1EE 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 (sourceand drain) a channelbetween source and drain a thin dielectric, silicon dioxide, on top of the channel (gate oxide) a conductive gateon top of the gate oxide In n-channelMOSFET (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-channelMOSFET (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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22. 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 = CoxV (V< 0 and hence Q< 0) where Q= charge stored across the capacitor per unit area (coul/cm2) Cox= ox/ toxis the capacitance per unit area (F/cm2) ox= oxide dielectric constant (3.9 o= 3.9 X 8.854 10-14F/cm) tox= thickness of oxide (cm) metaloxidemetalgateoxidep-semiconductor- - - - - - -metaloxidemetalgateoxidep-semiconductorV < 0V < 0- - - - - - -+ + + + + + ++ + + + + + +