Semiconductor

Subject: Semiconductor Physics Insulators: A material or an object that does not easily allow heat, electricity, light, or sound to pass through it. Air, cloth and rubber are good electrical insulators; feathers and wool make good thermal insulators. Compare conductor. Some common insulators are glass, air, plastic, rubber, and wood. Conductor: A conductor is an object or type of material that allows the flow of an electrical current in one or more directions. A metal wire is a common electrical conductor. Some common conductors are copper, aluminum, gold, and silver. Semiconductors: A solid substance that has a conductivity between that of an insulator and that of most metals, either due to the addition of an impurity or because of temperature effects. Devices made of semiconductors, notably silicon, are essential components of most electronic circuits. Materials that have the resistance levels between those of a conductor and an insulator are referred to as semiconductors. They are quite common, found in almost all electronic devices. Good examples of semiconductor materials are germanium, selenium, and silicon. Elemental Semiconductor: Single element semiconductor; element from the group IV of periodic table: Si, Ge, C, Sn. AIII-BV, III-V, semiconductors. III-V semiconductors are synthesized using elements from 3rd and 5th group of periodic table; e.g. GaAs, GaP, GaN, GaAlAs, InP, InSb, etc. Compound Semiconductor: In contrast to a semiconductor composed of a single element, one composed of two or more elements is called a compound semiconductor. Typical examples of compound semiconductors include gallium arsenide (GaAs), gallium nitride (GaN), indium phosphide (InP), zinc selenide (ZnSe), and silicon carbide (SiC) Elemental Semiconductor: Silicon Si, germanium Ge, and diamond C are important group IV elemental semiconductors. These group IV elemental materials all of them have diamond crystal structure. Another group IV elemental semiconductor having such a structure is alpha tin a-Sn, which is also referred as gray Sn. Other elemental structures differing from diamond structure include group III element boron (Rhombohedral), group V material phosphorus, and group VI materials such as sulphur S, selenium Se, and tellurium Te. Currently silicon is the most important semiconductor material used in electronic devices. Some of the important advantages of silicon Si over other semiconductors are • A relative ease of passivating the surface by oxidizing in a controlled manner forming a layer of stable native oxide that substantially reduces the surface recombination velocity. • Its hardness that large wafers to be handled safely without damaging it. • It is thermally stable up to 11000 C that allows high-temperature processes like diffusion, oxidation, and annealing. • It is relatively low cost due to established processes. The basic limitations of silicon are the magnitude and type of its energy bandgap. Its energy band-gap is 1.12eV. It is a direct semiconductor that limits the application in optoelectronics, and it has relatively low carrier mobility as compared to other semiconductor such as gallium arsenide GaAs. Emerging materials based on Si nanostructures e.g., Si nanocrystals, quantum wires and dots, and porous Si, and Si- 2 -1-x Ge layers grown on Si substrate, appear to be promising materials in various applications. In nanostructures because of quantum confinement of carriers, it leads to increase of electron hole wave function overlap and hence, it increases photon emission efficiency. There is a high energy shift toward the emission blue peak.x Porous Si can be obtained from the anodic etching of crystalline silicon in aqueous hydrofluoric acid HF. It contains a network of pores and crystallites (microscopic crystal) with sizes in the order of several nanometers. Thismaterial exhibits relatively efficient luminescence, which is several orders of magnitude higher than that in crystalline Si, and it is believed to be related to the quantum confinement effects in nanocrystalline Si. In principle, many semiconductors can be grown on Si substrates. For example, the growth of III-V compounds on silicon substrate is attractive since such heterostructures would enable to integrate optical devices in the III-V compound with silicon circuitry on a monolithic chip. III-V compound semiconductors offer a wide range of applications in optoelectronic devices, whereas silicon offers both a convenient electronic device technology and a large area substrate that is mechanically stronger than, III-V compound like GaAs and also has a larger thermal conductivity. The issues related to how to obtain high quality epitaxial heterostructures like GaAs/Si are: the presence of high dislocation densities due to the lattice constant mismatch between the epitaxial layer and substrate; residual stresses in the epitaxial layers due to the difference in thermal expansion coefficients of the epitaxial layer and substrate, and the formation of structural defects like antiphase boundaries due to the epitaxial growth of a polar crystal in the case of GaAs on a nonpolar substrate like silicon Various approaches are being used to overcome these problems. These include