Category Archives: FET

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FET

The field effect transistor was proposed by Julius Lilienfeld in US patents in 1926 and 1933

A field effect transistor (FET) is a unipolar device, conducting a current using only one kind of charge carrier.

  • The JFET source, gate, and drain correspond to the BJT’s emitter, base, and collector, respectively.
  • Application of reverse bias to the gate varies the channel resistance by expanding the gate diode depletion region.

FET OPERATION

A voltage applied to the gate, input element, controls the resistance of the channel, the unipolar region between the gate regions.

  • In an N-channel device, a heavy P-type region on both sides of the center of the slab serves as a control electrode, the gate. The gate is analogous to the base of a BJT.

  • The depletion region extends more deeply into the channel side due to the heavy gate doping and light channel doping.
  • The thickness of the depletion region can be increased by applying moderate reverse bias.
  • Increasing the reverse bias VGS will pinch-off the channel current. The channel resistance will be very high. This VGS at which pinch-off occurs is VP, the pinch-off voltage.
  • The source and drain are interchangeable, and the source to drain current may flow in either direction for low level drain battery voltage (< 0.6 V).

JFET Circuit Symbol

Figure (b) shows the schematic symbol for an N-channel field effect transistor compared to the silicon cross-section at (a).

  • The gate arrow points in the same direction as a junction diode.
  • The “pointing” arrow and “non-pointing” bar correspond to P and N-type semiconductors, respectively.

Discrete FETs

Discrete devices are manufactured with the cross-section as shown in figure but only the FET part as above image shows the FET along with Battery terminals.

FET ICs

All three FET terminals are available on the top of the die for the integrated circuit version so that a metalization layer (not shown) can interconnect multiple components.

  • Integrated circuit FET’s are used in analog circuits for the high gate input resistance.