Lecture 15 - Lecture 15 MOSFET structure...

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1 Lecture 15 – Slide 1 EE105, Fall 2013 Prof. Ming C. Wu Lecture 15 § MOSFET structure & operation (qualitative) § Review of electrostatics § The (N)MOS capacitor Electrostatics Charge vs. voltage characteristic § Reading: Chapter 6.1-6.2.1 Lecture 15 – Slide 2 EE105, Fall 2013 Prof. Ming C. Wu The MOSFET: Metal-Oxide-Semiconductor Field-Effect Transistor § Current flowing through the channel between the source and drain is controlled by the gate voltage. § “N-­‐channel” & “P-­‐channel” MOSFETs operate in a complementary manner § “CMOS” = Complementary MOS Substrate Gate Source Drain GATE LENGTH, L g OXIDE THICKNESS, T ox JUNCTION DEPTH, X j M. Bohr, Intel Developer Forum , September 2004 |GATE VOLTAGE| CURRENT V TH
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2 Lecture 15 – Slide 3 EE105, Fall 2013 Prof. Ming C. Wu N-Channel MOSFET Structure § The conventional gate material is heavily doped polycrystalline silicon (referred to as polysilicon or poly-Si or poly ) Note that the gate is usually doped the same type as the source/drain, i.e. the gate and the substrate are of opposite types. § The conventional gate insulator material is SiO 2 . § To minimize current flow between the substrate (or body ) and the source/drain regions, the p-type substrate is grounded. Circuit symbol Lecture 15 – Slide 4 EE105, Fall 2013 Prof. Ming C. Wu Review: Charge in a Semiconductor § Negative charges: Conduction electrons (density = n) Ionized acceptor atoms (density = N A ) § Positive charges: Holes (density = p) Ionized donor atoms (density = N D ) § The net charge density [C/cm3] in a semiconductor is § Note that p, n, N D , and N A each can vary with position. § The mobile carrier concentrations (n and p) in the channel of a MOSFET can be modulated by an electric field via VG. ( ) A D N N n p q + = ρ
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3 Lecture 15 – Slide 5 EE105, Fall 2013 Prof. Ming C. Wu § As the gate voltage ( V G ) is increased, holes are repelled away from the substrate surface. The surface is depleted of mobile carriers. The charge density within the deple&on region is determined by the dopant ion density. § As V G increases above the threshold voltage V TH , a layer of conducNon electrons forms at the substrate surface. For V G > V TH , n > N A at the surface. à The surface region is inverted to be n-­‐type. à The electron inversion layer serves as a conducNve path (channel) for current to flow between the heavily doped (i.e. highly conducNve) source and drain regions.
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