- Over 600 semiconducting materials are known. They may be elements or compounds and they have a resistivity somewhere between insulators and conductors. Good conductors have resistivity between 10-7Ωm to 10-8 Ωm at room temperature while the resistivity of insulators is in the range1010 Ωm and 1014 Ωm. Semiconductors fall in between with resistivities between 10-6Ωm to 107Ωm, a range of 14 orders of magnitude.
- Pure semiconductors behave like insulators at 0°Kelvin, however at normal temperatures, in contrast to metals, semiconductors have a negative coefficient of resistance due to the increase in the concentration of charge carriers as the temperature rises as explained below.
- The properties which make semiconductors interesting are
- Controlled amounts of other elements, misleadingly called “impurities” can be inserted into the crystal lattice of the semiconductor in a process known as “doping” to modify its electrical properties by creating positive or negative charge carriers, selectively increasing its conductivity in the doped region.
- The electrical behaviour of the doped semiconductor can be varied by the the effects of various external stimuli such as heat, light and electric or magnetic fields.
- By doping with precise amounts of different impurities in precise locations very small structures can be built in which the positive and negative charge carriers can interact allowing the creation of a wide range of passive and active electronic devices which in turn can be used as building blocks to create even more complex components.
- The Bohr model of the atom tells us that electrons can only have certain discrete amounts of energy corresponding to certain electron orbits around the atomic nucleus. The electrons have a minimum energy state and only certain discrete higher energy states are allowed. A fixed amount of energy is needed to pump the electron up into a higher energy state and when the electron falls back into a lower energy state that energy is given up as electromagnetic radiation. The highest filled band is called the valence band. The next higher band is the conduction band, which is separated from the valence band by an energy gap, also called a band gap. This band gap represents the energy required to knock electrons out of atoms into the conduction band. In any material, for conduction to occur, there must be electrons available in the highest energy band. The energy required for an electron to jump the band gap can be provided by heat or some form of radiation or from an electric field and is usually expressed in electron Volts (eV) where 1 eV is equivalent to 1.6 X 10-19 Joules (J).
- In a metal the minimum energy needed to liberate an electron from the surface of the metal is called the “work function”.(Note that the band gap is the difference between two energy levels, not a physical space.)
- Good conductors already have free electrons in the conduction band which are available to carry the current and a small band gap which makes it easy to pump more electrons into the conduction band. Metals have a positive coefficient of resistance since the thermal agitation of the electrons increases with temperature and impedes electron flow.
- Insulators on the other hand have a wide band gap typically higher than 5 eV (electron Volts) with no electrons in the conduction band. Conduction can occur if a high enough field is used to force electrons into the conduction band but this usually results in the breakdown of the insulating material.
- Pure semiconductors have few electrons in the conduction band making them poor conductors but their relatively low band gap (0.67 eV for Germanium and 1.12 eV for Silicon) permits their conductivity to be increased by using an external stimulus to raise the energy level of the electrons.
- When this occurs the higher energy electron it breaks free from the covalent bond between the semiconductor atoms creating an electron-hole pair of new charge carriers thus increasing the conductivity of the semiconductor material.
- Adding small numbers of new current carriers by doping allows dramatic changes to the conductivity of the semiconductor.
The unique physical properties of certain materials we now recognise as semiconductors have been known for over a hundred years but it was only in the 1940’s that materials were classified as such and their properties were explained. Some examples of semiconductors frequently used in electronics are given below.
- Lead Sulphide crystals (Galena) were noted for their rectifying properties.
- Selenium was first used in the 1870’s to create photo voltaic cells.
- Silicon carbide crystals(carborundum) were the crystals used with the “cats whisker” to detect the signals in early radio sets.
- Germanium was the material on which early transistors were based. It has a low melting point and is easy to work with and a low resistivity which helps in achieving high frequency response. Unfortunately it also has a low maximum working temperature of 75 °C and suffers from inherent high leakage currents due to its low band gap of only 0.67 eV.
- Silicon is the material used in the vast majority of semiconductor devices today. Manufacturing processes are more difficult but it can work at much higher operating temperatures and suffers less from leakage currents. Silicon is readily oxidised to form silicon dioxide (SiO2) which is an insulator, essential to the planar manufacturing process. (The oxide of Germanium is water soluble making it unsuitable for this purpose). Silicon dioxide is the compound from which Silicon is derived. It is the most abundant compound in the earth’s crust and commonly takes the form of ordinary sand, but also exists as quartz, rock crystal, amethyst, agate, flint, jasper and opal.