thermal cycle annealing, growth interrupts, selective area growth, and insertion of strained-layer superlattices. Carbon, silicon, germanium and tin are atoms in ascending order of atomic number from column IV A of the period table. Each is characterised by having four valence electrons in its outermost shell of electrons, and requires a further four to make up the full complement of the shell. All can solidify to form elemental, covalently bonded crystals where the four valence electrons of one atom are shared between its four nearest neighbours so that every atom effectively gains eight electrons in its valence shell. A group IV atom and its four nearest neighbours from a tetrahedron as shown in Figure 1. Figure 1: Schematic diagram to show the orientation of covalently bonded group 4 atoms. A tetrahedron is formed by the nearest neighbours, with the principal atom located at its centre. Figure 2: Unit cell of a crystal such as silicon or germanium. Taking a larger scale perspective of the arrangement of the atoms, or crystal lattice, it is found that they organise themselves into two interpenetrating face centred cubic (fcc) sub-lattices, one displaced from the other by 1/4(a0, a0, a0) along a diagonal of the unit cell. a0 is called the lattice constant or lattice parameter and is a measure of the size of the unit cell, often expressed in Ångstrom (Å) units (1Å=0.1nm or 1x10-10m). It is determined by techniques such as X-ray diffractometry. Figure 2 shows a complete unit cell for a group 4 crystal covalently bonded with the diamond structure. This structure is of course difficult to visualise and draw, hence it is usually represented by an equivalent 2-D, "square" arrangement shown in figure 3. Figure 3: 2-D representation of a covalently bonded crystal at 0K, eg Si. Note that the heavy lines between adjacent atoms depict the covalent bonds which contain TWO electrons and are all completely filled. In the example shown, it is assumed that the solid, Si say, is both crystallographically perfect and pure. At 0 K, all the covalent bonds are complete and there are no free charge carriers moving around randomly through the lattice; the crystal is an insulator. Before commenting further on the elemental "semiconductors", it is worth mentioning another group of technologically important solids which possess semiconducting properties to varying degrees, namely the III-V compounds. These are formed when equal numbers of group III and group V elements combine with the same basic arrangement as the group IV elemental solids. The difference lies in the fact that whereas the elemental solids contain only one type of atom such that every atom in the (perfect) lattice is bonded to four identical nearest neighbour atoms, in the III-V compounds a group III atom is bonded to four group V nearest neighbours, and a group V atom is bonded to four group III nearest neighbours. The two interpenetrating fcc sub-lattices now contain either all group III atoms or all group V atoms. Figures 4 and 5 show the 3-D and 2-D representations, respectively, for these materials. Figure 4 : Diagram to show the 3D unit cell of a III-V semiconductor compound (eg. GaAs, gallium arsenide) with the zinc blende lattice. Figure 5 : 2-D representation of a III-V semiconductor. Note the way in which the group III and V alternate through the lattice on their individual fcc sub-lattices. Although the Silicon Devices and Technology 3 course will concentrate on Si, it is important to remember that the electronic band structure of Si (and Ge) makes it unsuitable for certain applications; a prime example is light emitting devices. Most semiconductor lasers and LEDs are made from the III-V materials; it is NOT possible to get efficient light emission from Si. (There is now a great deal of interest world-wide in the II-VI compounds because it has been shown that blue and green LEDs and laser diodes can made from them, but that's yet another story!). There are two types of semiconductors (i)  Intrinsic or pure semiconductor (ii)  Extrinsic or impure semiconductor Intrinsic or pure semiconductor: The conduction band of silicon and germanium is empty and the valence band is fully filled up with electrons at very low temperature. Germanium and silicon have four valence electrons. Each atom of germanium silicon shares one electron with its neighboring atom. Thus covalent bond is made. So, there is no free electron in germanium and silicon. For this reason there is no conduction of electricity in them. Such pure semiconductors are known as intrinsic semiconductors. If pure semiconductors are heated at high temperature due to thermal agitation electrons of pure semiconductors becomes free by breaking the bonds. The electrons can pass forbidden gap if the energy of their electrons is very large and transferred into conduction band. When an electron goes into conduction band from valence band there a vacancy occurs. The vacancy makes a hole and this hole is equal to a positive charge. Due to thermal vibration a bound electron next to a hole can move across to fill the gap. The net motion of the negative charge from a bound position to another being in effect equivalent to the motion of a hole in the opposite direction. This