JFET

Question 1

N-JFET characteristics : Resistencia variavel

Answer 1

The JFET can be used as a variable resistor, where VGS controls the drain-source resistance (rd). As VGS becomes more negative, the resistance (rd) increases.

Question 1

MESFET

Answer 1

– MESFET = Metal Semiconductor Field Effect Transistor = Schottky gate FET.

– The MESFET consists of a conducting channel positioned between a source and drain contact region.

– The carrier flow from source to drain is controlled by a Schottky metal gate.

– The control of the channel is obtained by varying the depletion layer width underneath the metal contact which modulates the thickness of the conducting channel and thereby the current.

– The operation is very similar to that of a JFET.

– The p-n junction gate is replaced by a Schottky barrier, and the lower contact and p-n junction are eliminated because the substrate is now a semi-insulating one.

Question 2

Estrutura

Answer 2

– GaAs MESFETs are the most commonly used

– The base material on which the transistor is fabricated is a GaAs substrate.

– A buffer layer is epitaxially grown over the GaAs substrate to isolate defects in the substrate from the transistor.

The channel or the conducting layer is a thin, lightly doped (n) conducting layer of semiconducting material epitaxially grown over the buffer layer.

Question 3

Operação do MESFET

Answer 3

-By applying a bias to the gate junction, the depletion depth and therefore the resistance of the current flow between the source and drain and the saturation current can be controlled

-If a large enough negative gate bias is applied, the depletion region depth will equal the channel depth, or the channel will be pinched off.

-For the gate to have effective control of the channel current, the gate length L must be larger than the channel depth, d.

Question 4

Enhancement JFET and MESFET

Answer 4

• So far, our n-type JFETs and MESFETs required negative VG to deplete channel charge and bring devices to OFF state (NORMALLY ON )
  -But if a lightly doped and narrow conducting channel is used it is possible that the depletion
  layer established by the built-in potential is wide enough to pinch off the channel without VG. (NORMALLY OFF OR ENHANCEMENT MODE FET)

Question 5

Channel length modulation in JFET and MESFET

Answer 5

When VD increases above VP, more free carriers are depleted from the channel. Length of depleted region increases and length of neutral channel decreases.