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Introduction to Semiconductor Material and Devices. The Valence Band.
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Introduction to Semiconductor Material and Devices
The Valence Band The valence band is the band made up of the occupied molecular orbitals and is lower in energy than the so-called conduction band. It is generally completely full in semi-conductors When heated, electrons from this band jump out of the band across the band gap and into the conduction band, making the material conductive.
Conduction Band • The conduction band is the band of orbitals that are high in energy and are generally empty. In reference to conductivity in semiconductors, it is the band that accepts the electrons from the valence band.
Silicon Lattice • Silicon atoms form covalent bonds and can crystallize into a regular lattice. The illustration below is a simplified sketch; the actual crystal structure of silicon is a diamond lattice. • This crystal is called an intrinsic semiconductor and can conduct a small amount of current. Pure silicon is not capable to conduct current very well.
Intrinsic Semiconductor • 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.
Intrinsic Semiconductor • 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 that electrons can reach the conduction band and contribute to electrical conduction.
The term intrinsic here distinguishes between the properties of pure "intrinsic" silicon and the dramatically different properties of doped n-type or p-type semiconductors.
Valence Electrons • The electrons in the outermost shell of an atom are called valence electrons; they dictate the nature of the chemical reactions of the atom and largely determine the electrical nature of solid matter. • The electrical properties of matter are pictured in terms of how much energy it takes to free a valence electron.
Germanium and Silicon • In solid state electronics, either pure silicon or germanium may be used as the intrinsic semiconductor which forms the starting point for fabrication. Each has four valence electrons, but germanium at a given temperature have more free electrons and a higher conductivity. Silicon is by far the more widely used semiconductor for electronics, partly because it can be used at much higher temperatures than germanium. • Germanium is temperature sensitive and Silicon is temperature efficient. • Silicon is most widely used material for fabrication because of its characteristics.
Doping • The property of semiconductors that makes them most useful for constructing electronic devices is that their conductivity may easily be modified by introducing impurities into their crystal. The process of adding controlled impurities to a semiconductor is known as doping. The amount of impurity, or dopant, added to an intrinsic (pure) semiconductor varies its level of conductivity. Doped semiconductors are often referred to as extrinsic semiconductor.
The Doping of Semiconductors The addition of a small percentage of foreign atoms in the regular crystal Lattice of pure silicon or germanium produces dramatic changes in their electrical properties, producing: n-type and p-type semiconductors.
Pentavalent impurities:Impurity atoms with 5valence electrons produce n-type semiconductors by contributing extra electrons.Trivalent impurities: Impurity atoms with 3valence electrons produce p-type semiconductors by producing a "hole" or an electron deficiency.
N-Type Semiconductor 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.
P-Type Semiconductor 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.
Bands for Doped Semiconductors • The application of band theory to n-type and p-type semiconductors shows that extra levels have been added by the impurities. In n-type material there are electron energy levels near the top of the band gap so that they can be easily excited into the conduction band. In p-type material, extra holes in the band gap allow excitation of valence band electrons, leaving mobile holes in the valence band.
Doping is the process of add impurities to intrinsic semiconductors to alter their properties. Normally Trivalent and Pentavalent elements are used to dope Silicon and Germanium. When a intrinsic semiconductor is doped with Trivalent impurity it becomes a P-Type semiconductors. The P stands for Positive, which means the semiconductor is rich in holes or Positive charged ions.
When we dope intrinsic material with Pentavalent impurities we get N-Type semiconductor, where N stands for Negative. N-type semiconductors have Negative charged ions or in other words have excess electrons.
Now lets see what will happen when we pop in a pentavalent element into the lattice. As you can see the image (Figure : N-type), we have doped the silicon lattice with Phosphorous, a pentavalent element. Now pentavalent element has five electrons, so it shares a electron with each of the four neighboring silicon atoms, hence four atoms are tied up with the silicon atoms in the lattice.
This leaves an electron extra. This excess electron is free to move and is responsible conduction. • Hence N-type (Negative Type) extrinsic semiconductor (silicon in this case) is made by doping the semiconductor with pentavalent element.
P-Type • To create a P-type semiconductor, all we must do is to pop in a trivalent element into the lattice. A trivalent element has three electrons in its valence shell. It shares three electrons with three neighboring silicon atoms in the lattice, the fourth silicon atom demands an electron but the trivalent atom has no more electron to share.
P-Type • This creates a void in lattice which we call it has hole. Since the electron is deficient, the hole readily accepts an electron, this makes it a P-type (Positive type) extrinsic semiconductor. • As you can see at image (Figure: P-type), we have doped in boron (trivalent element) in silicon lattice. This has created a hole making the semiconductor a P-type material.
The case is no different in Germanium. It behaves same as silicon how ever some properties do differ which makes germanium based devices used in certain application and silicon based devices used in other applications.
Fermi Level • Fermi Level is the energy level at which an average of 50% of the available quantum states are filled by an electron. It was named after the famous physicist Enrico Fermi, who had a significant hand in developing the atom bomb. The Fermi Level relates the probable location of electrons in a band diagram.
Fermi Level • If you are looking at a band diagram of a substance (usually a semiconductor) the Fermi Level tells you where the averageelectron is. For metals the Fermi Level lies in the conduction band and for insulators the Fermi Level lies in the valence band and for semiconductors the Fermi Level lies in the band gap.
Fermi Level • Semiconductors are unique because the Fermi Level lies in the band gap which cannot contain electrons. This, however, doesn't prevent the statistical location of the Fermi Level lying in the band gap.
Fermi level • Energy ^ | Conduction Band | | ----------------------- | | Band | - - - - - - - - Fermi Level | Gap | | | ----------------------- | O O O O O O O O O O O O | Valence Band \ | \ electrons |------------------> x direction
Fermi Level • Note how the valence band is full of electrons and there are relatively few electrons in the conduction band, placing the Fermi Level right in the band gap. It is worth mentioning that when a piece of semiconductor (or any substance) is at equilibrium, with no net current or applied field, then the Fermi Level will be flat.