means the transfer of a hole is the transfer of positive charge. A hole is positive charge so flowing of holes mean flowing of current. Holes and electrons acts as charge carriers. Transferring of a hole and electron is shown in a figure. The number of electrons and holes are equal in pure on intrinsic semiconductor. Electron-hole bond pair forms due to thermal agitation. Hence the number of holes and electrons becomes equal in pure semiconductor. (ii) Extrinsic semiconductors or impure: We know that due to thermal agitation charger carrier are produced in pure semiconductor. At normal temperature the charge carrier in pure semiconductors is very small. So the electrical conductivity is also very low in this situation. The small current has no importance practically. It has been found that if a small amount of impurity is added with the pure semiconductor the charge carrier increases. The mixing amount is one part in million. Hence the conductivity of the semiconductor increases. This system of mixing suitable impurity is called doping or doping semiconductor. Semiconductor doping is the process of making impure semiconductor. This type of mixed semiconductor is called extrinsic semiconductor or impure semiconductor. Aluminum, arsenic, antimony, gallium etc impurities are mixed in silicon or germanium. Extrinsic or impure semiconductors are classified into two types, viz n-type and p-type semiconductors. n-type semiconductor means negative type semiconductor and p-type semiconductor means positive type semiconductor. binary semiconductor:. Semiconductor compound consisting of two elements; e.g. SiC, GaAs, CdS. ... ternary semiconductor. semiconductor compound consisting of three elements; e.g. AlGaAs, CdHgTe. The range of possible formulae is quite broad because these elements can form binary (two elements, e.g. gallium(III) arsenide (GaAs)), ternary (three elements, e.g. indium gallium arsenide (InGaAs)) and quaternary (four elements, e.g. aluminium gallium indium phosphide (AlInGaP)) alloys. Bonding in semiconductors: The electrons surrounding each atom in a semiconductor are part of a covalent bond. A covalent bond consists of two atoms "sharing" a single electron. Each atom forms 4 covalent bonds with the 4 surrounding atoms. Therefore, between each atom and its 4 surrounding atoms, 8 electrons are being shared. Valence and conduction bands The valence band and conduction band are the bands closest to the Fermi level and thus determine the electrical conductivity of the solid. The valence band is the highest range of electron energies in which electrons are normally present at absolute zero temperature, while the conduction band is the lowest range of vacant electronic states. On a graph of the electronic band structure of a material, the valence band is located below the Fermi level, while the conduction band is located above it. This distinction is meaningless in metals as the highest band is partially filled, taking on the properties of both the valence and conduction bands. Valence and conduction bands in semiconductors Semiconductor band structure: In solids, the ability of electrons to act as charge carriers depends on the availability of vacant electronic states. This allows the electrons to increase their energy (i.e., accelerate) when an electric field is applied. This condition is only satisfied in the conduction band, since the valence band is full in non-metals. As such, the electrical conductivity of a solid depends on its capability to flow electrons from the valence to the conduction band. Hence, in the case of a semimetal with an overlap region, the electrical conductivity is high. If there is a small band gap (Eg), then the flow of electrons from valence to conduction band is possible only if an external energy (thermal, etc.) is supplied; these groups with small Eg are called semiconductors. If the Eg is sufficiently high, then the flow of electrons from valence to conduction band becomes negligible under normal conditions; these groups are called insulators. There is some conductivity in semiconductors, however. This is due to thermal excitation—some of the electrons get enough energy to jump the band gap in one go. Once they are in the conduction band, they can conduct electricity, as can the hole they left behind in the valence band. The hole is an empty state that allows electrons in the valence band some degree of freedom. Fermi energy and Fermi level: The Fermi energy is a concept in quantum mechanics usually referring to the energy difference between the highest and lowest occupied single-particle states in a quantum system of non-interacting fermions at absolute zero temperature. Fermi Level: "Fermi level" is the term used to describe the top of the collection of electron energy levels at absolute zero temperature. This concept comes from Fermi-Dirac statistics. Electrons are fermions and by the Pauli exclusion principle cannot exist in identical energy states. Semiconductors increase in temperature: It is more complicated for semiconductor. There are basically 3 different regions of interest; undoped or intrinsic semiconductor, lightly doped and heavily doped: 1) Undoped or intrinsic semiconductor; conductivity goes up or resistivity