- Gallium Arsenide has high electron mobility giving it a much better high speed switching performance than Silicon. It also has better high temperature performance but it is hard to work with and consequently more expensive. Gallium Arsenide is also used in the manufacture of LED’s. Gallium by the way is a three valence element while Arsenic is a five valence element. Whereas most semiconductors are four valence elements or compounds, Gallium Arsenide is known as a III-V valence semiconductor. Its crystal structure however permits doping in the same way as a IV valence semiconductor. Gallium Arsenide is considered to be highly toxic and carcinogenic .
Over 200 device structures have been identified and the principles of operation can be illustrated from some basic types shown here. The list is not exhaustive and many variations on these themes are possible.
Note: It must be emphasised that the following explanations are merely a shorthand way of representing the major semiconductor behaviours, sufficient to understand the device functions and the tradeoffs and performance limitations involved in semiconductor device designs. Some minor effects have been omitted. Furthermore, the actual mechanisms involved in the functioning of semiconductor devices are much more complex and are based on quantum physics taking into account the energy levels of the charge carriers and their concentrations and distributions within the semiconductor crystal lattice.
Electrons and Holes
Semiconductor materials such as Germanium or Silicon are characterized by a valence shell containing four electrons. Each atom has four electrons in its outer orbit and shares these electrons with its four neighbours. Each shared electron pair constitutes a covalent bond. The force of attraction for the electrons by both nuclei holds the two atoms together forming tight crystal structures with no free electrons available to conduct a current. Pure Silicon crystals are therefore insulators. These materials can however be doped to create an excess or lack of electrons, turning the semiconductor into a conductor. Pure semiconductor material without doping is called an intrinsic semiconductor. Heavy doping provides low resistivity materials. Light doping is used for controlled high resistivity. See diagram below.
N type materials
If small quantities of a different element such as Phosphorus or Arsenic which have five valence electrons in its outer ring are introduced into the crystal lattice of a semiconductor such as Silicon, four of the five electrons will bind with the Silicon atoms leaving one electron unbound and free to move. Semiconductors doped in this way have a surplus of free electrons, or negative charge carriers, available to carry current and are consequently called N type materials. In N type materials the electrons are said to the majority carriers and holes the minority carriers.
P type materials
In a similar way semiconductors can be doped with elements such as Boron or Indium with only three valence electrons in its outer ring. In this case there will be a shortage of electrons to bind with the Silicon lattice leaving unfilled spaces known as “holes”. Electrons can propagate through the crystal like a bubble by filling the neighbouring vacant hole leaving behind a hole in the location it just left. This is equivalent to the propagation of a hole in the opposite direction. Since a hole is essentially the lack of an electron in a neutral material, it must have a net positive charge. Materials with such positive charge carriers are called P type materials. In P type materials the holes are said to be the majority carriers and electrons the minority carriers.
In the example above, the Phosphorus atoms create a surplus of electrons in the N type material and the Boron atoms create a surplus of holes or positive carriers in the P type material. Each of these materials standing alone is electrically neutral. However, when a single crystal is doped in the form of a junction, with N type material on one side and P type material on the other side the electrons are attracted by the holes on the other side of the junction and migrate across to bond with them. This leaves the N type material with a net positive charge and the P type material with a net negative charge. The result is that a permanent inherent or “sweeper” electric field is created across the junction. (This region where electrons have diffused across the junction is called the “depletion region” because it no longer contains any mobile charge carriers. It is also known as the “space charge region”.)
Almost all semiconductor devices depend in some way on the operation of the P-N junction. The simplest of these devices is the diode and we can use the diode to illustrate what happens at the P-N junction.
When the diode is forward biased the applied electric field across the diode causes the negative charge carriers (electrons) to move across the junction towards the positive terminal and the positive charge carriers (holes) likewise move the other way across the junction towards the negative terminal. When the electrons and holes reach eachother they recombine resulting in a current flow through the diode.
Although the electrons and holes flow in opposite directions, the current only flows one way because the charge carriers have opposite polarities.
