121B_1_EE121B_Slide08 - MOS1

121B_1_EE121B_Slide08 - MOS1 - EE 121B Principles of...

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Slide 8-1 © 2006 Marko Sokolich All Rights Reserved EE 121B – Chi On Chui EE 121B Principles of Semiconductor Device Design Metal-Oxide-Semiconductor Capacitors - I Professor Chi On Chui Electrical Engineering Department University of California, Los Angeles Email: chui@ee.ucla.edu Slide 8-2 © 2006 Marko Sokolich All Rights Reserved EE 121B – Chi On Chui Outline Photoelectric Effect – Band Diagram of a Metal Metal-Insulator-Semiconductor Structure Equilibrium Conditions Band Bending Under Applied Bias Accumulation Depletion Inversion Charge vs. Surface Potential Gate Capacitance vs. Gate Voltage Threshold Voltage
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Slide 8-3 © 2006 Marko Sokolich All Rights Reserved EE 121B – Chi On Chui Experimental Observations: Photoelectric Effect 0 vacuum Battery - + Ammeter Emitter Collector Blue Light - current flows Red Light - no current Red Light Electrons are bound to the metal with energy eV o eV o is called the Work Function of the metal Frequency of Light Battery Voltage eV = h ν -eV o Red Green Blue V o Slide 8-4 © 2006 Marko Sokolich All Rights Reserved EE 121B – Chi On Chui Work Function: Simplified Conceptual Model of a Metal electrons metal vacuum Metal Work Function Aluminum 4.2 eV Cesium 1.81 Copper 4.6 Germanium 4.56 Gold 5.22 Silicon 4.1 Titanium 4.17 Tungsten 4.56 What wavelength of light can free an electron from the surface of Cesium? Is this wavelength in the visible spectrum? “sea of electrons” with the highest energy at –eV 0 –eV 0 photon electron
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Slide 8-5 © 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 a 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 Slide 8-6 © 2006 Marko Sokolich All Rights Reserved EE 121B – Chi On Chui MIS Structure In the general case the junction is called a Metal-Insulator- Semiconductor (MIS) Junction If the semiconductor is silicon and the insulator is SiO 2 then the junction is called a Metal-Oxide-Semiconductor (MOS) junction E c E v E Fs E Fm E i
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Slide 8-7 © 2006 Marko Sokolich All Rights Reserved EE 121B – Chi On Chui Apply Negative Bias on the Metal Fermi Level in Semiconductor Stays Flat…no current flow through oxide Bands Bend in both semiconductor and insulator In the insulator the potential is a straight line (why?) E Fm E Fs The case shown is called accumulation . The interface is strongly p-type. Free holes have accumulated there. E i ++ + Slide 8-8 © 2006 Marko Sokolich All Rights Reserved EE 121B – Chi On Chui Apply Some Positive Bias on the Metal For positive bias on the metal the Fermi level in the semiconductor is above the Fermi level in the metal An easy way to remember is that electrons are attracted to the positive bias therefore the bands bend down ( electrons roll down ) E Fm E Fs The case shown is called depletion .
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121B_1_EE121B_Slide08 - MOS1 - EE 121B Principles of...

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