121B_1_EE121B_Slide09 - MOS2

121B_1_EE121B_Slide0 - EE 121B Principles of Semiconductor Device Design Metal-Oxide-Semiconductor Capacitors II Professor Chi On Chui Electrical

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Slide 9-1 © 2006 Marko Sokolich All Rights Reserved EE 121B – Chi On Chui EE 121B Principles of Semiconductor Device Design Metal-Oxide-Semiconductor Capacitors - II Professor Chi On Chui Electrical Engineering Department University of California, Los Angeles Email: [email protected] Slide 9-2 © 2006 Marko Sokolich All Rights Reserved EE 121B – Chi On Chui Outline Non-ideal MOS Capacitors Non-zero Flat Band Voltage Charges in the Oxide Deep Depletion Applications of MOS Capacitors Charge Couple Devices
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Slide 9-3 © 2006 Marko Sokolich All Rights Reserved EE 121B – Chi On Chui Flat Band Metal-Semiconductor Line-up Start with a Metal and Semiconductor We have chosen a semiconductor such that both Fermi level and vacuum level line up. This is the Flat Band Condition E c E v E Fs E Fm q Φ m q Φ s q χ Interpose a very wide bandgap insulator Work Function Electron Affinity = 4.05V in Silicon Slide 9-4 © 2006 Marko Sokolich All Rights Reserved EE 121B – Chi On Chui Relaxing the Flat Band Condition Start with a Metal and Semiconductor We have chosen a semiconductor such that the Fermi levels and vacuum levels DO NOT line up Φ m < Φ s E c E v E Fs E Fm q Φ m q Φ s q χ If we put the metal and the semiconductor together the bands in the semiconductor must bend in equilibrium
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Slide 9-5 © 2006 Marko Sokolich All Rights Reserved EE 121B – Chi On Chui Band Bending in Equilibrium If the work functions in the metal and semiconductor are not the same, the bands in the semiconductor will bend in equilibrium In equilibrium the Fermi level must be a constant but the vacuum level only needs to be continuous The band bending is equal to the difference in the work functions E Fm E Fs - - - q ϕ s q F q Φ m q Φ s ms s m s Φ Φ Φ = Slide 9-6 © 2006 Marko Sokolich All Rights Reserved EE 121B – Chi On Chui Voltage Applied for Flat Band If we apply a voltage on the metal equal to the difference in work functions the bands will be flat again E c E v E Fs E Fm E i qV V = V In this case the flat band voltage is negative
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Slide 9-7 © 2006 Marko Sokolich All Rights Reserved EE 121B – Chi On Chui Polysilicon “Metal” In modern MOS transistors the “Metal” in MOS is really heavily doped n+ or p+ polysilicon Polysilicon is pure silicon with dopant (in this case n+ dopant… Phosphorus or Arsenic) but it is not a single crystal like the substrate Instead, “Poly” is made up of many crystal grains in different orientations It behaves like a metal because it is degenerately doped. The Fermi level is well into the conduction band and there are 10 19 to 10 20 electrons/cm 3 .
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This note was uploaded on 11/11/2010 for the course EE 121b taught by Professor Bjt-gamma during the Winter '08 term at UCLA.

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121B_1_EE121B_Slide0 - EE 121B Principles of Semiconductor Device Design Metal-Oxide-Semiconductor Capacitors II Professor Chi On Chui Electrical

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