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Note for Micro Electronics - ME By Ayush Agrawal

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Unit 1 – Chapter 4 MOS Field-Effect Transistors (MOSFETs) UNIT 1 OUTLINE 1.1 Device Structure and Physical Operation 1.2 Current – Voltage Characteristics 1.3 MOSFET Circuits at DC 1.4 Biasing in MOS amplifier circuits 1.5 Small Signal Operation and Models 1.6 The MOSFET as an Amplifier and as a Switch 1.7 Single Stage MOS amplifiers 1.8 SPICE MOSFET models and examples LEARNING OUTCOMES: At the end of this chapter one can clearly get to know the following:       Understanding Physical construction and operation of an Enhancement MOSFET Drawing the V-I characteristics of n and p channel E-MOSFET DC operation or biasing of MOSFETs AC Operation: Small signal modeling of MOSFETs Single stage MOS amplifiers : Common Source, Common Drain and Common Gate amplifiers SPICE modeling of MOSFETs INTRODUCTION Along with the Junction Field Effect Transistor (JFET), there is another type of Field Effect Transistor available whose Gate input is electrically insulated from the main current carrying channel and is therefore called an Insulated Gate Field Effect Transistor or IGFET. The most common type of insulated gate FET which is used in many different types of electronic circuits is called the Metal Oxide Semiconductor Field Effect Transistor or MOSFET for short. The IGFET or MOSFET is a voltage controlled field effect transistor that differs from a JFET in that it has a "Metal Oxide" Gate electrode which is electrically insulated from the main semiconductor N-channel or P-channel by a thin layer of insulating material usually silicon dioxide (commonly known as glass). This insulated metal gate electrode can be thought of as one plate of a capacitor. The isolation of the controlling Gate makes the input resistance of the MOSFET extremely high in the Mega-ohms ( MΩ ) region thereby making it almost infinite. 3

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As the Gate terminal is isolated from the main current carrying channel "NO current flows into the gate" and just like the JFET, the MOSFET also acts like a voltage controlled resistor were the current flowing through the main channel between the Drain and Source is proportional to the input voltage. Also like the JFET, this very high input resistance can easily accumulate large amounts of static charge resulting in the MOSFET becoming easily damaged unless carefully handled or protected. MOSFETs are three terminal devices with a Gate, Drain and Source and both P-channel (PMOS) and N-channel (NMOS) MOSFETs are available. The main difference this time is that MOSFETs are available in two basic forms: 1. Depletion Type - the transistor requires the Gate-Source voltage, ( VGS ) to switch the device "OFF". The depletion mode MOSFET is equivalent to a "Normally Closed" switch. 2. Enhancement Type - the transistor requires a Gate-Source voltage, ( VGS ) to switch the device "ON". The enhancement mode MOSFET is equivalent to a "Normally Open" switch. Basic operating principle of a MOSFET:   Use of the voltage between two terminals to control the current flowing in the third terminal Also, the control signal can be used to cause the current in the third terminal to change from zero to a large value, thus allowing the device to act as a switch. The FET differs from BJT in the following important characteristics: 1. 2. 3. 4. 5. 6. 7. 8. It is a unipolar device It is simpler to fabricate Occupies less space in Integrated form, packaging density is high(>200 million) It has higher input resistance It can be used as a symmetrical Bilateral switch It functions as a memory device It is less noisy than a BJT It exhibits no offset voltage at zero input, hence making an excellent signal chopper THE ONLY DISADVANTAGE IS IT HAS SMALLER GAIN- BANDWIDTH PRODUCT THAN BJT The symbols and basic construction for both configurations of MOSFETs are shown below. 4

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