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Chapter 6 Lecture - Part 1

As in some situations ei can bend below ef thus thus

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Unformatted text preview: e redistribution of charge as a function of position. redistribution Band Diagrams for MOS Capacitors Band • The Depletion Situation (V > 0) – – – – Ei bends towards EF in order to make the depletion region be more n-like depletion The band bending is voltage dependent such that The as the voltage increases, so does the bending. as In some situations, Ei can bend below EF, thus thus implying a large electron concentration in Ec implying When the positive voltage gets high enough, the When region very near the interface inverts from p-type to n-type. to Band Diagrams for MOS Capacitors Band • The Inversion Situation (V >> 0) – – How high does the positive voltage need to go? Let qΦs = the amount of band bending at the semiconductor surface relative to Ei When V = 0, qΦs = 0 (called the flat-band situation) situation) For qΦs < 0, accumulation (bands bend up) 0, – For qΦs > 0, depletion (band bends down) – When Φs > ΦF, then Ei < EF and inversion has When occurred occurred – – Band Diagrams for MOS Capacitors Band • Strong Inversion – Occurs when the inversion region is as strongly Occurs n-type as the bulk of the semiconductor is p-type n-type The Debye screening length (LD) – • • • It is an indication of the distance in which charge It imbalances are screened as to appear neutral imbalances LD is inversely dependent on doping concentration. LD can be derived by examining the E field at the surface of the semiconductor surface Figure 6—12 Band diagram for the ideal MOS structure at: structure (a) equilibrium; (b) negative voltage causes hole accumulation in the p-type semiconductor Figure 6—12 Band diagram for the ideal MOS structure at: (c) positive voltage depletes holes from the semiconductor surface: (d) a larger positive voltage causes inversion — a “ntype” layer at the semiconductor surface....
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