Field Effect Transistor (FET) Basics

Field Effect Transistor (FET)

• "Field effect" relates to the depletion region formed in the channel of a FET when voltage is applied on one of its terminals (gate).

Types of FETs

• Two main types of FETs are:
  • Junction Field Effect Transistor (JFET)
  • Metal Oxide Semiconductor Field Effect Transistor (MOSFET)

JFET

• JFET has two channel types:
  • n channel
  • p channel

JFET Structures and Symbols

• JFET structures and symbols are represented in a diagram.

Basic Operation of JFET

• VDD provides a drain-to-source voltage and supplies current from drain to source.
• VGG sets the reverse-bias voltage between gate and source.
• JFET is always operated with the gate-source pn junction reverse biased.
• Reverse-bias of gate-source junction with negative gate voltage produces a depletion region along pn junction, increasing resistance by restricting the channel width.
• Greater VGG narrows the channel, increasing the resistance of the channel and decreasing ID.
• Less VGG widens the channel, decreasing the resistance of the channel and increasing ID.
• The channel width and resistance can be controlled by varying the gate voltage, controlling the amount of drain current, ID.

Drain Characteristic

• JFET with VGS=0 V and variable VDS (VDD) shows a drain characteristic curve.
• Pinch-off occurs where constant current begins.

JFET Characteristic, VGS = 0

• As VDD (and thus VDS) is increased from 0V, ID will increase proportionally in the ohmic region.
• In this area, the channel resistance is essentially constant because of the depletion region.
• IG = 0, an important characteristic for JFET.
• At point B, the curve levels off and enters the active region where ID is constant.
• The value of VDS at which ID becomes constant is the pinch-off voltage, VP.
• As VDD increases from point B to point C, the reverse-bias voltage from gate to drain produces a depletion region large enough to offset the increase in VDS, keeping ID relatively constant.

VGS = 0

• VDS increases above VP, produce almost constant ID called IDSS.
• IDSS (drain to source current with gate shorted) is the maximum drain current at VGS = 0V.
• Breakdown occurs at point C when ID begins to increase very rapidly with any further increase in VDS, which can result in irreversible damage to the device.
• JFETs are always operated below breakdown and within the constant-current area.

VGS Controls ID

• As VGS is set to increasingly more negative values by adjusting VGG, a family of drain characteristic curves is produced.
• ID decreases as the magnitude of VGS is increased to larger negative values because of the narrowing of the channel.
• For each increase in VGS, the JFET reaches pinch-off at values of VDS less than VP.

JFET at Cutoff

• The more negative VGS is, the smaller ID becomes in the active region.
• When VGS has a sufficiently large negative value, ID is reduced to zero.