goes down with temperature goes up. 2) lightly doped, up to about 10E20, conductivity goes down or resistivity goes up with temperature goes up. 3) Heavily doped >>10E20, conductivity turns up again or resistivity goes down with temperature goes up. Doped semiconductors: Doping is the process of adding impurities to intrinsic semiconductors to alter their properties. Normally Trivalent and Pentavalent elements are used to dope Silicon and Germanium. When an intrinsic semiconductor is doped with Trivalent impurity it becomes a P-Type semiconductor. P-Type semiconductor: Trivalent element like gallium or aluminum is mixed with pure semiconductor like silicon or germanium to make p type semiconductor. Three valence electrons of aluminum make bond with neighboring electrons of silicon. Here three covalent bonds are made. But the fourth bond is not completed. It is needed one electron to complete the bond. One electron of neibouring bond jumps into the vacancy and make a covalent bond with the vacancy. Now the bond is completed. A positively charge produces with covalent bond. In this way for each impurity atom accepts an election and a positive charge or hole produces in the semiconductor. Thus each aluminum atom has one hole which is eager to accept an electron. For this cause trivalent aluminum impurity atom is called acceptor. In this way impurity atom gets one extra electron and it becomes a negatively charged ion. A large number of holes are produced by the impurity atom. Due to thermal energy some covalent bonds break and electron-hole pairs are made. The number of holes is larger than the electrons. So the holes are called majority carriers and the electrons are called minority carriers. The extrinsic semiconductor is called p type semiconductor because the holes are large in number here. A figure is shown for energy band diagram for p type semiconductor. Additional energy level EA is produce by the acceptor impurity atom in the band gap. This level is very near to valence band EV . The different of energy gap between EA and EV is less than 0.1eV. So electron can go to new level from valence band easily. Thus hole or vacancy is created by going electron from valence band to new level show in the right figure. Electron-hole pair form due to thermal agitation. Some electrons go to conduction band from valence band and creates hole in valence band and thus free electrons in the conduction band. N-Type semiconductor: A pentavalent impurity like arsenic, phosphorous is added with pure semiconductor like silicon or germanium for making n type semiconductor. The most important requirement is that the size of the impurity atom should be nearly equal to the size of the atoms in the pure semiconductor silicon or germanium in this case. At a high temperature arsenic or antimony pentavalent atoms are added by special technology. The main structure of silicon or germanium should not be changed so the amount of impurity is controlled at the time of mixing rather these atoms are incorporated in the crystal lattice. Arsenic or antimony has five valence electrons. Four electron of them makes bond with neighboring silicon or germanium. Fifth electron of them remains free. Every arsenic atom donates a free electron such way. The impurity atom called donor. Here some bonds break for the thermal energy. Which produces electron-hole pairs i.e. equal number of electrons and holes per cubic cm 107 free electrons contains in crystal formed. In n type semiconductor negatively charged electron plays the initial role for electrical conduction. So that electrons are majority carriers and the holes are minority carriers here. Because electrons are large in number, so this type of extrinsic semiconductor is called n type semiconductor. The figure shows the energy band diagram of n type semiconductor. Donor impurity atom makes extra energy level ED. this energy level is very near to the conduction level. Difference between ED and EC is very tiny about 0.1eV. So electrons can go easily to the conduction from this new level. Some extra electrons come from the valence band by breaking covalent band due to thermal agitation. This make equal number of holes in the valence band. Energy bands: Energy bands consisting of a large number of closely spaced energy levels exist in crystalline materials. The bands can be thought of as the collection of the individual energy levels of electrons surrounding each atom. The wavefunctions of the individual electrons, however, overlap with those of electrons confined to neighboring atoms. The Pauli exclusion principle does not allow the electron energy levels to be the same so that one obtains a set of closely spaced energy levels, forming an energy band. The energy band model is crucial to any detailed treatment of semiconductor devices. It provides the framework needed to understand the concept of an energy bandgap and that of conduction in an almost filled band as described by the empty states. In this section, we present the free electron model and the Kronig-Penney model. Then we discuss the energy bands of semiconductors and present a simplified band diagram. We also introduce the concept of holes and the effective mass.  