Semiconductors

What are Semiconductors?

Semiconductors are a separate class of electrical materials. They are neither conductors nor insulators.

One can draw an analogy between semiconductors and oxygen-producing plants. Plants emit oxygen to sustain life on earth. Without them, there would neither be any animal species nor any humans. In regard to the techonology of modern society, without the use of semiconductors there would neither be any home appliances nor any industrial machinery.

Semiconductors are therefore at the very heart of our technology. Although we do not usually have direct contact with them, it is hard to imagine everyday life without them.

Why are Semiconductors Important?

Why are Semiconductors Important?

What will you learn?

Basic Concepts

The basic electrical terms are:

  • Electric charge (Q)
  • Electric current (I)
  • Voltage(U)

Electric charge is the fundamental unit of matter that is responsible for the electric phenomenon. The charge is expressed in coulombs (C).

Electric current is the rate of flow of the electric charge. The current is measured in amperes (A). One ampere is 1 coulomb per second.

Voltage describes the energy that is required to force the charge to flow. It is measured in volts (V).

Electric Circuit

An electric circuit (or electric network) is an interconnection of electrical elements in a serial order to form a closed path.

Three well known electrical elements are: a battery, a bulb, and a switch. A simple electric circuit is created by connecting these three elements with a wire.
To flow continuously, the current will require the following preconditions:

  • a steady supply of free (mobile) charges,
  • a closed path,
  • and a voltage difference that forces the charge to flow.

Should any of these preconditions not be met anymore, the circuit will be interrupted and there will be no more current.

Conductivity

Conductivity is the ability of a material to conduct an electric current .

Conductors, such as metals, have high values of conductivity. They permit a current to flow relatively unrestricted.

Insulators, in contrast, have low values of conductivity. Materials such as wood, plastic and glass are very good electric insulators. They will stop currents from flowing.

Semiconductors like silicon, on the other hand, have medium values of conductivity at room temperature. Therfore they have been named semiconductors.

The electric characteristics of a material directly depend on the underlying atomic structure.

Atomic Structure

Each atom consists of a nucleus and of electrons.

Electrons circle in orbits around the nucleus analog to planets around the sun.

A nucleus consists of positively charged protons and neutral particles called neutrons.

While the nucleus has a positive charge its surrounding electrons have negative charges. As a whole, however, the atom is altogether neutral.

Silicon Atom Structure

Let us have a closer look at the structure of a silicon (Si) atom which is shown in this figure.

The nucleus of a silicon atom has 14 protons and 14 neutrons.

Because it has 14 Protons the silicon atom also has 14 electrons which are distributed in three different orbits.

The most outer orbit is referred to as the valence orbit. It determines the electrical properties of the atom. In the valence orbit all electrons are called valence electrons. The silicon atom itself has four valence electrons.

The inner orbits and the nucleus are, on the other hand, referred to as the core of the atom. Normally atoms are illustrated with two concentric circles - an internal one which indicates the core and an external one which indicates the valence orbit. Thus the core of a silicon atom consists of a nucleus and 10 electrons that are distributed in the atom's two inner orbits.

Energy Levels

Electrons can only have discrete values of energy, which are called energy levels (E1, E2, E3). An electron will always have an energy level that is proportionate to the size of its orbit. Extra energy is accordingly needed to move an electron to an outer orbit with a higher energy level.

In an analog situation, people in this antique theater could only occupy the seats in rows that have vacancies. Usually the rows that are close to the stage are occupied first. Later on, free seats will only be found in the outer rows. But in order to move to the back seats, one will need an extra amount of energy for climbing the stairs. In a similar manner an electron too will need an extra supply of energy for moving into an outer orbit. Thus an outer orbit is equal to an higher energy level.

Crystals

A crystal is a solid material whose atoms are arranged in an orderly pattern.

This pattern is called a crystal lattice.

When atoms are combined to form a crystal, each of them is connected with strong bonds to its neighboring atoms.

