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Introduction to FET’s

Introduction to FET’s. Current Controlled vs Voltage Controlled Devices. Types of Field Effect Transistors (The Classification) . JFET MOSFET (IGFET). n-Channel JFET p-Channel JFET. FET. Enhancement MOSFET. Depletion MOSFET. n-Channel DMOSFET . p-Channel DMOSFET .

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Introduction to FET’s

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  1. Introduction to FET’s

  2. Current Controlled vs Voltage Controlled Devices

  3. Types of Field Effect Transistors (The Classification) • JFET MOSFET(IGFET) n-Channel JFET p-Channel JFET FET Enhancement MOSFET Depletion MOSFET n-Channel DMOSFET p-Channel DMOSFET n-Channel EMOSFET p-Channel EMOSFET

  4. The Junction Field Effect Transistor (JFET) Figure: n-Channel JFET.

  5. JFET Construction There are two types of JFET’s: n-channel and p-channel. The n-channel is more widely used. There are three terminals: Drain (D) and Source (S) are connected to n-channel Gate (G) is connected to the p-type material ELEC 121

  6. Drain Drain Drain Gate Gate Gate Source Source Source SYMBOLS n-channel JFET Offset-gate symbol p-channel JFET n-channel JFET

  7. N-Channel JFET Symbol

  8. Biasing the JFET Figure: n-Channel JFET and Biasing Circuit.

  9. Operation of JFET at Various Gate Bias Potentials Figure: The nonconductive depletion region becomes broader with increased reverse bias. (Note: The two gate regions of each FET are connected to each other.)

  10. Transfer Characteristics The input-output transfer characteristic of the JFET is not as straight forward as it is for the BJT In a BJT,  (hFE) defined the relationship between IB (input current) and IC (output current). In a JFET, the relationship (Shockley’s Equation) between VGS (input voltage) and ID (output current) is used to define the transfer characteristics, and a little more complicated (and not linear): As a result, FET’s are often referred to a square law devices

  11. JFET Operating Characteristics • There are three basic operating conditions for a JFET: • JFET’s operate in the depletion mode only • VGS = 0, VDS is a minimum value depending on IDSS and the drain and source resistance • VGS < 0, VDS at some positive value and • Device is operating as a Voltage-Controlled Resistor • For an n channel JFET, VGS may never be positive* • For an p channel JFET, VGS may never be negative*

  12. Saturation • At the pinch-off point: • • any further increase in VGS does not produce any increase in ID. VGS at pinch-off is denoted as Vp. • • ID is at saturation or maximum. It is referred to as IDSS. • • The ohmic value of the channel is at maximum.

  13. Specification Sheet (JFETs)

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