Lecture 15 OUTLINE The MOS Capacitor Energy band diagrams

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Presentation transcript:

Lecture 15 OUTLINE The MOS Capacitor Energy band diagrams Reading: Pierret 16.1-16.2, 18.1; Hu 5.1

MOS Capacitor Structure (cross-sectional view) MOS devices today employ: degenerately doped polycrystalline Si (“poly-Si”) film as the gate-electrode material n+-type for “n-channel” transistors p+-type, for “p-channel” transistors SiO2 as the gate dielectric band gap = 9 eV er,SiO2 = 3.9 Si as the semiconductor material p-type, for “n-channel” transistors n-type, for “p-channel” transistors GATE xo Semiconductor + VG _ EE130/230M Spring 2013 Lecture 15, Slide 2

MOS Equilibrium Band Diagram metal oxide semiconductor n+ poly-Si SiO2 EC p-type Si EC=EFM EFS EV EV EE130/230M Spring 2013 Lecture 15, Slide 3

MOS Band Diagrams: Guidelines Fermi level EF is flat (constant with x) within the semiconductor Since no current flows in the x direction, we can assume that equilibrium conditions prevail Band bending is linear within the oxide No charge in the oxide => dE/dx = 0 so E is constant => dEc/dx is constant From Gauss’ Law, we know that the electric field strength in the Si at the surface, ESi, is related to the electric field strength in the oxide, Eox: E E E EE130/230M Spring 2013 Lecture 15, Slide 4

MOS Band Diagram Guidelines (cont’d) The barrier height for conduction-band electron flow from the Si into SiO2 is 3.1 eV This is equal to the electron-affinity difference (cSi and cSiO2) The barrier height for valence-band hole flow from the Si into SiO2 is 4.8 eV The vertical distance between the Fermi level in the metal, EFM, and the Fermi level in the Si, EFS, is equal to the applied gate voltage: EE130/230M Spring 2013 Lecture 15, Slide 5

Special Case: Equal Work Functions FM = FS EE130/230M Spring 2013 Lecture 15, Slide 6

General Case: Different Work Functions EE130/230M Spring 2013 Lecture 15, Slide 7

Flat-Band Condition EE130/230M Spring 2013 Lecture 15, Slide 8