All semiconductor materials are crystals and they derive their electrical properties directly from their crystal structure.

Silicon Crystal

In a silicon crystal, each atom shares its four valence electrons with its neighboring atoms. These shared electrons form covalent bonds, which hold the entire atoms together in one structure.

On the top right we have a two-dimensional schematic representation illustrating the covalent bonds of a silicon crystal lattice.

Energy Bands

Atoms are very densely packed in crystals and strongly influence one another. Under these conditions, the electron energy levels of atoms, which are normally isolated, are transformed into energy bands with prohibited gaps between them (left figure).

The two highest energy bands - the so-called valence and conduction bands (right figure) - are very important for the conduction process. The gap between these bands is referred to as the bandgap energy Eg. Its size is an important parameter for every semiconductor material.

Energy Bands - Free Electrons

In order to be excited into the conduction band, valence electrons require an outside energy supply that is greater than the existing bandgap energy Eg .

Therefore, the greater the material bandgap energy is the more energy will be required to transport valence electrons to the conduction band.

In a crystal structure, electrons that are positioned in the conduction band are referred to as free electrons because they no longer belong to any atom.

Applying even the slightest voltage will cause these free electrons to flow through the crystal and constitute a current.

The following pages will focus on how free electrons can be created and on how they constitute a current in different types of semiconductors.

Semiconductor Types

There are two types of semiconductors:

  • Intrinsic - if the material contains no impurities
  • Impure or extrinsic - if impurity atoms are introduced in the crystal

The concentration of impurity atoms greatly influences the electrical behavior of semiconductors. By precisely controlling the level of impurity, manufacturers of semiconductors are able to produce devices with specific parameters.

Widely used semiconductor materials are silicon (Si), gallium arsenide (GaAs) and germanium (Ge).

Intrinsic Semiconductors

A typical example for an intrinsic semiconductor is a silicon crystal that only contains silicon atoms.

As already mentioned, atoms in semiconductor crystals are tied together by covalent bonds.

However, under certain conditions these covalent bonds can become a source for free charge carriers that are essential for electrical conductivity in intrinsic semiconductors.

Concentration of Carriers in Intrinsic Semiconductors

The concentration of carriers in intrinsic semiconductors is strongly dependent on temperature.

At zero Kelvin ( -273 oC) all covalent bonds will be complete. There will be no free carriers at this temperature level and the semiconductor will have the characteristics of an insulator.

However, should the temperature rise, then the atoms will increasingly begin to vibrate.
Along with the rising temperature, the number of electrons breaking away from their covalent bonds will grow at an exponential rate. Thus the conductivity of intrinsic semiconductors will also increase with rising temperatures.

Thermal energy produces holes and free electrons always in pairs. This process is referred to as thermo generation. Regardless of the temperature level, the concentration of free electrons ni will always be equal the concentration of holes pi. In the notation the subscript i indicates that the material is intrinsic.

Creation of Carriers- Break-up of Bonds

To enable conductivity, some of the valence electrons must gain enough energy to break away from their covalent bonds.

Heat from the surrounding air is the primary source for the required energy.

This heat energy will cause the atoms in the crystal to vibrate. At a sufficiently high temperature level, the vibrations will be able to dislodge electrons from their valence orbits, which will result in the covalent bonds breaking up.

Whenever an electron leaves a covalent bond and becomes a free electron, there will be a vacancy for a new bond. This vacancy is called a hole and it has a positive charge.

Free electrons and holes are often referred to as carriers because they carry a charge.

Creation of Carriers - Energy Required

The bandgap energy Eg is the minimum amount of energy necessary to break up a covalent bond and to excite an electron from the valence band to the conduction band.

Note that equal numbers of free electrons and holes are created during this process.

The creation of free electrons and holes is similar to a car leaving the above queue. To move a car out of the queue will take a certain amount of energy. But once the car has been pulled out, it will be able to move freely (like a free electron), and there will be a vacancy in the queue (similar to a hole).

