LECTURE # 8 FIELD EFFECT TRANSISTOR (FET) By: Muhammad Zahid

FIELD EFFECT TRANSISTOR (FET) FET is uni-polar device. Its operation depends on only one type of charge carriers (hole or electrons) . It is a Voltage controlled Device (gate voltage controls drain current)

FIELD EFFECT TRANSISTOR (FET) Few important advantages of FET over conventional Transistors. Very high input impedance (109-1012 ) Source and drain are interchangeable Low Voltage Low Current Operation is possible (Low-power consumption) Less Noisy No minority carrier storage (Turn off is faster) Very small in size, occupies very small space in ICs

FIELD EFFECT TRANSISTOR (FET) Types of Field Effect Transistors:- JFET MOSFET n-Channel JFET p-Channel JFET FET FET FET FET FET Enhancement MOSFET Depletion MOSFET n-Channel EMOSFET n-Channel DMOSFET p-Channel DMOSFET p-Channel EMOSFET

FIELD EFFECT TRANSISTOR (FET) Construction of junction Field Effect Transistor:- A single semiconductor substrate, doped to form p region & n region as shown in diagram. Two p-type regions are diffused in the n-type material to form a channel, and both p-type regions are connected to the gate lead. There are three terminals: Drain (D) and Source (S) are connected to n-channel Gate (G) is connected to the p-type material

FIELD EFFECT TRANSISTOR (FET) Operation of junction Field Effect Transistor:- The JFET is always operated with the gate-source pn junction reverse-biased. Reverse biasing of the gate-source junction with a negative gate voltage produces a depletion region along the pn junction,

FIELD EFFECT TRANSISTOR (FET) The depletion region extends into the n channel and thus increases its resistance by restricting the channel width. The channel width is controlled by varying the gate voltage, thereby controlling the amount of drain current, ID.

FIELD EFFECT TRANSISTOR (FET) SYMBOLS:- Gate Drain Source Gate Drain Source p-channel JFET n-channel JFET

FIELD EFFECT TRANSISTOR (FET) JFET CHARACTERISTICS AND PARAMETERS:- Consider the case when the gate-to-source voltage is zero (VGS= 0 V). As VDD is increased from 0V, ID will increase proportionally.

FIELD EFFECT TRANSISTOR (FET) PINCH-OFF VOLTAGE:- For Vgs = 0V, the value of Vds at which ID becomes essentially Constant is the pinch-off voltage, Vp. For a given JFET, Vp has a fixed value.

FIELD EFFECT TRANSISTOR (FET) A continued increase in VDS above the pinchoff voltage produces an almost constant drain current. This value of drain current is called IDSS (Drain to Source current with gate Shorted)

FIELD EFFECT TRANSISTOR (FET) BREAKDOWN:- When the voltage beyond specified limits breakdown occurs. At breakdown ID begins to increase very rapidly with out further increase in VDS.

FIELD EFFECT TRANSISTOR (FET) Breakdown can result in irreversible damage to the device. JFETs are always operated below breakdown within the active region (constant current).

FIELD EFFECT TRANSISTOR (FET) Cutoff Voltage:- The value of VGS that makes ID approximately zero is the cutoff voltage, VGS(off) The JFET must be operated between VGS 0V and VGS(off). ID will vary from a maximum of IDSS to a minimum of almost zero.

FIELD EFFECT TRANSISTOR (FET) JFET Universal Transfer Characteristic:- As VGS control ID, the change in ID with change in VGS is shown in general transfer characteristic curve graphically This curve is also known as a transconductance curve.

FIELD EFFECT TRANSISTOR (FET) JFET Universal Transfer Characteristic:- JFET transfer characteristic curve is expressed approximately as

FIELD EFFECT TRANSISTOR (FET) Example:- The datasheet for a 2N5459 JFET indicates that typically IDSS 9 mA and VGS (off)= -8V(maximum). Using these values, determine the drain current for 0V, -1V & -4V