Upload
hortense-osborne
View
244
Download
1
Tags:
Embed Size (px)
Citation preview
Introduction to FET’s
Current Controlled vs Voltage Controlled Devices
Types of Field Effect Transistors (The Classification)
» JFET
MOSFET (IGFET)
n-Channel JFET
p-Channel JFET
n-Channel EMOSFET
p-Channel EMOSFET
Enhancement MOSFET
Depletion MOSFET
n-Channel DMOSFET
p-Channel DMOSFET
FET
Figure: n-Channel JFET.
The Junction Field Effect Transistor (JFET)
ELEC 121
JFET ConstructionThere are two types of JFET’s: n-channel and p-channel.
The n-channel is more widely used.
There are three terminals: Drain (D) and Source (S) are connected to n-channelGate (G) is connected to the p-type material
Gate
Drain
Source
SYMBOLS
n-channel JFET
Gate
Drain
Source
n-channel JFETOffset-gate symbol
Gate
Drain
Source
p-channel JFET
N-Channel JFET Symbol
Figure: n-Channel JFET and Biasing Circuit.
Biasing the JFET
Figure: The nonconductive depletion region becomes broader with increased reverse bias. (Note: The two gate regions of each FET are connected to each other.)
Operation of JFET at Various Gate Bias Potentials
Transfer CharacteristicsThe input-output transfer characteristic of the JFET is not as straight forward as it is for the BJT
In a BJT, (hFE) defined the relationship between IB (input current) and IC (output current).
In a JFET, the relationship (Shockley’s Equation) between VGS (input voltage) and ID (output current) is used to define the transfer characteristics, and a little more complicated (and not linear):
As a result, FET’s are often referred to a square law devices
2GS
D DSSP
V I = I 1 -
V
JFET Operating Characteristics
There are three basic operating conditions for a JFET:JFET’s operate in the depletion mode onlyA. VGS = 0, VDS is a minimum value depending on IDSS and the drain and source
resistanceB. VGS < 0, VDS at some positive value andC. Device is operating as a Voltage-Controlled Resistor
For an n channel JFET, VGS may never be positive*For an p channel JFET, VGS may never be negative*
Saturation
At the pinch-off point: • any further increase in VGS does not produce any increase in ID. VGS at pinch-off is denoted as Vp. • ID is at saturation or maximum. It is referred to as IDSS. • The ohmic value of the channel is at maximum.
Specification Sheet (JFETs)