Flow of Carriers

A voltage applied to the crystal will force free electrons and holes to move in opposite directions.

Free electrons are able to move relatively fast through crystals.

Holes, on the other hand, move slower than electrons by drifting from one covalent bond to another.

Free electrons and holes are often referred to as mobile carriers.

Note that the movement of one hole requires the movement of many more valence electrons (gray circles). In the above animation the movement of one valence electron to the left is relating to the movement of several holes to the right.

Flow of Carriers - Analogy with Cars

A free electron moves like the fast van in the upper lane - unobstructed by any obstacles.

The hole's movement, however, is comparable to the movement of the gap between the taxi cabs below.

If a cab (valence electron) moves on to fill the empty space ahead, a new gap (equivalent to a hole) will appear behind the car. In the above animation the next cab will follow suit and so forth thus causing the gap to move to the right while the cars are moving to the left. This is almost the same as immediately moving the original gap itself to the end of the queue. The only difference is that moving the cabs one by one, which is equivalent to the movement of holes, will always be a lengthier process.

Conduction Process

If no voltage is applied to the semiconductor, the mobile carriers will move at random.

This means that there will be no net charge transfer per unit of time and therefore no current.

If voltage is applied, it will force all free electrons and holes to flow in one direction, which will result in an electric current.

A current in a semiconductor is made up by two components - an electron current In and a hole current Ip.

Conduction Process - Current Direction

The direction of the hole's flow is opposite to that of the free electrons.

The electric current as such is conventionally represented by the flow of positive charges.

According to this convention, the direction of the electric current runs from the left to the right (as shown in the picture), even though the electrons themselves actually move from the right to the left.

While moving, electrons and holes will be scattered due to their collisions with the atoms of the crystal lattice. This scattering will tend to distract the carriers from direction of their movement. Consequently, free electrons and holes will move with an average velocity in the electric field that is referred to as drift velocity.

Impure Semiconductors

Impure semiconductors have some of the atoms in their crystals replaced with impurity atoms.

Impurities can already in very low concentrations to have a great effect on a semiconductor's electrical properties.

A typical sample of impure silicon material will have an extremely small number of introduced impurity atoms in comparison to its number of silicon atoms.

Introducing impurities is called doping. Impurities that are used for doping are in general materials with either three or five valence electrons.

Disadvantages of Intrinsic Semiconductors

When it comes to producing semiconductor devices, however, intrinsic semiconductors are of little use. This is due to their following disadvantages:

  • At room temperature (25 oC) a silicon crystal almost behaves like an insulator, because it will only have a few free electrons and holes that are produced by thermal energy.
  • Since the number of charge carriers strongly depends on the temperature, the current will also be sensitive to temperature changes in the surrounding atmosphere and therefore vary.

n-Type Semiconductor

An impurity atom with five valence electrons is called a donor, because it will donate an extra electron to the silicon crystal.

In semiconductors with donor impurities, the concentration of free electrons n is much greater than the concentration of holes p. Consequently, the free electrons are called majority carriers whereas the holes are referred to as minority carriers.

Since most of the carriers are negatively charged, the material of a semiconductor that has been doped in this manner is called an n-type..

Donor types can be among others phosphorus (P), arsenic (As), and antimonial (Sb).

Creation of Carriers in n-Type Semiconductors

If an impurity donor atom is introduced into a silicon crystal, four of its valence electrons will enter into covalent bonds with their surrounding silicon atoms.

The fifth valence electron, however, will not participate in any covalent bond; for this electron is only very lightly attracted to the atom. Therefore it will become a free electron, even at very low temperatures. For every bond of one impurity donor atom with its surrounding silicon atoms one free electron will be created, but without a hole.

Whenever a neutral donor atom looses one electron, it will become a positively charged ion. This process is called donor ionization. However, ions cannot participate in the conduction process because they are fixed to the crystal lattice.
But once the temperature level becomes high enough, several of the covalent bonds themselves will break up and add pairs of free electrons and holes beside the free electrons from the impurity atoms.

