Lecture 4 4521 semiconductor device physics - metal-semiconductor system

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Transcript of Lecture 4 4521 semiconductor device physics - metal-semiconductor system

  • *Prof. Ming-Jer ChenDepartment of Electronics EngineeringNational Chiao-Tung UniversityNovember 6, 2014 DEE4521 Semiconductor Device Physics

    Metal-Semiconductor System: Contact

  • *

    PP. 331 338

    Section 6.4 Metal-Semiconductor JunctionsTextbook pages involved

  • *How to establish device physics for this circuitry? Even for the poor contact case?(given doping concentration, two Ohmic contacts, a metal line, a supply voltage source, and a grounding system)

  • *6-22Comparison of the I-Va characteristics of a Schottky diode and a pn junction diode. The scale for the reversecharacteristic is compressed compared with the scale for forward bias.

    Figure 6.22Ohmic ContactCan you derive an analytic model for these I-V?and Can you derive their small-signal models?

  • *Four Situations of Band Bending at Semiconductor Surfaces Contact Case:Depletion (suitable for a metal-semiconductor interface, as suggested by many and many experiments done before)

    Accumulation (not suitable for a metal-semiconductor interface)

    Inversion (not suitable for a metal-semiconductor interface)

    Flatband (not suitable for a metal-semiconductor interface)

  • *6-23Low-resistance metal-semiconductor contacts using degenerate surface layers. Metal-n+n contact (a) and metal-p+p contact (b). The Ohmic barrier is thin enough to permit tunneling.

    Figure 6.23 of textbook by AndersonsHow do holes and electrons communicate with each other at the interface?Not so clear

  • *Metal-Semiconductor Contact System (or Junction):

    Ohmic Contact -- Two-way conducting (on) -- Nearly zero resistance or potential drop -- Equilibrium at both sides

    Schottky Contact -- Usually for one-way conducting, with the other way off -- Considerable potential drop -- Fermi level may split off

  • *6-18Energy band diagram as predicted by the electron affinity model for an Al:n-Si metal semiconductor junction: (a) Neutrality (b) equilibrium. The predicted barrier of 0.10 eV from metal to semiconductor is much less than the experimental value of about 0.7 eV. A more refined model is required.

    Figure 6.18These two diagrams are wrong!Band bending must go upward, NOT downward, for n-type.accumulation

  • *6-19(a) The neutrality diagram for the Al:n-Si Schottky barrier diode including the tunneling-induced dipoleeffect. (b) The equilibrium energy band diagram for an Al:n-Si Schottky barrier diode.

    Figure 6.19This is the depletion case by bending band upward for n-type semiconductor.Wrong band diagramCorrect band diagram

  • *6-20 Energy band diagrams for a metal: n-type semiconductor Schottky barrier. (a) For forward bias, electrons flow from semiconductor to metal. (b) For reverse bias, only a small leakage current flows. (c) For the first-order model, the metal-semiconductor barrier (EB(0) = EC(x = 0) - Efm) is independent of applied voltage.

    Figure 6.20Xm = (2sVj/qND)1/2Thermionic injection

  • *6-21A Schottky barrier diode made with a P-type semiconductor. (a) Equilibrium; (b) forward bias; (c) reverse bias.

    Figure 6.21 band bending down, for p-type.Electron-hole recombinationLowered barrier seen by holes Forward biasReverse biasRaised barrier seen by holes Too wide to tunnel

  • *6-22Comparison of the I-Va characteristics of a Schottky diode and a pn junction diode. The scale for the reversecharacteristic is compressed compared with the scale for forward bias.

    Figure 6.22Ohmic ContactFrom the energy band diagram,we EE people can now derive an analytic model for I-V,as well as for small-signal equivalent circuits.Junction Conductance Junction CapacitanceBulk (Series) Resistance