MosfetBasics

MosfetBasics - Basics of MOSFET Transistors S Tewksbury Jan...

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Basics of MOSFET Transistors S. Tewksbury Jan, 2006 Table of Contents 1. INTRODUCTION. ................................................................................................................................................ 1 2. MOSFET BEHAVIOR . ........................................................................................................................................ 2 2.1 NMOS/PMOS SCHEMATIC SYMBOLS . ............................................................................................................ 2 2.2 CURRENT-VOLTAGE RELATIONSHIPS FOR NMOS AND PMOS TRANSISTORS. .............................................. 3 2.3 OTHER "SCHEMATIC" SYMBOLS (NOT FORMAL) SHOWING BEHAVIOR. ......................................................... 5 3 THE NMOS AND PMOS TRANSISTOR STRUCTURE. ................................................................................ 7 4. FABRICATING THE NMOS CIRCUIT: CLASSIC SELF-ALIGNED POLYSILICON GATE. .......... 10 4.1 SOME CONSTRAINTS IMPACTING FABRICATION. .......................................................................................... 10 4.2 NMOS TRANSISTOR. ...................................................................................................................................... 11 4.3 PMOS TRANSISTOR. ....................................................................................................................................... 18 4.4 SUBSTRATE CONTACTS . .................................................................................................................................. 19 5. FROM TRANSISTOR SCHEMATIC TO MASK LAYOUT. ...................................................................... 21 6. A COUPLE OF COMMENTS. .......................................................................................................................... 22 1. Introduction The basic principles underlying MOS Field Effect Transistors (MOSFETs) are summarized here. Silicon MOSFETs have become the dominant devices for VLSI digital and analog circuits and are expected to continue their dominance for some time to come. As seen later, the outputs of MOSFETs in digital logic cells typically drive the gates of MOSFETs appearing in other logic cells. That leads to the outputs seeing a capacitive load, rather than the resistive load seen in technologies using bipolar circuits. Once the capacitance driven by the logic cells have charged/discharged to their steady state values, no further current needs (ideally) to flow in the circuits, with no static power (P = I*V) being dissipated once the capacitors have been charged. The term "ideally" is used above since very small leakage currents do flow and lead to a very small steady state power. The discussions here start with the electronic symbol for a MOSFET and extend down into the physical structure and behavior of a MOSFET device. This is the inverse of most presentations but is better suited for a class more interested in using MOSFETs to build circuits than in understanding the detailed physics underlying their operation. After presenting the basic structure of a MOSFET, the general steps used to fabricate the MOSFET are reviewed. The design of the mask layout is directly tied to the sequence of microfabrication steps used to generate the device structure.
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There are two distinctive types of MOSFET. For one type, the N MOS transistor, a gate voltage to create a thin conducting layer between the N -type source and drain regions. For the second type, the P MOS, a gate voltage is used to turn on and off a thin conducting later between the P -type source and drain regions. These NMOS/PMOS transistor are characterized by threshold voltages, namely the gate voltage at which the transistor's source-to-drain connection turns ON and OFF. The NMOS transistor turns ON for gate voltages greater than its threshold voltage whereas the PMOS transistor turns ON for gate voltages less than its threshold voltages. In this sense, the control of the PMOS transistor is opposite to (the inverse of) that of the NMOS transistor.
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This note was uploaded on 10/27/2009 for the course ECE 5 taught by Professor Chavez during the Spring '09 term at Stevens.

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MosfetBasics - Basics of MOSFET Transistors S Tewksbury Jan...

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