When the diode is reverse biased the applied electric field is in the opposite direction and the mobile charge carriers must also move in the opposite direction. Electrons still move towards the positive terminal and holes towards the negative terminal but this time it is in a direction away from the junction, depleting it of mobile charge carriers in its immediate vicinity and creating a barrier to the flow of further electric current. This narrow region across the junction, devoid of charge carriers is called the space charge layer. Reverse biasing the diode also sets up back voltage across the junction or depletion layer with a surplus of electrons on the negative side of the layer and a surplus of holes on the positive side so that the space charge layer acts like a charged capacitor.
There will however always be a small, troublesome, temperature dependent leakage current across the junction due to the liberation of new charge carriers caused by thermal effects on the semiconductor material whichever way the diode is biased.
LED’s, solar cells, lasers and tunnel diodes are among the many versions of the P-N junction.
A varactor diode is a variable reactance device. Increasing the reverse bias voltage on a P-N junction forces the charges on either side of the depletion or space charge layer further apart effectively increasing its capacitance. Conversely reducing the bias reduces the capacitance. This property makes the varactor diode ideal for use as a variable capacitor in tuned circuits. Being reverse biased no current flows.
Schottky diodes use a metal-semiconductor junction instead of a P-N junction as in conventional diodes. It is electrically similar to a P-N junction, but the current flow in the diode is due primarily to “majority carriers” which means that if the semiconductor body is doped N-type, then the current is carried by only the N-type carriers (mobile electrons ). No slow, random recombination of electrons and holes is involved. This results in both very fast switching times and low forward voltage drop.
This same effect however results in poor reverse-bias voltage performance and high reverse leakage currents.
Schottky metal-semiconductor contacts can also be used to provide non rectifying, ohmic contacts with a negligible resistance, regardless of the polarity of the applied voltage, to make connections to other devices in an electronic system.
A P-I-N diode is a P-N junction with a wide intrinsic (undoped) I layer sandwiched between the P and N layers. The intrinsic layer acts as an insulator providing high reverse breakdown voltage, high power handling and a low capacitance. At low levels of reverse bias the depletion layer becomes fully depleted and once fully depleted the diode capacitance is independent of the level of bias since there is little remaining net charge in the intrinsic layer.
When the diode is forward biased both types of current carrier are injected into the intrinsic layer where they combine enabling the current to flow across the insulating layer. The resistance value of the PIN diode is determined only by the forward biased DC current which makes it useful as a distortion free variable resistor at RF and microwave frequencies for fast switching and attenuator applications
The avalanche effect is caused by the breakdown of the P-N junction due to a high reverse voltage field across the depletion layer. As the reverse voltage is increased the very high field across the depletion layer accelerates any charge carriers randomly generated by heat in the layer. In doing so the charge carriers pick up enough energy to raise the energy levels of more electrons when they collide with the crystal lattice giving rise to even more electron-hole pairs creating a self sustaining avalanche effect and a low impedance across the diode. Removing the voltage turns off the current.
Zener diodes are very similar to avalanche diodes (above) but they do not depend on the acceleration of the charge carriers randomly generated by heat in the depletion layer. Rather the high electric field directly breaks down the bonds in the crystal lattice to create electron-hole pairs. Practical devices need heavy doping and a very thin space charge layer which is too thin to accelerate the thermally generated charge carriers to sufficient energy to start the avalanching process. Zener diodes are thus less temperature dependent than avalanche diodes. By controlling the doping levels the onset of the avalanche effect can be made to occur at different voltages making the Zener diode suitable as a voltage reference device.
Like avalanche and Zener diodes, tunnel diodes are heavily doped and have an extremely narrow depletion or space charge layer of less than 5 to 10 nanometers thick – only a few atoms deep. Similarly they are in breakdown when reverse biased however unlike the other two devices they remain in breakdown for a small initial region of forward biasing, with the breakdown current superimposed on the normal diode forward current.
As the forward voltage increases the diode slowly comes out of breakdown and the total current consequently decreases until breakdown stops and only the normal diode forward current flows, whereupon the current starts to increase again.
The possibility of unusual applications and very high switching speeds led to great expectations for the tunnel diode but difficulties in manufacturing and advances in other semiconductor technologies have now made it almost obsolete.
Gunn Diode – Transferred Electron Device (TED)
An Gunn diode has a voltage current characteristic similar to the tunnel diode (above) but depends on a completely different principle. It has no P-N junction but consists only of N type semiconductor material with different doping concentrations in three distinct regions. The two outer regions connected to the terminals are heavily N doped and a thin layer of lightly doped material is sandwiched in between.
