Transcript OtherTransistorTopologies30MarchAnd1April

Other Transistor Topologies
30 March and 1 April 2015
JFET
Junction Field
Effect Transistor
• The two gate terminals are tied together to form single gate connection; the
source terminal is grounded
• The flow of electric charge through a PN JFET is controlled by constricting the
current-carrying channel; the width of the channel is controlled by the gate voltage
through varying the depletion region at the PN junction at the interface between
the gate and the channel
• The current also depends on the electric field between source and drain
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Structure of JFET
N-channel JFET
• a long channel of n-type (N-channel) or p-type (p-channel) semiconductor. Two
ohmic contacts with each at one end of the channel: the source and the drain
• The gate (control) terminal has doping opposite to that of the channel, so there is a
PN junction at the interface between the junction and the channel. The contact from
gate to outside is also ohmic.
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I-V relationship of PN JFET
Nonsaturation region
(linear region)
Saturation region
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Other Features of PN JFET
• JFET is unipolar device since only majority carriers transport in the channel
• The source and drain region are interchangeable
• N-channel devices have greater conductivity than p-channel types, since
electrons have higher mobility than holes
• The gate current is approximately zero since the PN junction is reverse
biased
Symbols of JFET (arrow represents the polarity of the PN junction)
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Comparisons of Transistors
BJT
Structure
NPN: n++p+n
PNP: p++n+p
Current
transport
Diffusion
Carriers
involved in
current
transport
Bipolar: electrons
and holes
Current at
terminals
IC=βIB= (1/α) IE
(Forward active
mode)
IC=f(VBE, VBC)
MOSFET
n(p)-type inversion layer
structure as channel
from S to D
Drift
Unipolar
NMOS: electrons
PMOS: holes
IG=0
ID=f(VGS, VDS)
JFET
Inversed biased PN junction
between the gate and channel
from S to D
Drift
Unipolar:
N-channel: electrons
P-channel: holes
IG≈0
ID=f(VGS, VDS)
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Comparisons of Transistors (Cont’d)
BJT
MOSFET
JFET
Symbols
NPN
Applications
PNP
• Current-controlled
current amplifier
• Switch for digital
signal
• Discrete circuits
NMOS
PMOS
• Voltage-controlled
current amplifier
• Switch for digital
signal
• IC circuits
N-channel
P-channel
• Voltage-controlled current amplifier
• Switch for digital signal
• IC circuits
• MESFET can be used in higherfrequency than PN-JFET due to higher
electron mobility in GaAs
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