Transcript chapter 5 - Portal UniMAP

CHAPTER 5
FIELD EFFECT TRANSISTORS(part a)
(FETs)
Introduction of FET
•
FETs (Field-Effect Transistors) are much like BJTs (Bipolar Junction
Transistors).
•
FETs sometimes called unipolar transistor operates only with one type
charge carrier.
•
The two main types of FETs are
JFET (Junction Field Effect
Transistor)
MOSFET (Metal Oxide
Semiconductor Field Effect
Transistor)
i. D-MOSFET –– Depletion MOSFET
ii. E-MOSFET –– Enhancement MOSFET
•
The terms ‘Field Effect’ relates to the depletion region formed in the
channel of a FET as a result of a voltage applied on one of its
terminal(gate).
FETs vs BJTs
Similarities:
•
Amplifiers
•
Switching devices
•
Impedance matching circuits
Differences:
FET
BJT
• Unipolar device – operate use only
one type of charge carrier
• voltage controlled devices
• higher input impedance
• Bipolar device – operate use both
electron & hole
• current controlled devices
• higher gains
•
FETs are less sensitive to temperature variations and because of there
construction they are more easily integrated on ICs.
•
FETs are also generally more static sensitive (faster when turn on and
off ) than BJTs.
•
FETs are usually smaller than BJTs and particularly useful for IC chips.
FETs vs BJTs
(a) Current-controlled and (b) voltage-controlled amplifiers.
BJT – current controlled, IC is direct
function of IB
FET – voltage controlled, ID is a direct
function of the voltage VGS applied to the
input circuit.
JFET
There are two types of JFET : n-channel and p-channel
A representation of the basic structure of the two types of JFET.
JFET Symbol
Basic Operation of a JFET

The channel width and the channel resistance can be
controlled by varying the gate voltage – controlling the
amount of drain current, ID.

The depletion region (white area) created by reverse bias.
Wider toward the drain-end of the channel – reverse-bias
voltage between gate and drain is greater than voltage
between gate and source.

JFET Analogy

JFET operation can be compared to a water spigot.
The source of water pressure is the
accumulation of electrons at the negative
pole of the drain-source voltage.
The drain of water is the electron
deficiency (or holes) at the positive pole
of the applied voltage.
The control of flow of water is the gate
voltage that controls the width of the nchannel and, therefore, the flow of charges
from source to drain.
Basic Operation of JFET
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JFET Characteristics and Parameters,
VGS = 0
Let’s first take a look at the effects with a VGS of 0V. ID increases proportionally
with increases of VDD (VDS increases as VDD is increased). This is called the ohmic
region (point A to B).
In this area (ohmic region) the channel resistance is essentially constant
because of the depletion region is not large enough to have sufficient effect 
VDS and ID are related by Ohm’s law
In JFET, IG = 0  an important characteristic for JFET
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JFET Characteristics and Parameters,
VGS = 0
At point B the ID cease to increase regardless of VDD increases. This called
pinch-off voltage.
As VDD increase from point B to point C, the reverse-bias voltage from gate to
drain (VGD) produces a depletion region large enough to offset the increase in
VDS, thus keeping ID relatively constant.
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JFET Characteristics and Parameters,
VGS = 0
Continue increase in VDS above the pinch-off voltage produces an almost constant
drain current  this drain current is IDSS (drain to source current with gate shorted).
Breakdown occurs at point C when ID begins to increase very rapidly with any further
increase in VDS. It can result irreversible damage to the device, so JFETs are always
operated below breakdown and within the constant-current area (between points B
and C on the graph)
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JFET action for VGS = 0V
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JFET Characteristics and Parameters,
VGS < 0
As VGS is set to increasingly more negative by adjusting VGG. A family of drain
characteristic curves is produced as shown in (b).
Notice that ID decrease as the magnitude of VGS is increased to larger negative 
causing the pinch-off is lowered as well (Boystead – lower in parabolic manner)
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JFET Characteristics and Parameters,
VGS < 0; VGS (off)
As VGS becomes more negative:
•
The JFET experiences
pinch-off at a lower voltage
(Vp).
•
ID decreases (ID < IDSS) even
though VDS is increased.
•
Eventually ID reaches 0A.
VGS at this point is called Vp
or VGS(off) ( VGS (off) = VP)
•
Take note at Ohmic &
Saturation Region
FLOYD  VGS (off) = - VP ; reverse polarity
Also note that at high levels of VDS the
JFET reaches a breakdown situation. ID
increases uncontrollably if VDS > VDSmax.
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JFET Characteristics and Parameters,
VGS < 0; VGS (off)
For cutoff voltage (VG(off)). The field (in white) grows such that it
allows practically no current to flow through.
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JFET Transfer Characteristics

The transfer characteristic of input-to-output is not as straightforward in
a JFET as it is in a BJT.

In a BJT,  indicates the relationship between IB (input) and IC (output).

IC = IB

In a JFET, the relationship of VGS (input) and ID (output) is a little more
complicated:
 V 
I D  I DSS  1  GS 

V P 

2
JFET Transfer Curve
This graph shows the value of ID for a given value of VGS.
When VGS = 0; ID = IDSS
When VGS = VGS (off) = VP; ID = 0 mA
Plotting the JFET Transfer Curve
Using IDSS and Vp (VGS(off)) values found in a specification sheet, the transfer curve
can be plotted according to these three steps:
Step 1
Solving for VGS = 0V

V
I D  I DSS  1  GS
VP

ID = IDSS
Step 2



2

V
I D  I DSS  1  GS
VP

Solving for VGS = Vp (VGS(off)) ID = 0A



2
Step 3
Solving for VGS = 0V to Vp

V
I D  I DSS  1  GS
VP




2
P-channel JFET operation

Same as n-channel JFET
except required negative
VDD and positive VGS.
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EXAMPLE
VGS(off)= -4V and IDSS=
12mA. Determine the
minimum value of VDD
required to put the device
in constant-current region
of operation when VGS=
0V.
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JFET Transfer Characteristic
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JFET Transfer Curve
This graph shows the value of ID for a given value of VGS.
When VGS = 0; ID = IDSS
When VGS = VGS (off) = VP; ID = 0 mA
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EXAMPLE
JFET with IDSS = 9 mA and VGS(off) = -8V (max). Determine
ID for VGS = 0V, -1V and -4V.
ANSWER:
VGS = 0V, ID = 9mA
VGS = -1V, ID = 6.89mA
VGS = -4V, ID = 2.25mA
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