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Fets

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Transcript

  • 1. Field-Effect Transistors
  • 2. Field-effect transistors
    • General overview
    • Types of field-effect transistors
    • Uses
  • 3. 1. General overview
    • Field-effect transistors (FETs) follow an other principle than bipolar junction transistors
    • They rely on an electric field to control the shape and hence the conductivity of a „channel“ in a semiconductor material
    • FETs controlled by voltage (  BJTs controlled by current)
  • 4. 1. General overview
    • The concept of the field effect transistor predates the bipolar junction transistor (BJT)
    • Was not physically implemented until after BJTs, due to limitations of semiconductor materials and relative ease of manufacturing BJTs compared to FETs at that time.
  • 5. 1. General overview
    • Basic principle of FETs
    • The terminals of a FET refer to their function: Gate (G), Source (S), Drain (D)
    control terminal current channel of charge carriers for charge carriers control voltage
  • 6. 1. General overview
    • FETs are voltage-controlled by the voltage between gate and source terminal
    • Voltage effects the electric field of the transistor which enlarges or diminishes the channel
    •  Effects the current stream through the FET
  • 7. 2. Types of field-effect transistors
    • Junction Field-Effect Transistors
    • JFETs
    • Simplest type of FET
    • Long channel semiconductor
    • Either p- or n-doped (  p-type, n-type)
    • A contact at each end  source and drain terminals
    • Gate terminal surrounds the channel and is doped opposite to the doping of the channel
  • 8. 2. Types of field-effect transistors
    • Junction Field-Effect Transistors
    • JFETs
    • N-channel types and P-channel types
  • 9. 2. Types of field-effect transistors
    • Metal-Oxide-Semiconductor FETs
    • MOS-FETs
    • Most common field-effect transistor
    • traditional metal–oxide–semiconductor (MOS) structure is obtained by depositing a layer of silicon dioxide and a layer of metal on top of a semiconductor die. As the silicon dioxide is a dielectric material its structure is equivalent to a planar capacitor, with one of the electrodes replaced by a semiconductor (substrate)
  • 10. 2. Types of field-effect transistors
    • Metal-Oxide-Semiconductor FETs
    • MOS-FETs
    • Metal electrode is used as the gate, the other forms the so called body/bulk/substrate terminal of a MOS-FET
    • Opposite doted semiconductors placed within the substrate form the drain and source terminals
    • Modulating the MOS-Capacitance forms a channel between source and drain terminals
    • This controls the current flow through the MOS-FET
  • 11. 2. Types of field-effect transistors
    • Metal-Oxide-Semiconductor FETs
    • MOS-FETs
  • 12. 2. Types of field-effect transistors
    • Metal-Oxide-Semiconductor FETs
    • MOS-FETs
    depletion-mode enhancement-mode P-Channel N-Channel
  • 13. 3. Uses
    • As well as BJTs, FETs can be used as switching and/or amplifying elements
    • JFETs are also often used as variable, controlable resistors
    • FETs, especially JFETs consumpt almost no current  high input impedance
    • Makes JFETs very suitable for pre-amplifiers
    • FETs behave like electron tubes from the old days
    • Integrated circuits (CMOS, other modern elements) use MOS-FETs as the switching element
    • Power-MOS-FETs are used in HiFi power amplifiers and produce a sound comparable to tube amplifiers
    • Many, many more uses…