Electron physics,Atomic structure,Electrons and electric currents,Structure of matter,Motion of an electron in a magnetic field,Structure of matter and Impurity semiconductors

Electron physics

All matter consists of molecules, which are defined as the smallest portion of a substance capable of independent existence and having the properties of the substance. Studies by Dalton and others in the early part of the nineteenth century showed that molecules consist of groupings of various types of atoms. These atoms relate to the basic elements of which all matter is constructed. There are over I 00 elements, from hydrogen (the lightest) to uranium (one of the heaviest).

A molecule of table salt, for example, consists of one atom of sodium and one atom of chlorine. A molecule of copper sulphate consists of one atom of copper, one atom of sulphur and four atoms of oxygen.

Atoms are far too small to be observed directly with a micro­ scope, but their existence can be inferred by experiments.

Atomic structure

Experimental work on gas discharge effects suggested that an atom is not a single entity but is itself composed of smaller particles. These were termed elementary particles. The atom appeared as a small solar system with a heavy nucleus composed of positive particles and neutral particles. These were named protons and neutrons. Around this nucleus, clouds of negatively charged particles, called electrons, circle.

As an atom is electrically neutral, the negative charge carried by the electrons must be equal in magnitude (but opposite in sign) to the positive charge carried by the protons. Experiments with electrostatic charges show that unlike charges attract, so it can be considered that electrostatic forces hold an atom together.

The difference between various atoms is therefore determined by

their composition. A hydrogen atom consists of one proton and one electron; a helium atom of two protons, two neutrons and two electrons.

Electron physics

All matter consists of molecules, which are defined as the smallest portion of a substance capable of independent existence and having the properties of the substance. Studies by Dalton and others in the early part of the nineteenth century showed that molecules consist of groupings of various types of atoms. These atoms relate to the basic elements of which all matter is constructed. There are over I 00 elements, from hydrogen (the lightest) to uranium (one of the heaviest).

A molecule of table salt, for example, consists of one atom of sodium and one atom of chlorine. A molecule of copper sulphate consists of one atom of copper, one atom of sulphur and four atoms of oxygen.

Atoms are far too small to be observed directly with a micro­ scope, but their existence can be inferred by experiments.

Atomic structure

Experimental work on gas discharge effects suggested that an atom is not a single entity but is itself composed of smaller particles. These were termed elementary particles. The atom appeared as a small solar system with a heavy nucleus composed of positive particles and neutral particles. These were named protons and neutrons. Around this nucleus, clouds of negatively charged particles, called electrons, circle.

As an atom is electrically neutral, the negative charge carried by the electrons must be equal in magnitude (but opposite in sign) to the positive charge carried by the protons. Experiments with electrostatic charges show that unlike charges attract, so it can be considered that electrostatic forces hold an atom together.

The difference between various atoms is therefore determined by their composition. A hydrogen atom consists of one proton and one electron; a helium atom of two protons, two neutrons and two electrons.

Work by Bohr and others in the early part of the present century demonstrated that the electron orbits are arranged in shells, and that each shell has a defined maximum number of electrons it can contain. The first shell can contain two electrons, the second eight electrons. The number in each shell is given by:

2n2, where n =I, 2, 3 and so on.

Chemical reaction and electrical effects are all concerned with the behaviour of electrons in the outer shell of any particular atom. If a shell is full, for example, the atom is unable to react with any other atom and is, in fact, one of the inert gases such as helium.

Electrons and electric currents

If there are few electrons in the outermost shell, the forces binding them to the nucleus are weak. Thermal effects easily detach these electrons, leaving a positively charged atom. These detached electrons drift around inside the substance until they meet another positively charged atom, at which they become captured again. The process of free electron production and recapture is going on continuously, and the substance can be considered as being permeated with a negatively charged gas.

If an electrical potential is now applied across the substance, the free electrons will start to accelerate towards the positive connection. As they move they will collide with atoms in the substance, releasing energy which we observe as heat. The net effect is a drift of electrons at a roughly constant speed towards the positive connection. The motion of electrons is an electric current.

As electrons are removed by the electrical potential source at the positive connection, electrons are being injected at the negative connection. The potential can be considered as a form of electron 'pump'.