When a voltage is applied to the device, the electrical gradient will be largest across the thin middle layer because it is less heavily doped and thus has the highest resistance. Eventually, this layer will start to conduct due to the charge carriers created by the high electric field. The presence of the charge carriers however reduces its resistance and hence the gradient across it, thus preventing further conduction. In practice, this means a Gunn diode has a region of negative differential resistance. Once the current pulse has passed through the middle layer, its resistance of the layer and hence the voltage gradient across it rises again allowing conduction to occur once more.
IMPact Avalanche Transit Time (IMPATT) diodes are two-terminal semiconductor devices that generate RF power by introducing a 180° phase shift between current and voltage waveforms at microwave frequencies.
IMPATT diodes can produce very high power at microwave frequencies but because they depend on the avalanche process they are hampered by the high level of phase noise they generate.
Light Emitting Diode (LED)
LED’s depend for their effect on the recombination of hole-pairs and the reforming of the covalent bonds in a forward biased diode. When an electron falls back to its unexcited energy level it loses its excess energy which is emitted as a photon of light. The energy in the photon and hence the colour of the light depends on the band gap of the semiconductor. Different materials with different band gaps are needed to produce different colours. This light energy is transmitted out from the device through the sides of the junction region. The light intensity depends on the rate of recombination of the charge carriers and thus the forward current.
Not all semiconductors are suitable for making LED’s – the conduction band must be directly above the valence band for photon emission. For this reason Gallium Arsenide is a suitable candidate but not Silicon.
The core of a laser diode is a P-N junction creating spontaneous emission of photons due to the recombination of electron hole pairs in the same way as in an LED described above. Spontaneous emission is necessary to initiate laser oscillation, but it creates light with random phase and polarisation and is a source of inefficiency once the laser is oscillating.
Laser operation depends on stimulated emission of photons, rather than spontaneous emission which occurs when an atom in a high energy, or excited, state can return to the lower state spontaneously.
Stimulated emission occurs when a photon of light interacts with the excited atom, causing it to return to its lower state. One photon interacting in this way with an excited atom results in two photons being emitted giving rise to an optical gain. Furthermore, the two emitted photons are in phase resulting in a fixed phase relationship between light radiated from different atoms enabling the production of a monochromatic and coherent light output. The output is further enhanced by coupling the laser to some form of optical resonator usually by means of a pair of optically flat and parallel mirrors at the ends of the junction created by cleaving and polishing the semiconductor crystal. Photons are reflected back and forth between the mirrors stimulating further emissions while in transit thus increasing the light output. If the round trip distance between the mirrors is an integral number of wavelengths the optical wave will be reinforced.
Photovoltaic Diode (Solar Cell)
- Photodiodes, also known as PV cells or solar cells, generate an electric current when light energy of sufficient magnitude impinges on the semiconductor lattice near to a P-N junction.
- If the photon energy in the light beam is less than the band gap, the energy is simply dissipated as heat and no electrons are released into the conduction band and no current flows.
- If however the energy level of the photons is equal to, or higher than, the band gap of the semiconductor material, it will cause the covalent bonds in the semiconductor to be broken as electrons jump the band gap into the conduction band.
- Both the electron and the vacant site left behind by the electron in the valence band (the hole) then act as free charge carriers and contribute to the possible current.
- Once a photon has caused the release of an electron, any photon energy it had in excess of the band gap energy will be dissipated in the form of heat.
- Photons will thus pass through the crystal lattice until they are absorbed as heat or until they give up their energy by causing the generation of electron hole pairs and the release of an electron across the band gap.
- In the absence of an electrical field both the electron and the hole move about until they find each other and recombine. An important requirement for the functioning of the photovoltaic cell is the existence of an internal electrical field that will drive the photo-excited charge carriers into the external circuit before they recombine.
- The “sweeper field” in the PN junction noted above provides the necessary field which causes the charge carriers to flow across the junction giving rise to a current if an external circuit is connected across the junction. Electrons will flow in the external circuit until the charge carriers causing the field are depleted.
- If charge carriers are replenished due to continuous illumination as in a PV cell, then a continuous current will flow. The current which flows is directly related to the rate at which the photons are absorbed into the semiconductor lattice and is thus proportional to light intensity.
- The cell output voltage follows the band gap voltage and is typically around 0.5 Volts.