TOPIC 5: ELECTRONIC
The Concept of Energy Band in Solids
Explain the concept of energy bands in solids
In solid-state physics, the electronic band structure (or simply band structure) of a solid describes those ranges of energy that an electron within the solid may have (called energy bands, allowed bands, or simply bands) and ranges of energy that it may not have (called band gaps or forbidden bands).
Band theory derives these bands and band gaps by examining the allowed quantum mechanical wave functions for an electron in a large, periodic lattice of atoms or molecules. Band theory has been successfully used to explain many physical properties of solids, such as electrical resistivity and optical absorption, and forms the foundation of the understanding of all solid-state devices (transistors, solar cells, etc.).
Difference between Conductors, Semiconductors and Insulators
Distinguish between conductors, semiconductors and insulators
An electrical insulator is a material whose internal electric charges do not flow freely, and therefore make it impossible to conduct an electric current under the influence of an electric field. This contrasts with other materials, semiconductors and conductors, which conduct electric current more easily.
The property that distinguishes an insulator is its resistivity; insulators have higher resistivity than semiconductors or conductors.
A perfect insulator does not exist, because even insulators contain small numbers of mobile charges (charge carriers) which can carry current. In addition, all insulators become electrically conductive when a sufficiently large voltage is applied that the electric field tears electrons away from the atoms. This is known as the breakdown voltage of an insulator.
Some materials such as glass, paper and Teflon, which have high resistivity, are very good electrical insulators. A much larger class of materials, even though they may have lower bulk resistivity, are still good enough to prevent significant current from flowing at normally used voltages, and thus are employed as insulation for electrical wiring and cables. Examples include rubber-like polymers and most plastics.
A conductor is an object or type of material that allows the flow of electrical current in one or more directions. For example, a wire is an electrical conductor that can carry electricity along its length.
In metals such as copper or aluminum, the movable charged particles are electrons. Positive charges may also be mobile, such as the cationic electrolyte(s) of a battery, or the mobile protons of the proton conductor of a fuel cell. Insulators are non-conducting materials with few mobile charges and support only insignificant electric currents.
A semiconductor material has an electrical conductivity value falling between that of a conductor, such as copper, and an insulator, such as glass. Semiconductors are the foundation of modern electronics. Semiconducting materials exist in two types: elemental materials andcompound materials.
The modern understanding of the properties of a semiconductor relies on quantum physics to explain the movement of electrons and holes in a crystal lattice. The unique arrangement of the crystal lattice makes silicon and germanium the most commonly used elements in the preparation of semiconducting materials.
An increased knowledge of semiconductor materials and fabrication processes has made possible continuing increases in the complexity and speed of microprocessors and memory devices. Some of the information on this page may be outdated within a year because new discoveries are made in the field frequently.
Examples of semiconductors are Silicon, Germanium.
The Effects of Temperature on the Conductivity of Conductors, Semiconductors and Insulators
Describe the effect of temperature on the conductivity of conductors, semiconductors and insulators
The conductivity of pure defect free metal decreases with increase in temperature. With increased temperature in a metal, thermal energy causes atoms in metal to vibrate, in this excited state atoms interact with and scatter electrons.
Thus decreasing the mean free path, and hence the mobility of electrons too decreases, and resistivity increases.
Since, resistivity = 1/conductivity
The electrical conductivity of a semiconductor will increase exponentially with an increase in temperature, as temperature increases the electrons in the valance band will gain energy and go into the higher energy levels in the conduction band where they become charge carriers.
The increase in conduction can also be explained, I guess,due to the formation of Cooper pairs and hence the creation of Phonon field.
Types of Semiconductors
Identify types of Semiconductors
There are two types of semiconductors
- Intrinsic semiconductors
- Extrinsic semiconductors
An intrinsic semiconductor material is chemically very pure and possesses poor conductivity. It has equal numbers of negative carriers (electrons) and positive carriers (holes). Examples are Silicon and Germanium.
