July09LessonFour - Key Lecture Concepts for CoE225/EE 271...

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Key Lecture Concepts for CoE225/EE 271 (Mostly Digital Electronics) LESSON 4: The MOS Field-Effect Transistor (MOSFET); MOSFET Circuit Analysis Examples; Logic Circuit Concepts: Voltage Transfer Characteristics, Noise Margins and Digital Gain; Introduction to the Dynamic Response of Logic Gates. Appendices: Characteristics of other FET Devices; Introduction to the Properties of Semiconductor Materials; Review of the Historical Development of Computer Hardware Lesson Overview: A major goal of this course is to develop an understanding of how computers work from a hardware point of view. Because digital voltage signals lose amplitude and their rise and fall times become less steep as they propagate from the input of a digital system to its outputs, there is a need to “regenerate” the signals to full amplitude and to sharpen the rise and fall times by providing digital signal amplification using three- terminal “transistor” devices. Transistors also provide the isolation necessary to prevent voltage signals intended to propagate towards an output of a digital system from propagating back and influencing the input to the transistor. The Metal-Oxide-Semiconductor field-effect transistor, MOSFET, has proven to be an outstanding device for providing two requirements of digital logic circuits: digital signal amplification and signal directionality. By directionality it is meant that signals can travel towards the output but not back towards the input, which would interfere with other input signals. Millions of MOSTs required for computer logic and memory can be mass-fabricated on the surface of inexpensive silicon material, at pennies for thousands of devices. The MOSFET is a three-terminal device with the voltage on a gate terminal controlling the flow of charge from a source terminal to a drain terminal. The charge can be electrons (for the n-type transistor) or holes (for the p-type transistor ). The lesson presents the two I/V characteristics of MOSFETs. They are: 1) the dependence of the output current on the voltage difference between the input gate terminal and the source terminal; 2) the dependence of the output current on the voltage difference between the drain and the source terminals. Modern digital circuits were invented by improving and exploiting these two characteristics. For digital circuits the MOSFET serves as a switch; for analog circuits it serves as an amplifier of voltage applied between gate and the source terminals. In this lesson, basic n or p-channel MOSFET circuits with a resistor “load” are introduced. The resistor load serves to isolate the MOSFET switch/amplifier output terminal from the power supply. In the following lessons the modern logic circuits which use MOSFET versus resistor loads will be studied. A key property of inverter circuits is their
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July09LessonFour - Key Lecture Concepts for CoE225/EE 271...

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