Fermi Band diagram In MOS Devices

  In the earlier blogs we have seen how fermi energy level shifts in case of intrinsic, extrinsic semiconductor as well as how it acts when we add doping impurity to it. Now we will see how actually band bending takes place in case of metals, oxides and semiconductors. EI = Vacuum energy level Ec= Conduction band energy Efp = Fermi energy level of mos Ev = Valence band energy  Φm = Work function of metal Xs = Electron affinity Eg = band gap energy In the full energy band diagram of MOS ,  the most important point to be noticed are: Continuity Stability The vacuum energy level or EI should be continuous for the complete device. The reason behind this can be explained in the figure below If the band is not continuous and some electron is present just above the vacuum level of metal , it might travel into the oxides conduction band which will disrupt the charge present in oxide. For stability it is must that the fermi energy level is constant through

  Fermi energy band diagrams in PMOS Modes:                   a. Accumulation                   b. Turned off                    c. Depletion                    d. Inversion  a. When a positive voltage is applied at the gate terminal of the p type MOSFET, the opposite charged particles i.e electrons are attracted towards the gate oxide. Due to which there is increase in concentration of majority charge carriers near the oxide. As the doping intensity of n-type particles increase the energy banbends towards the valence band. b. When there is no gate voltage applied , there is no bending in the fermi energy bands. c. When a negative voltage is applied at the gate, electrons are repelled away from the region near the gate oxide - semiconductor interface. Hence the net charge near the gate oxide becomes zero. As the concentration of majority charge carrier lowers the intrinsic fermi energy shifts away from the valence band . The band bend upwards. d. When the negative vo

Fermi energy band diagrams in NMOS Depletion Mode              When a positive gate voltage is applied to a n type MOSFET , holes near the gate oxide and substrate interface are pushed away due to the same charges. Hence there exist no excess holes in the region and net charge there becomes zero. Thus forming a depletion region with all charged particles in pair (electron hole pairs). Due to the formation of  depletion region the intrinsic energy level becomes equal to the fermi energy . The shift in the bands is nearly equal to the gate voltage applied. Inversion Mode When further positive voltage is applied at the gate , free electrons start to get attracted towards the gateocide - substrate interface. Thus near the interface the concentration of n type impurity appears in a p-type substrate. It suggest inversion of the substrate type and so termed as inversion mode. The intrinsic energy band (Ei) crosses the fermi energy level as Ei lies below Ef for negative doped impurity. Accumu

            Energy band Diagrams What is conduction band and Valence band? These bands define the specific amount of energy present in given shell of electrons.Valence bands specify the energy level of electrons in valence shell of atomic structure whereas conduction band specifies the energy of electrons in conduction band and are responsible for conduction.Valence and conduction bands are separated by certain amount of energy level which is known as energy gap.Energy gap plays an important role for all the mechanisms of energy band diagrams.                        The upper,lower and yellow boxes represent the conduction band, valence band and occupancy level of electrons respectively Energy band Diagrams for metals,semiconductors, I nsulators Above Diagram depicts the following Features : a & b)Metals consisting of one or two valence electrons c)Semiconductor in which band is close enough with valence band d)Insulator having larger energy gap   Let's come down to our topic

Fermi Band diagram In MOS Devices Always wondered how an Nmos device functions on electron level?  This blog will answer all your questions. Fermi energy is a quantum mechanic concept which refers to the energy difference between the highest and lowest energy state occupied by a single particle in a quantum system of non-interacting fermions at absolute zero temperature.Fermi level is the concept used to describe the top of  collection of electron energy levels at absolute zero temperature.Fermi level is “surface of sea” at absolute zero where no electrons will have enough energy to rise above  that surface. Fermi energy is the value of the Fermi level at absolute zero temperature (−273.15 °C). It can also be referred as the maximum kinetic energy an electron can attain at 0K. So the Fermi Function gives the probability that a given available electron energy state will be occupied  at a given temperature.