Handout_6_-_MOSFET

Handout_6_-_MOSFET - EE 101A / Winter 10 EE 101A MOSFET...

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1 EE 101A / Winter 10 Handout #6 EE 101A MOSFET Reading – Sedra & Smith, Chapter 4, Sections 4.1 – 4.3 Metal-Oxide-Semiconductor Field-Effect-Transistor (MOSFET) is the most common form of transistor (> 90%) in today’s integrated circuits. 1. Physical Structure The physical structure of a MOSFET is shown below: a silicon substrate (body, bulk) two heavily doped regions ( source and drain ) a channel between source and drain a thin dielectric, silicon dioxide, on top of the channel ( gate oxide ) a conductive gate on top of the gate oxide In n-channel MOSFET (N-MOSFET, NMOS) (as shown above) substrate is p-type source and drain are heavily doped n-type (n + ) channel is p-type when the MOSFET is off, and becomes n-type when the MOSFET is on (to be discussed later) In p-channel MOSFET (P-MOSFET, PMOS) (not shown) substrate is n-type source and drain are heavily doped p-type (p + ) channel is n-type when the MOSFET is off, and becomes p-type when the MOSFET is on
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2 2. Basic Operations We will first discuss the operations of n-channel MOSFET, NMOS. 2.1 MOS Capacitor The metal-oxide-semiconductor (MOS) capacitor structure (right) is similar to a metal-oxide-metal (MOM) capacitor (left). When a negative voltage is applied to the top electrode of the MOM (left) with the bottom electrode grounded, there will be negative charges in the top electrode and positive charges in the bottom electrode. When a negative voltage is applied to the gate of the MOS (right) with the substrate grounded, there will be negative charge in the gate and positive charge (holes) in the substrate. For both cases, Q = C ox V ( V < 0 and hence Q < 0) where Q = charge stored across the capacitor per unit area (coul/cm 2 ) C ox = ox / t ox is the capacitance per unit area (F/cm 2 ) ox = oxide dielectric constant (3.9 o = 3.9 X 8.854 10 -14 F/cm) t ox = thickness of oxide (cm) metal oxide metal gate oxide p-semiconductor - - - - - - - metal oxide metal gate oxide p-semiconductor V < 0 V < 0 - - - - - - - + + + + + + + + + + + + + +
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3 When a positive voltage is applied to the top electrode of the MOM (left) with the bottom electrode grounded, there will be positive charges in the top electrode and negative charges in the bottom electrode (left). When a positive voltage is applied to the gate of the MOS (right) with the substrate grounded, there will be positive charge in the gate. However, because the p-semiconductor has many holes but few electrons, initially the holes will be pushed away from the surface. The surface of the semiconductor will be less p-type, depleted of holes, but with few electrons.
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This note was uploaded on 03/30/2010 for the course EE 101A taught by Professor Wong during the Winter '08 term at Stanford.

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Handout_6_-_MOSFET - EE 101A / Winter 10 EE 101A MOSFET...

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