Carriers Concentration in n-Type Semiconductor

At room temperature essentially all donor atoms will release their free electrons. Yet only few covalent bonds will be broken.

Therefore the number of electrons created through donor ionization will far exceed the number of mobile carriers that have been created through the break-up of covalent bonds.

Since each donor atom creates only one free electron, the concentration of electrons n will equal the concentration of added donor atoms ND. The more impurities that are introduced, the greater will be the conductivity of the material.

All donor atoms will be ionized in the operational temperature range (-55 to +125oC) of semiconductor devices. The concentration of donor electrons is therefore insensitive to any changes in the operational temperature.

p-Type Semiconductor

An impurity atom with only three valence electrons is called an acceptor.

It will create one hole in the crystal structure and therefore tend to accept one valence electron.

A semiconductor is called a p-type if its number of holes (the positively charged carriers) far exceeds the number of free electrons. Here the holes are the majority carriers and the free electrons the minority carriers.

Examples for acceptors are boron (B), aluminum (Al) and gallium (Ga).

Creation of Carriers in p-Type Semiconductor

If an acceptor atom is introduced into a silicon crystal, it will only have three valence electrons that will join with their neighboring silicon atoms to form covalent bonds.

Therefore the possibility of one additional covalent bond (the fourth covalent bond) will remain open and constitute a hole.

Valence electrons from adjacent atoms are already at very low temperatures able to fill this vacancy. This process is referred to as acceptor ionization; for once a neutrally charged acceptor gains an extra electron it will become a fixed negatively charged ion.

As soon as the temperature should rise far enough, further covalent bonds in the silicon structure will be able to break apart and create many pairs of free electrons and holes.

Concentration of Carriers in p-Type Semiconductor

Essentially all acceptor atoms are ionized at room temperature but only a few covalent bonds in the structure are broken.

Therefore the concentration of holes created by acceptor ionization will far exceed the concentration of mobile carriers that are generated through covalent bonds.

Since each acceptor atom produces one hole, the number of holes p will equal the number of added acceptor atoms NA. The more impurities that are introduced, the greater will be the conductivity of the material.

All acceptor atoms are ionized in the semiconductor device's operational temperature range which runs from -55 to +125oC. Within this range the concentration of holes is insensitive to any temperature changes.

The Conduction Process in Impurity Semiconductors

n-Type Semiconductor

Introduced donor impurities will create free electrons and provide a current.

In n-type semiconductors, electrons are the majority carriers. Here the value of the electron current In will by far exceed the value of the hole current Ip.

If a voltage should be applied, the electrons will move toward the positive potential (in the illustration above this will be to the left).

However, a number of holes will also flow. But, by moving as minority carriers into the opposite direction, they will be in very small numbers and almost have no effect on the circuit.

Conduction Process in Impurity Semiconductors

p-Type Semiconductor

Introducing acceptor impurities will also provide a current, but with holes.

With the introduction of acceptor impurities, majority carriers (holes) of the p-type material will move toward the negative potential (which is to the right). Likewise, there will be a negligible flow of minority carriers running in the opposite direction. These minority carriers, which are electrons, will also have almost no effect on the circuit.

In p-type semiconductors the hole current Ip will by far exceed the electron current In.

For both types of impure semiconductors, majority carriers will be the main component of the current.

Advantages of Impure Semiconductors

  • The conductivity of impure semiconductors depends on the concentration level of their impurities in the material. At room temperature relatively few majority carriers should be enough to constitute a current.
  • Impure semiconductors have electrical characteristics that can be precisely controlled. For impurities can be deliberately introduced into every crystal during the manufacturing process.
  • The concentration of majority carriers is insensitive to temperature changes.

Therefore impure semiconductors are almost exclusively used for the production of modern semiconductor devices.