FIELD EFFECT TRANSISTOR

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FIELD EFFECT TRANSISTOR

• Characteristics

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What you need to know?

• What FET stands for?
• How many terminals does a FET have?
• What is those terminals?
• What are the differences between BJT and FET?
• Why FET is known as voltage controlled device?
• How many type of FETs channels available in use?
• Why FET is a unipolar device? (e.g. compared to
  BJT as bipolar device?

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What you need to know?

• FET stands for Field Effect Transistor
• FET has 3 terminals
• Those terminals are gate, source, drain
• To be discuss!!
• Voltage controlled device VGS applied will
  affect the ID values
• FET has 2 type of channels n-channel, p-channel
• FET depending solely on EITHER electron
  (n-channel) or hole (p-channel) as charge carrier

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What you need to know? -Symbols and short forms

• VGS gate to source voltage
• VDS drain to source voltage
• VGG voltage supply at gate terminal
• VDD voltage supply at drain terminal
• VP pinch off voltage
• ID drain current
• IDSS drain to source saturation current
• n-channel, n-type material
• p-channel, p-type material
• rd FET resistance when VGS 0
• ro FET resistance at a particular level of VGS

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What you need to understand?-Related to
SEMICONDUCTOR

• P-N junction
• Depletion region
• Forward biased
• Reversed biased
• Magnetic principles same charges repel, opposite
  charges attract
• N-type material has electron as majority charge
  carrier and hole as minority charge carrier
• P-type material has hole as majority charge
  carrier and electron as minority charge carrier

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DEPLETION REGION

• Depletion layer appears when positive and
  negative ions are created at the pn-junction due
  to free electrons and holes combinations

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FORWARD-BIAS

• Forward-bias is established by applying the
  positive potential to the p-type material
  negative potential to the n-type material.
• This will pressure electrons in n-type material
  holes in p-type material to recombine with ions
  near p-n junction and reduce the depletion region
  width.

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REVERSE-BIAS

• Reverse-bias is established by applying the
  positive potential to the n-type material
  negative potential to the p-type material.
• This will attract the electrons in n-type
  material towards end of positive supply terminal
  and thus widening the depletion region width.

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Construction of fets

• 3 type of FET will be discussed in this subject
• Junction Field Effect Transistor (JFET)
• i) n-channel
• ii) p-channel
• Depletion-type MOSFET
• i) n-channel
• ii) p-channel
• Enhancement-type MOSFET
• i) n-channel
• ii) p-channel

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JUNCTION FIELD EFFECT TRANSISTOR (jfet)

• JFET n-channel type has n-type channel which
  connecting drain (D) and source (S).
• The main charge carrier for this type of channel
  is electron.
• The depletion layer appears along the p-type
  material at gate (G) and n-channel.

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JUNCTION FIELD EFFECT TRANSISTOR (jfet)

• JFET p-channel type has p-type channel which
  connecting drain (D) and source (S).
• The main charge carrier for this type of channel
  is hole.
• The depletion layer appears along the n-type
  material at gate (G) and p-channel.

p-channel
p
n
n
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DEPLETION-TYPE MOSFET

• Depletion-type MOSFET n-channel type has p
  substrate, one n-doped region on each source and
  drain terminal, and an n-channel connected both
  n-doped regions.
• All terminals (drain, gate, source) are connected
  through metal contact.
• The gate is insulated from the n-channel using a
  thin SiO2 layer.

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DEPLETION-TYPE MOSFET

• Depletion-type MOSFET p-channel type has n
  substrate, one p-doped region on each source and
  drain terminal, and an p-channel connected both
  p-doped regions.
• All terminals (drain, gate, source) are connected
  through metal contact.
• The gate is insulated from the p-channel using a
  thin SiO2 layer.

p-channel
p
n-
p
p
p-doped regions
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ENHANCEMENT-TYPE MOSFET

• Enhancement-type MOSFET n-type has p substrate,
  one n-doped region on each source and drain
  terminal.
• Channel is not present between n-doped regions
• All terminals (drain, gate, source) are connected
  through metal contact.
• The gate is insulated from the n-channel using a
  thin SiO2 layer.

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ENHANCEMENT-TYPE MOSFET
p

• Enhancement-type MOSFET p-type has n substrate,
  one p-doped region on each source and drain
  terminal.
• Channel is not present between p-doped regions
• All terminals (drain, gate, source) are connected
  through metal contact.
• The gate is insulated from the n-channel using a
  thin SiO2 layer.

p
n
p
p
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JFET n-CHANNEL
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What you need to understand?JFET Operating
Characteristics

• There are three basic operating conditions for a
  JFET
• A. VGS 0, VDS increasing to some positive
  value
• B. VGS lt 0, VDS at some positive value
• C. Voltage-Controlled Resistor

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What you need to understand?JFET Operating
Characteristics

• A. VGS 0, VDS increasing to some positive
  values

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What you need to understand?JFET Operating
Characteristics

• Refer to the figure, ID flows from positive
  supply to the drain terminal of the JFET.
• Refresh back formula VIR as electrons (charge
  carriers) flow, there is a voltage drop along the
  n-channel.
• We can see a higher voltage near the drain
  terminal than at the source terminal.
• A higher supply for reverse-bias near the drain
  terminal.

