Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Energy Bands: • The Fermi level: o This is an energy level in the energy gap that represents the point where the probability of finding an electron is = 0.5 . o For intrinsic semiconductors, the Fermi level EF, is in the middle of the energy gap and it is called intrinsic Fermi-level Ei. Conduction Band EC EF = Ei Energy Gap EV Valence Band EV o For N-type semiconductors, EF is closer to conduction band EC. As a result, as ND increases Î EF get closer to EC or ϕ F increases. ϕ F = Fermi potential = (EF - Ei)/q = Vtln(ND/ni) Æ positive Vt ≈ 0.025 V, Thermal Voltage = constant/T Conducting Band EC EF Ei qϕ F EV Valence Band N-type o For P-type semiconductors, EF is closer to Valence Band EV. As a result, as NA increases Î EF get closer to Ev. ϕ F = (EF - Ei)/q =-Vtln(NA/ni) Æ negative Conducting Band EC Ei qϕ F EF EV Valence Band P-type o In the absence of current flaw, the Fermi-level would be continuous and constant. • Energy Bands under Bias: o An applied electric field pushes the bands in its direction, e.g Electric field Є EC EF EV o Electrons act like liquid Î accumulate in lowest region in conduction Band. While holes act like bubbles Î accumulate in highest region in Valence Band. o This effect could be used to invert a certain region in a semiconductor by applying a large enough voltage with the appropriate polarity. This region is inverted (EF is closer to EC than EV) Æ N-type Є Electrons accumulated EC EF EV Holes are depleted P-Type Electron Depletion EC EF EV Holes accumulation Є This region is inverted (EF is closer to EV than EC) Æ P-type N-Type The P-N Junction (Diode): o The Diode is made by bringing into a contact a p-type semiconductor with an N-type semiconductor. o Ferric level will align. As a result, the band will bend around the junction by q(ϕ FN – ϕ FP). This is called the Build-in potential. Vbi = ϕ FN – ϕ FP = Vt ln[(NA*ND)/ni2] ND = net Donor concentration on the N-side NA = net acceptor concentration on the P-side Vbi is typically ≈ 0.5 V to 0.8 V Before: P N EC Ei EFP EC qϕ FN qϕ FP EV EFN Ei EV After: P N EC q(ϕ FN – ϕ FP) EV EC EF EV