This model explains many observed effects. If the magnitude of electrical potential increases, the electrons accelerate faster and their mean velocity is higher i.e. the current increases. The collisions between electrons and atoms transfer energy to the atoms which manifests itself as heat. This effect is known as Joule heating.

Materials such as these are termed ohmic conductors, as they obey the well-known Ohm's law;

The constant is the resistance of the material. If V is in volts, and I is in amps, the constant (R) is in ohms.

Not all electrical conduction is ohmic; heating and other effects cause some materials to have complex VII relationships.

If electrons in the outer orbit are tightly bound, negligible amounts of free electrons are formed. If an electric potential is applied, very few electrons move and the current is small. Sub­ stances with these characteristics are called insulators.

Work by Bohr and others in the early part of the present century demonstrated that the electron orbits are arranged in shells, and that each shell has a defined maximum number of electrons it can contain. The first shell can contain two electrons, the second eight electrons. The number in each shell is given by:

2n2, where n =I, 2, 3 and so on.

Chemical reaction and electrical effects are all concerned with the behaviour of electrons in the outer shell of any particular atom. If a shell is full, for example, the atom is unable to react with any other atom and is, in fact, one of the inert gases such as helium.

Electrons and electric currents

If there are few electrons in the outermost shell, the forces binding them to the nucleus are weak. Thermal effects easily detach these electrons, leaving a positively charged atom. These detached electrons drift around inside the substance until they meet another positively charged atom, at which they become captured again. The process of free electron production and recapture is going on continuously, and the substance can be considered as being permeated with a negatively charged gas.

If an electrical potential is now applied across the substance, the free electrons will start to accelerate towards the positive connection. As they move they will collide with atoms in the substance, releasing energy which we observe as heat. The net effect is a drift of electrons at a roughly constant speed towards the positive connection. The motion of electrons is an electric current.

As electrons are removed by the electrical potential source at the positive connection, electrons are being injected at the negative connection. The potential can be considered as a form of electron 'pump'.

This model explains many observed effects. If the magnitude of electrical potential increases, the electrons accelerate faster and their mean velocity is higher i.e. the current increases. The collisions between electrons and atoms transfer energy to the atoms which manifests itself as heat. This effect is known as Joule heating.

Materials such as these are termed ohmic conductors, as they obey the well-known Ohm's law;

The constant is the resistance of the material. If V is in volts, and I is in amps, the constant (R) is in ohms.

Not all electrical conduction is ohmic; heating and other effects cause some materials to have complex VII relationships.

If electrons in the outer orbit are tightly bound, negligible amounts of free electrons are formed. If an electric potential is applied, very few electrons move and the current is small. Sub­ stances with these characteristics are called insulators.

Motion of electron in an electric field

If an electric potential is applied between two plates in a vacuum, and an electron in introduced, the electron experiences an attractive force to the positive plate.

This force causes the electron to accelerate towards the positive plate in a straight line. It suffers no collisions because the area between the plates is in vacuo. This effect is used in thermionic valves.

If the electron is given some motion and the electron field is applied perpendicular to the motion, interesting effects occur. In the system below, a beam of electrons is emitted from a device called an electron gun. These electrons are moving in the x direction. As they emerge they pass between two plates which have a potential applied across them in the y direction.

As the electrons pass between the plates they are accelerated in the y direction, as explained before, but their velocity in the x direction is unaltered. The electron beam is thus deflected as shown. By varying the potential applied to the plates, the angle of deflection can be controlled. This effect is the basis of the cathode ray oscilloscope.

Motion of an electron in a magnetic field

A moving electron is effectively an electric current. Experiments with electric motors demonstrate that magnetic fields exert a force on wires carrying current, and similar effects may be expected to occur with moving electrons.

Direction of the force can be predicted from Fleming's left hand rule. An electron experiences a force when moving perpendicular to a magnetic field. This force is at right angles to both the field and the direction of the electron's motion. It follows that electrons moving parallel to a magnetic field are unaffected.

There is one important difference between the motion of an electron in a magnetic field and its motion in an electric field. In an electric field the force is a fixed direction, whereas in a magnetic field the force is always at right angles to the electron's motion.

It follows that an electron injected into a suitable magnetic field can be made to spiral along the field axis. This effect is used in magnetic focusing coils in a television tube.

Structure of matter

Matter can exist in three states; solid, liquid and gaseous. In the liquid and gaseous states molecules can move around freely. In the solid state, however, molecules are fixed and can only vibrate about their mean positions. These vibrations we interpret as heat.