Energy bands of semiconductors: The energy band diagrams of semiconductors are rather complex. The detailed energy band diagrams of germanium, silicon and gallium arsenide . The energy is plotted as a function of the wavenumber, k, along the main crystallographic directions in the crystal, since the band diagram depends on the direction in the crystal. The energy band diagrams contain multiple completely-filled and completely-empty bands. In addition, there are multiple partially-filled band Simple energy band diagram of a semiconductor: The energy band diagrams shown in the previous section are frequently simplified when analyzing semiconductor devices. Since the electronic properties of a semiconductor are dominated by the highest partially empty band and the lowest partially filled band, it is often sufficient to only consider those bands. This leads to a simplified energy band diagram for semiconductors as shown in Figure 2.3.4: A simplified energy band diagram used to describe semiconductors. Shown are the valence and conduction band as indicated by the valence band edge, Ev, and the conduction band edge, Ec. The vacuum level, Evacuum, and the electron affinity, c, are also indicated on the figure. The diagram identifies the almost-empty conduction band by a set of horizontal lines. The bottom line indicates the bottom edge of the conduction band and is labeled Ec. Similarly, the top of the valence band is indicated by a horizontal line labeled Ev. The energy band gap, Eg, is located between the two bands. The distance between the conduction band edge, Ec, and the energy of a free electron outside the crystal (called the vacuum level labeled Evacuum) is quantified by the electron affinity, c multiplied with the electronic charge q. An important feature of an energy band diagram, which is not included on the simplified diagram, is whether the conduction band minimum and the valence band maximum occur at the same value for the wavenumber. If so, the energy bandgap is called direct. If not, the energy bandgap is called indirect. This distinction is of interest for optoelectronic devices since direct bandgap materials provide more efficient absorption and emission of light. For instance, the smallest bandgap of germanium and silicon is indirect, while gallium arsenide has a direct bandgap. Metals, insulators and semiconductors: Once we know the bandstructure of a given material we still need to find out which energy levels are occupied and whether specific bands are empty, partially filled or completely filled. Empty bands do not contain electrons. Therefore, they are not expected to contribute to the electrical conductivity of the material. Partially filled bands do contain electrons as well as available energy levels at slightly higher energies. These unoccupied energy levels enable carriers to gain energy when moving in an applied electric field. Electrons in a partially filled band therefore do contribute to the electrical conductivity of the material. Completely filled bands do contain plenty of electrons but do not contribute to the conductivity of the material. This is because the electrons cannot gain energy since all energy levels are already filled. In order to find the filled and empty bands we must find out how many electrons can be placed in each band and how many electrons are available. Each band is formed due to the splitting of one or more atomic energy levels. Therefore, the minimum number of states in a band equals twice the number of atoms in the material. The reason for the factor of two is that every energy level can contain two electrons with opposite spin. To further simplify the analysis, we assume that only the valence electrons (the electrons in the outer shell) are of interest. The core electrons are tightly bound to the atom and are not allowed to freely move in the material. Four different possible scenarios are shown in Figure Figure Possible energy band diagrams of a crystal. Shown are: a) a half filled band, b) two overlapping bands, c) an almost full band separated by a small bandgap from an almost empty band and d) a full band and an empty band separated by a large bandgap. A half-filled band is shown in Figure a). This situation occurs in materials consisting of atoms, which contain only one valence electron per atom. Most highly conducting metals including copper, gold and silver satisfy this condition. Materials consisting of atoms that contain two valence electrons can still be highly conducting if the resulting filled band overlaps with an empty band. This scenario is shown in b). No conduction is expected for scenario d) where a completely filled band is separated from the next higher empty band by a larger energy gap. Such materials behave as insulators. Finally, scenario c) depicts the situation in a semiconductor. The completely filled band is now close enough to the next higher empty band that electrons can make it into the next higher band. This yields an almost full band below an almost empty band. We will call the almost full band the valence band since it is occupied by valence electrons. The almost empty band will be called the conduction band, as electrons are free to move in this band and contribute to the conduction of the material.