Chapter 1 Boylestad

• semiconductor materials fall into one of two classes: single-crystal and compound.
• Semiconductors are a special class of elements having a conductivity between that of a good conductor and that of an insulator.
• Single-crystal semiconductors such as germanium (Ge) and silicon (Si) have a repetitive crystal structure.
• whereas compound semiconductors such as gallium arsenide (GaAs), cadmium sulfide (CdS), gallium nitride (GaN), and gallium arsenide phosphide (GaAsP) are constructed of two or more semiconductor materials of different atomic structures.
• The three semiconductors used most frequently in the construction of electronic devices are Ge, Si, and GaAs.
• Germanium


  was used almost exclusively because it was relatively easy to find and was available in fairly large quantities.
• Germanium


  It was also relatively easy to refine to obtain very high levels of purity, an important aspect in the fabrication process.
• Diode and transistors constructed using germanium as the base material suffered from low levels of reliability due primarily to its sensitivity to changes in temperature
• silicon less temperature sensitive, but it is one of the most abundant materials on earth, removing any concerns about availability.
• the development of the first GaAs transistor in the early 1970s. This new transistor had speeds of operation up to five times that of Si.
• GaAs was more difficult to manufacture at high levels of purity, was more expensive
• silicon has 14 orbiting electrons
• germanium has 32 electrons
• gallium has 31 electrons
• arsenic has 33 orbiting electrons
• Atoms that have four valence electrons are called tetravalent , those with three are called trivalent , and those with five are called pentavalent
• Covalent bonds are also called molecular bonds. Sharing of bonding pairs will ensure that the atoms achieve stability in their outer shell
• This bonding of atoms, strengthened by the sharing of electrons, is called covalent bonding
• The term free is applied to any electron that has separated from the fixed lattice structure and is very sensitive to any applied electric fields such as established by voltage sources or any difference in potential.
• The external causes include effects such as light energy in the form of photons and thermal energy (heat) from the surrounding medium.
• The term intrinsic is applied to any semiconductor material that has been carefully refined to reduce the number of impurities to a very low level—essentially as pure as can be made available through modern technology.
• The free electrons in a material due only to external causes are referred to as intrinsic carriers
• One such factor is the relative mobility of the free carriers in the material, that is, the ability of the free carriers to move throughout the material
• The ability to change the characteristics of a material through this process is called doping , something that germanium, silicon, and gallium arsenide readily and easily accept
• One important and interesting difference between semiconductors and conductors is their reaction to the application of heat
• For conductors, the resistance increases with an increase in heat. This is because the numbers of carriers in a conductor do not increase significantly with temperature, but their vibration pattern about a relatively fixed location makes it increasingly difficult for a sustained flow of carriers through the material.
• The farther an electron is from the nucleus, the higher is the energy state, and any electron that has left its parent atom has a higher energy state than any electron in the atomic structure.
• The energy gap also reveals which elements are useful in the construction of light-emitting devices such as light-emitting diodes (LEDs)
• the energy gap, the greater is the possibility of energy being released in the form of visible or invisible (infrared) light waves
• A semiconductor material that has been subjected to the doping process is called an extrinsic material.
• There are two extrinsic materials of immeasureable importance to semiconductor device fabrication:


  n -type and p -type materials.
• Both n -type and p -type materials are formed by adding a predetermined number of impurity atoms to a silicon base
• An n -type material is created by introducing impurity elements that have five valence electrons ( pentavalent ), such as antimony , arsenic , and phosphorus.
• Diffused impurities with five valence electrons are called donor atoms
• An electron in the valence band of silicon must absorb more energy than one in the valence band of germanium to become a free carrier. Similarly, an electron in the valence band of gallium arsenide must gain more energy than one in silicon or germanium to enter the conduction band.
• This difference in energy gap requirements reveals the sensitivity of each type of semiconductor to changes in temperature.
• as the temperature of a Ge sample increases, the number of electrons that can pick up thermal energy and enter the conduction band will increase quite rapidly because the energy gap is quite small
• For conductors, the overlapping of valence and conduction bands essentially results in all the additional energy picked up by the electrons being dissipated in the form of heat
• Note that a discrete energy level (called the donor level) appears in the forbidden band with an E g significantly less than that of the intrinsic material.
• Those free electrons due to the added impurity sit at this energy level and have less difficulty absorbing a sufficient measure of thermal energy to move into the conduction band at room temperature.