Sep 172014
 

The Field Effect Transistor (FET)

Field effect transistor or FET is a particularly interesting transistor and it can be of two types: the Junction Field Effect Transistor or JFET and the Metal-Oxide Semiconductor Field Effect Transistor (MOSFET). These devices have a high input impedance (1012 ohms) and they are controlled by voltage.

Both devices are used as an amplifier or a switch in analog and digital circuits.

The Field Effect Transistor (FET) symbol

Electrical characteristics of JFET and MOSFET are similar although their technology and physical structure are totally different.

Advantages of FETs

  1. They are devices controlled by voltage with a very high input impedance (107 to 1012 ohms)
  2. FETs generate a lower noise level than the Bipolar Junction Transistor (BJT)
  3. FETs are more stable than BJT with temperature
  4. FETs are easier to manufacture than the BJT, because they require fewer steps to be built and they allow more integrated devices in the same IC
  5. FETs behave like resistors controlled by voltage for small drain-source voltage values
  6. The high input impedance of FET allows them to withhold loads long enough to allow its usage as storage elements
  7. Power FETs can disipate higher power and can switch very large currents

Disadvantages of FETs

  1. FETs have a poor frequency response due to its high input capacitance
  2. FETs have a very poor linearity, and generally they are less linear than Bipolar Juntion Transistor
  3. FETs can be damaged due to the static electricity
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