Lecture 19 - MOS Intro and Electrostatics

Lecture 19 - MOS Intro and Electrostatics - Metal-Oxide...

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1 Metal-Oxide Semiconductor (MOS) Introduction Objectives: Understand the physics and models of semiconductor devices including … field-effect transistors. Questions to be answered: • What is a MOS structure and how is it formed? • What are the biasing regions and what do the EE360 – Lecture 19 corresponding band diagrams look like? • How do we describe the charge density and electric field in a MOS structure? Metal-Oxide Semiconductor Structure •MO S = M etal” gate + insulating O xide + S emiconductor MOS Structure • More generally called a Metal- Insulator Semiconductor (MIS) structure (or diode or capacitor) • Precursor to the Metal-Oxide Semiconductor Field-Effect MOSFET From Sze, pg. 363 EE360 – Lecture 19 Transistor (MOSFETs were proposed in the 1920s, but practical devices were not fabricated until the 1960s) From Sze, pg. 434
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2 Making a MOS Structure • Start with a very clean semiconductor wafer – we want the best interface possible • Thermally oxidize the material (standard for Si-SiO 2 structures, but few other semiconductors have high-quality thermal oxides) or deposit an insulator layer (i.e. HfO 2 for high dielectric constant From Pierret p. 564. EE360 – Lecture 19 constant) • Deposit a “metal” - typically aluminum or heavily doped polycrystalline silicon Equilibrium Band Diagram • Similar to the MS Contact • Fermi-level is uniform throughout the device in equilibrium Metal work function and semiconductor and insulator Metal work function and semiconductor and insulator electron affinities determine band diagram EE360 – Lecture 19 Isolated Metal, Insulator, and Semiconductor Band Diagrams From Pierret p. 565.
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3 Equilibrium Band Diagram: Ideal Structure Ideal case (definition from Pierret): 1. Metallic gate is at a uniform potential under d.c. and a.c. conditions. 2 Nt t h h t h i l t Band Diagram for Ideal MOS Structure with n-type Semi. 2. No current through the insulator 3. Only charges that can exist are those in the semiconductor and those at the metal surface (no charges are present in the oxides or fixed at the interfaces). 4. Uniformly doped 5. Thick substrate (no electric field present at the back contact) 6 An ohmic contact is established on the EE360 – Lecture 19 6. An ohmic contact is established on the back of the device. 7. MOS-C is a 1D structure 8. Metal and semiconductor work functions are equal; thus, Φ ms = Φ m - Φ s = 0.
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This note was uploaded on 04/20/2008 for the course EE 360 taught by Professor Hastings during the Spring '08 term at Kentucky.

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Lecture 19 - MOS Intro and Electrostatics - Metal-Oxide...

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