A silicon crystal is different from an insulator because at any temperature above absolute zero temperature, there is a finite probability that an electron in the lattice will be knocked loose from its position, leaving behind an electron deficiency called a "hole."
If a voltage is applied, then both the electron and the hole can contribute to a small current flow.The conductivity of a semiconductor can be modeled in terms of the band theory of solids.
The band model of a semiconductor suggests that at ordinary temperatures there is a finite possibility thatelectrons can reach the conduction band and contribute to electrical conduction. The term intrinsic heredistinguishes between the properties of pure "intrinsic" silicon and the dramatically different properties ofdoped n-type or p-type semiconductors.
The current flow in an intrinsic semiconductor is influenced by the density of energy states which in turn influencesthe electron density in the conduction band. This current is highly temperature dependent. The electrical conductivityof intrinsic semiconductors increase with increasing temperature.
Extrinsic semiconductor is an improved intrinsic semiconductor with a small amount of impurities added by a process,known as doping, which alters the electrical properties of the semiconductor and improves its conductivity.
Introducing impurities into the semiconductor materials (doping process) can control their conductivity.Doping process produces two groups of semiconductors:
- The negative charge conductor (n-type).
- The positive charge conductor (p-type).
Semiconductors are available as either elements or compounds. Silicon and Germanium are the most common elemental semiconductors. Compound Semiconductors include InSb, InAs, GaP, GaSb, GaAs, SiC, GaN. Si and Ge both have a crystalline structure called the diamond lattice. That is, each atom has its four nearest neighbors at the corners of a regular tetrahedron with the atom itself being at the center.
In addition to the pure element semiconductors, many alloys and compounds are semiconductors.The advantage of compound semiconductor is that they provide the device engineer with a wide range of energy gaps and mobilities, so that materials are available with properties that meet specific requirements. Some of these semiconductors are therefore called wide band gap semiconductors.
The Mechanism of Doping Intrinsic Semiconductors
Describe the mechanism of doping intrinsic semiconductors
The addition of a small percentage of foreign atoms in the pure crystal lattice of silicon or germanium produces dramatic changes in their electrical properties, producing n-type and p-type semiconductors.
The addition of pentavalent impurities such as antimony,arsenic or phosphorous contributes free electrons, greatly increasing the conductivity of the intrinsic semiconductor. Phosphorous may be added by diffusion of phosphine gas (PH3).(5 valence electrons) produce n-type semiconductors by contributing extra electrons.
(3 valence electrons) produce p-type semiconductors by producing a "hole" or electron deficiency.
The addition of pentavalent impurities such as antimony, arsenic or phosphorous contributes free electrons,greatly increasing the conductivity of the intrinsic semiconductor. Phosphorous may be added by diffusion of phosphine gas (PH3).
The addition of trivalent impurities such as boron, aluminum or gallium to an intrinsic semiconductor creates deficiencies of valence electrons,called "holes". It is typical to use B2H6 diborane gas to diffuse boron into the silicon material.
P-n junctions are formed by joining n-type and p-type semiconductor materials.
Since the n-type region has a high electron concentration and the p type a high hole concentration, electrons diffuse from the n-type side to the p-type side. Similarly, holes flow by diffusion from the p-type side to the n-type side.
If the electrons and holes were not charged, this diffusion process would continue until the concentration of electrons and holes on the two sides were the same, as happens if two gasses come into contact with each other. However, in a p-n junction, when the electrons and holes move to the other side of the junction, they leave behind exposed charges on dopant atom sites, which are fixed in the crystal lattice and are unable to move.
On the n-type side, positive ion cores are exposed. On the p-type side, negative ion cores are exposed. An electric field Ê forms between the positive ion cores in the n-type material and negative ion cores in the p-type material. This region is called the "depletion region" since the electric field quickly sweeps free carriers out, hence the region is depleted of free carriers.
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