named Hans Christian Oersted began a series of experiments involving electricity. He reasoned that if electric current was allowed to flow through a wire, it should turn the wire into a kind of magnet with the property of north and south poles. In one of his experiments, he placed a wire with current flowing in it parallel with a compass needle. He discovered that the compass needle moved, swinging around a quarter of a turn until it was positioned precisely at right angles to the wire. Electric current did, indeed, create a magnetic field around a wire.

Andre Marie Ampere

Around the year 1808, a French physicist by the name of Andre Marie Ampere discovered that one electrical wire can produce a magnetic effect upon another electrical wire next to it. When an electrical current flows through two nearby parallel conductors, the conductors will be attracted to each other if the current is flowing in the same direction and will repel one another if the current flow is in opposite directions.

The next question to be answered: could a magnetic field create an electric current?

Joseph Henry

In 1830, Joseph Henry in America and, a year later, Michael Faraday in London discovered that a magnetic field could indeed induce a current – provided the magnetic field was kept in motion.

Michael Faraday

Electromagnetic induction occurs when a conductor is moved through the lines of force in a magnetic field. The magnetic field will act on the free electrons in the conductor, displacing them and creating a potential difference and a flow of current in the conductor. Therefore, electromagnetic induction will occur when either the conductor is moved in a stationary magnetic field or a magnetic field is moved near a stationary conductor.


Electromagnetic waves are radiations that require no material medium for propagation and travel in the form of a transverse wave at the speed of light in a vacuum. Such waves consist of oscillating electric and magnetic fields of force that are of widely varying frequencies.

The electromagnetic spectrum extends from the low frequency (50/60Hz) emissions from power lines to the ultra-high frequency waves associated with cosmic rays. The spectrum is divided into a number of overlapping frequency bands. Some of the frequency bands generally recognized are known as radio waves, infrared radiation, visible light, ultraviolet radiation, x-rays and gamma rays. All the components present in the electromagnetic spectrum regardless of frequency have in common the typical properties of wave motion including interference and diffraction.

James Clerk Maxwell

The theory of electromagnetic waves originated with the Scottish physicist James Clerk Maxwell. In a series of papers published in the 1860’s, he analyzed mathematically the theory of electromagnetic fields and was correct when he predicted that visible light was an electromagnetic phenomenon.

The wave nature of light was already well known before Maxwell published his theory. However, physicists had originally assumed that the waves were mechanical in nature and required some form of medium for propagation, similar to sound waves which require air or another medium in order to propagate. Since experiments indicated that light could move easily through a vacuum, the nature of this medium or “ether” as it was called had to be extremely diffuse, present even in the best laboratory vacuum.

Maxwell’s theory of electromagnetic waves indicated that the wave motion was not of a material substance, but of oscillations of both electric and magnetic fields of the same frequency and in phase with each other and also at right angles to each other and to the direction of propagation.

Maxwell had showed that a pulsating electric current would create a pulsating magnetic field in the space around it, and that this magnetic field would create another electric field and so on in a self propagating manner. This chain of electric and magnetic disturbances would flow across space until it contacted another piece of matter where it would generate an electric current similar to the one that started the process.

Maxwell did not directly dismiss the concept of ether, however, his theory made it clear that such a medium for the transmission of electromagnetic waves was not necessary. The concept of ether was eventually discarded with the release of Einstein’s Special Theory of Relativity. (Electromagnetic Spectrum Chart)

Electromagnetic Wave Propagation



When a conductor is moved back and forth in a magnetic field, the flow of current will change directions as often as the motion of the conductor changes direction. This is the basic idea behind the electrical generator which powers our modern electrical grid. (Electrical Grid Diagram)

The most important reason for the use of alternating current is that the voltage of the current can be easily changed to almost any value by the means of a simple electromagnetic device called a transformer. (Transformer Diagram)

When an alternating current is passed through a coil of wire, the magnetic field that surrounds the coil expands and collapses and then expands in a field of opposite polarity and then again collapses. If another conductor or coil is placed in the magnetic field of the first, but not in direct connection with it, the movement of the magnetic field induces an alternating current in the second coil.

If the second coil has a greater number of turns than the first, the voltage induced in the second coil will be greater than the first because the magnetic field is acting upon a greater number of individual conductors. Conversely, if the number of turns in the second coil is less than the primary, the induced voltage in the secondary will be lower than in the primary.

This transformer action is what allows the transmission of electrical power over long distances. By transmitting electricity at higher voltages, say at 200,000 volts instead of 2000 volts, the resistance of the transmission lines will have a smaller effect on the amount of power delivered to the load.

Nikola Tesla

No discussion of alternating current or our modern power grid would be complete without mentioning Nikola Tesla. Born in July of 1856 in Smiljan, Austria – Hungary, Tesla was a brilliant electrical engineer and inventor. He was educated at the Polytechnic School in Graz, Austria and also at the University of Prague. After working for several years as an electrical engineer for a telephone company, he immigrated in 1884 to the United States, where he later became a naturalized citizen.

Tesla was briefly employed by the American inventor Thomas A. Edison in New York. He later abandoned that position to devote himself to experimental research and invention. This was about the time the difference in opinion over Edison’s use of direct current and the advantages in the use of alternating current began.

In 1888, Tesla designed the first practical system for generating and transmitting alternating current (A.C.) for electrical power and subsequently held numerous U.S. patents for his A.C. system. The system consisted of a polyphase electrical generator, a set of step-up and step-down transformers and a polyphase electric motor. American inventor George Westinghouse purchased the patents and the rights to Tesla’s A.C. electrical system after a visit to Tesla’s laboratory. Tesla and Westinghouse demonstrated the A.C. system for the first time at the Chicago World’s Fair in 1893. Tesla then went on to design the first hydroelectric power plant at Niagara Falls in 1895. With this accomplishment, an alternating current power grid was soon destined to become the world standard for electrical power distribution.

Tesla is also well known for his experiments with high voltage, high frequency electricity. In 1899 Tesla built a giant coil (well known today as a “Tesla Coil”) in Colorado Springs, Colorado that could transmit electrical energy for miles without the use of wires. It could also generate sparks more than 30 feet long. He used this device to wirelessly light a string of lamps that were 25 miles away.

Tesla will always be recognized as one of the outstanding pioneers in the field of electric power. He was also active in many other scientific areas as well with over 700 patents registered worldwide. One area that he was a leading pioneer in was the basic system for radio transmission: describing the elements of a radio transmitter and also constructing a device that could receive radio waves.

Over the years Tesla received numerous awards including honorary doctoral degrees from Columbia University and Yale University. In 1931, at the age of 75, Tesla appeared on the cover of Time Magazine. Tesla’s numerous discoveries are the basis for many of the electrical devices we take for granted today and these devices will continue to be with us far into the future.

Tesla died in New York in 1943 at the age of 86. The state of New York and other states as well proclaim July 10, his birthday, as Nikola Tesla Day.

Note: The photo below was actually created with a double exposure.




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