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What you need to understand?JFET Operating
Characteristics

• A. VGS 0, VDS increasing to some positive
  values

2V
1.5V
2V
1V
0.5V
0V
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What you need to understand?JFET Operating
Characteristics

• A. VGS 0, VDS increasing to some positive values

Ohmic region
Active region
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What you need to understand?JFET Operating
Characteristics

• A. VGS 0, VDS increasing to some positive
  values

• The charge carriers in this circumstance are
  electrons.
• As the VDS is increased from 0 to higher volts,
  the ID current will increase
• Look at the drain curve at previous slide.
• As VDS increased and approaches VP value, the
  depletion regions will widen and almost touching.
• At the same time, the conducting n-channel
  becomes narrower and therefore prevents a further
  ID current from increases.
• in logic, the channel should cut off and no
  more current could flow, but in fact, ID maintain
  at saturation level because ID keep flowing (a
  very small channel still exist) at very high
  density.
• Any further increment in VDS will not increase
  the ID value.

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What you need to understand?JFET Operating
Characteristics

• B. VGS lt 0, VDS at some positive values

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What you need to understand?JFET Operating
Characteristics

• B. VGS lt 0, VDS at some positive value

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What you need to understand?JFET Operating
Characteristics

• C. Voltage Controlled Resistor (VDS lt VP)

• The region to the left of pinch-off locus is
  called ohmic region or voltage controlled
  resistor region.
• In this region, JFET can be used as a variable
  resistor (for automatic gain control system).
• The resistance values is controlled by VGS.
• When VGS more negative, the curve slopes become
  more horizontal.
• This is a sign of an increasing resistance level.

VGS 0 V
VGS -1 V
VGS -2 V
VGS -3 V
VDS (V)
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What you need to understand?JFET Operating
Characteristics
C. Voltage Controlled Resistor (VDS lt VP)

• To show the resistance increased when the slope
  of the curve become more horizontal
• 1) refresh equation ymxc and VIR
• 2) from drain curve we can see ID at y-axis and
  VDS at x-axis.
• 3) then from eq VIR, we can manipulate the
  equation becoming IV/R
• 4) the eq IV/R is equal to I(1/R)V
• where I at y-axis
• V at x-axis
• 1/R slope
• 5) as the slope become more horizontal by
  reducing VGS values, R value is increasing
  because slope 1/R

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What you need to understand?JFET Operating
Characteristics
C. Voltage Controlled Resistor (VDS lt VP)

• The following equation will provide a good
  approximation to the resistance level in terms of
  applied voltage VGS.
• Where r0 is the resistance with VGS 0 V and
• rd is the resistance at particular level of
  VGS

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What you need to understand?JFET Operating
Characteristics
Transfer Characteristics

• Transfer characteristic for FET is different from
  BJT.
• For BJT the relationship is shown below
• But for JFET, the relationship between ID and VGS
  is defined by Shockleys equation
• We can see here, the relationship is not linear
  between input and output quantities.

Control variable
Constant
Control variable
Constants
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What you need to understand?JFET Operating
Characteristics

• Plotting the transfer curve (ID vs VGS)

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What you need to understand?JFET Operating
Characteristics

• Steps to plot the transfer curve (ID vs VGS)

At y-axis VGS 0 V
At x-axis ID 0 mA

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JFET BIASING
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What you need to understand?JFET biasing

• Purpose of biasing
• To select the proper dc gate-to-source voltage to
  establish a desired value of drain current and
  thus a proper Q-point
• 2 types of bias circuits
• Self bias
• Voltage divider bias

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Exercise 1

• Determine ID and VGS for the JFET with the
  voltage divider in the figure, given that for
  this particular JFET the internal values are such
  that VD 7V

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What you need to understand?JFET biasing

• Steps to determine the Q-point
• VGS -IDRS
• Calculate VGS when ID is zero
• Get IDSS from data sheet
• Calculate VGS when IDIDSS
• Draw the load line
• The point where the line intersects the transfer
  characteristic curve is the Q-point

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DEPLETION-TYPE MOSFET n-TYPE
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What you need to understand?Depletion-type
mosfet Operating Characteristics

• A. VGS 0, VDS increasing to some positive
  values

VDS
VGS 0V
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What you need to understand?Depletion-type
mosfet Operating Characteristics

• Plotting the transfer curve (ID vs VGS)

At y-axis VGS 0 V
At x-axis ID 0 V
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What you need to understand?Depletion-type
mosfet Operating Characteristics

• B. VGS lt 0, VDS increasing to some positive values

VDS
VGS 0V to VP
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What you need to understand?Depletion-type
mosfet Operating Characteristics