There are several substances which are observed to form crystals:

table salt and copper sulphate are two common examples. Crystals form because the atoms arrange themselves into a geometrical pattern, and this pattern continues however large the crystal.

At the atomic level, however, atoms in most substances are

arranged in a crystalline pattern. A representation of the crystalline structure of germanium is shown, and the regular pattern is obvious.

Motion of an electron in a magnetic field

A moving electron is effectively an electric current. Experiments with electric motors demonstrate that magnetic fields exert a force on wires carrying current, and similar effects may be expected to occur with moving electrons.

Direction of the force can be predicted from Fleming's left hand rule. An electron experiences a force when moving perpendicular to a magnetic field. This force is at right angles to both the field and the direction of the electron's motion. It follows that electrons moving parallel to a magnetic field are unaffected.

There is one important difference between the motion of an electron in a magnetic field and its motion in an electric field. In an electric field the force is a fixed direction, whereas in a magnetic field the force is always at right angles to the electron's motion.

It follows that an electron injected into a suitable magnetic field can be made to spiral along the field axis. This effect is used in magnetic focusing coils in a television tube.

Structure of matter

Matter can exist in three states; solid, liquid and gaseous. In the liquid and gaseous states molecules can move around freely. In the solid state, however, molecules are fixed and can only vibrate about their mean positions. These vibrations we interpret as heat.

There are several substances which are observed to form crystals:

table salt and copper sulphate are two common examples. Crystals form because the atoms arrange themselves into a geometrical pattern, and this pattern continues however large the crystal.

At the atomic level, however, atoms in most substances are arranged in a crystalline pattern. A representation of the crystalline structure of germanium is shown, and the regular pattern is obvious.

The vast majority of electronic devices depend on conduction in solids. The ability of a substance to conduct electricity depends on its ability to produce free electrons, as we saw earlier.

The elements silicon and germanium both have four electrons in their outer orbit. This results in a tight-knit diamond-type crystal. Because of the tight bond, pure crystals of germanium and silicon are fairly good insulators.

Impurity semiconductors

Although pure crystals of silicon and germanium are fairly good insulators, their conductivity can be dramatically changed by the addition of small amounts of impurities.

Shown is a version of germanium crystal structure, with one atom of germanium replaced by one atom of arsenic. This is called doping. Arsenic has five electrons in this outer shell, and although it will 'sit' in the crystal it has one electron free from bonding.

The vast majority of electronic devices depend on conduction in solids. The ability of a substance to conduct electricity depends on its ability to produce free electrons, as we saw earlier.

The elements silicon and germanium both have four electrons in their outer orbit. This results in a tight-knit diamond-type crystal. Because of the tight bond, pure crystals of germanium and silicon are fairly good insulators.

Impurity semiconductors

Although pure crystals of silicon and germanium are fairly good insulators, their conductivity can be dramatically changed by the addition of small amounts of impurities.

Shown is a version of germanium crystal structure, with one atom of germanium replaced by one atom of arsenic. This is called doping. Arsenic has five electrons in this outer shell, and although it will 'sit' in the crystal it has one electron free from bonding.

Representation of n-type semiconductor

The surplus electrons are very mobile, and can easily become current carriers when a voltage is applied across the substance. The amount of doping can be controlled easily to control the amount of free electrons available. The impurities are known as donor atoms, and the substance as an impurity semiconductor.

Conduction in the substance is by free electrons, and the substance is called an n-type semiconductor (with n standing for negative).

If an atom with three electrons in its outer shell (e.g. boron, indium) is introduced, a similar effect occurs. The arrangement is shown below. The deficiency of electrons forms a 'hole' in the structure, and the corresponding unbonded electron is free to become a current carrier. If a voltage is applied to the substance, the electrons moves towards the positive connection, and the holes apparently move in the opposite direction. It is usual to consider the moving hole as the current carrier. This type of material is known as a p-type semiconductor (with p standing for positive) and the impurities are known as acceptor atoms.

As well as the current carriers formed in the semiconductor by the impurities, there are also current carriers formed by normal thermal action releasing free electrons. These latter current carriers obviously exist as complementary electrons/holes, and are known as minority carriers. The current carriers introduced by the impurities are known as majority carriers.