• Plotting the transfer curve (ID vs VGS)

At depletion region VGS 0 V to VP
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What you need to understand?Depletion-type
mosfet Operating Characteristics
B. VGS lt 0, VDS increasing to some positive
values Discussion on Drain Curve and Transfer
Curve

• By reducing VGS values below than 0 V, the
  n-channels width that connecting the n-region at
  drain (D) terminal and n-region at source (S)
  terminal, will then reduced.
• The reason is the negative supply at the gate
  will force the free moving electrons (same
  charges repel) along n-channel near the
  p-substrate. At the same time, holes from
  p-material substrate will attract towards gate
  (opposite charges attract).
• Hence, the recombination process occurs between
  electrons from n-channel and holes from the
  p-substrate.
• The effect is, the n-channel width will be
  reduced because less number of free electrons
  available for conduction.

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What you need to understand?Depletion-type
mosfet Operating Characteristics

• C. VGS gt 0, VDS increasing to some positive values

VDS
n
VGS 1V
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What you need to understand?Depletion-type
mosfet Operating Characteristics

• Plotting the transfer curve (ID vs VGS)

At enhancement region VGS gt 0 V
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What you need to understand?Depletion-type
mosfet Operating Characteristics
C. VGS gt 0, VDS increasing to some positive
values Discussion on Drain Curve and Transfer
Curve

• As the VGS values increase above 0 V, the gate
  terminal for the MOSFET is supplying positive
  voltage.
• This positive gate will draw additional electrons
  from the p-substrate due to reverse leakage
  current.
• By having more electrons as charge carriers,
  drain current will increase rapidly.
• Due to the rapid rise, we must be aware of the
  maximum drain current rating since it could be
  exceeded with a positive gate voltage.
• In this circumstance, the Shockleys equation
  will continue to be applicable in both depletion
  (VGS lt 0V) and enhancement (VGS gt 0V) regions.

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What you need to understand?Depletion-type
mosfet Operating Characteristics

• Drain Curve and Transfer Curve (n-channel)

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Exercise

• The data sheet for a certain D-MOSFET gives
  VGS(off) -5V and IDSS 8mA.
• Plot the transfer characteristic curve.
• Hint VGS(off) VP
• use Shorthand Method

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DEPLETION-TYPE MOSFET p-TYPE
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What you need to understand?Depletion-type
mosfet Operating Characteristics

• Drain Curve and Transfer
• Curve (p-channel)

Depletion mode
VGS gt 0V
VDS gt 0V
VDS gt 0V
VDS gt 0V
VGS lt 0V
VGS 0V
Enhancement mode
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What you need to understand?Depletion-type
mosfet Operating Characteristics

• Drain Curve and Transfer Curve (p-channel)

Enhancement mode
For VGS 0V
Depletion mode
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ENHANCEMENT-TYPE MOSFET n-TYPE
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What you need to understand?ENHANCEMENT-type
mosfet Operating Characteristics
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What you need to understand?ENHANCEMENT-type
mosfet Operating Characteristics
VGS gt VT, VDS increasing to some positive values
VDS
VDS
VGS gt VT
VGS 0V
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What you need to understand?ENHANCEMENT-type
mosfet Operating Characteristics
VGS gt VT, VDS increasing to some positive values

• For enhancement-type MOSFET, the drain current,
  ID remains cut off until VGS reaches a specific
  magnitude, which known as threshold voltage, VT.
• Saturation level for VDS is also defined as
• VDSsat VGS VT
• This type of transistor only operates in
  enhancement mode in which VGS values must
  exceeding VP that is higher than 0V (for n-type
  material).
• The transfer curve for the MOSFET is defined by
  different equation as given below

where
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What you need to understand?ENHANCEMENT-type
mosfet Operating Characteristics

• VGS gt VT, VDS increasing to some positive values

VDS (8-2)V
Given that VT 2V
VDS 6V
VDS (7-2)V
VDS 5V
VDS (6-2)V
VDS 4V
VDS (5-2)V
VDS 3V
VDS (V)
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What you need to understand?ENHANCEMENT-type
mosfet Operating Characteristics
VGS gt VT, VDS increasing to some positive values
VDS
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ENHANCEMENT-TYPE MOSFET p-TYPE
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What you need to understand?ENHANCEMENT-type
mosfet Operating Characteristics
VGS gt VT (more negative than), VDS decreasing
to some negative values
VDS
VGS gt VT
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What you need to understand?ENHANCEMENT-type
mosfet Operating Characteristics
VGS gt VT (more negative than), VDS decreasing
to some negative values
VDSsat VGS VT
VDSsat -6V (-2)V
VDSsat -4V
VDSsat -5V (-2)V
VDSsat -3V
VDSsat -4V (-2)V
VDSsat -2V
VDS (V)
-1 -2 -3 -4
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What you need to understand?ENHANCEMENT-type
mosfet Operating Characteristics
VGS gt VT (more negative than), VDS decreasing
to some negative values
Given that VT -2V
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The End