Field Effect Transistors

1
Field Effect Transistors

• Characteristics
• Common type of transistor, just like the bipolor
  ones covered in the previous section
• Use primarily where extremely high input
  impedance is required
• Most of today's transistors are "MOS-FETs", or
  Metal Oxide Semiconductor Field Effect
  Transistors.
• Types Covered
• JFET
• MOSFET
• VMOSFET

2
Field Effect Transistors

• JFET
• Theory of Doped Silicon
• Voltage across a lightly Doped silicon
• Small current flows
• Electrons enter the silicon through the Source
• Electrons exit through the Drain
• Bar of silicon acts as a resistor.
• Resistance dependent upon
• Amount of impurities (doping in the silicon) in
  the silicon bar
• Length and cross sectional area if the silicon
  bar

3
Field Effect Transistors

• JFET
• Theory of Doped Silicon w/Gate
• PN Junction on the side of the bar
• Called a Gate
• The area around the gate has few electrons and
  is called the depletion region
• Acts as a good insulator
• Reverse Biasing the PN Junction
• Increases the size of the depletion region
• Increases the resistance of the silicon bar
• The more reverse biasing voltage the greater the
  resistance
• Thus the current through the silicon bar can be
  varied by varying the biasing voltage

4
Field Effect Transistors

• JFET
• Comparison parameter - Transconductance (gm)
• Ratio of change in drain current
• to a change in gate-to-source voltage
• Units siemens (aka mho)
• Typically in µS or µmho
• Example A 2v change in VGS causes a 5mA change
  in ID
• Find gm
• Circuit Symbols

5
Field Effect Transistors

• JFET
• Circuit Symbols
• Arrows indicate direction of conventional current
  flow, thus the polarity required to reverse bias
  the junction
• Characteristic curve for a typical N-channel JFET
• Above 4 V VDS and less than VGS 0V
• ID is at its max value
• ID is controlled by changes in VGS
• Sample Problem
• Use chart and determine gm
• when changes from -2 to -3V

6
Field Effect Transistors

• JFET Biasing
• A separate supply could be used to bias the JFET
  but self-biasing by using a Source resistor is
  more economical
• Sizing the Source resistor (RS)
• Sample Problem
• Given Desired VGS -3V and ID 2mA
• FIND RS
• In Class exercises
• Page 83 4-2, 4-5, 4-6, 4-7

7
Field Effect Transistors

• JFET as an AC Amplifier
• Example Circuit
• With the circuit shown and the desire to prevent
  signal clipping
• Assume ID 2mA THEN VD VDD - ID RD 10V
• The input signal VS is coupled through CC

then
and
then
and
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Field Effect Transistors

• JFET as an AC Amplifier
• Example Circuit
• With the previous circuit shown and gm 3000µmho
• Add a load Resistor and coupling Cap
• The Gain changes to AV gm rL
• NOTICE gm rL NOT gm RL
• RL and RD are in parallel to make rL which is for
  AC signals
• Input resistance
• Since the Gate-to-Source
• resistance is so high the Amp
• Input resistance matches the
• resistor just before the gate

9
Field Effect Transistors

• JFET as an AC Amplifier
• In-class Exercises
• Page 84. Problems 4-10 and 4-12
• Metal Oxide Semiconductor Field Effect Transistor
  (MOSFET)
• Current through the device is controlled by the
  Gate voltage
• Construction is similar to IC construction in
  that the various layers are individually
  deposited

10
Field Effect Transistors

• MOSFET
• Types
• Depletion Mode
• Enhanced Mode
• Most Common
• Covered in this course
• Layers (continued)
• N layers are heavily doped
• N- areas are lightly doped
• Operation
• W/ 0volts on the gate the isnt enough electrons
  in the space between the source and drain for any
  significant current to flow

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Field Effect Transistors

• MOSFET
• Operation
• W/ voltage on the gate electrons from the
  substrata are drawn into the channel between the
  Drain and Source
• This enhancement makes the channel conductive
• Current flows between the Source and Drain
• Enhanced Mode Device
• No current flows through the gate
• The more positive the Gate voltage the higher the
  current

Demo http//www.pbs.org/transistor/science/info/t
ransmodernex.html
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MOSFET Symbols
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Field Effect Transistors

• MOSFET As Small Signal Amplifier
• Practically no drain current flows w/ 0V on gate
• VO depends upon
• How high the gate voltage goes
• Value of RD
• How low of resistance that the MOSFET appears to
  have
• RD and the MOSFET act as a voltage divider

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Field Effect Transistors

• MOSFET As Small Signal Amplifier
• Practically no drain current flows w/ 0V on gate
• Typical
• With a reasonable
• digital input of 5V
• VO would almost
• zero
• With a reasonable input
• of 0V
• VO would almost 15V
• The range is much
• better than for a bipolar
• transistor

of
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Field Effect Transistors

• Power MOSFET
• Typical non-power MOSFETs develop only a narrow
  channel between Source and Drain
• Typical only useful in low power applications due
  to the resistance of the channel
• Several Different types are used that have much
  less resistance Source to Drain
• Types
• Lateral Double Defused MOSFET (LDMOSFET)
• Shorter Channel between Source
• and Drain Less Resistance
• Thus higher currents

16
Field Effect Transistors

• Power MOSFET
• Several Different types (continued)
• Types
• Vertical MOSFET or V-Channel MOSFET
• (VMOSFET)
• HAS a shorter/wider channel
• Lower resistance
• Thus more current flows
• Has two Source and a Gate connection on top
• Drain on the bottom
• Lower Capacatance
• TMOSFET
• Similar to the VMOSFET
• No V Channel

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Field Effect Transistors

• Power MOSFET
• Several Different types (continued)
• Types
• TMOSFET
• Gate is embedded in the Silicon
• Dioxide Layer
• The contact from the gate is over a wider area
  than for the non-power MOSFETs
• Has smaller physical size than VMOSFETs
• Key Characteristics
• Lower Source to Drain resistance
• Higher currents
• The text covers VMOSFET in more detail

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Field Effect Transistors

• Vertical MOSFET
• Becomes conductive
• with a charge on the
• Gate
• Channel resistance
• increases with
• temperature, thus
• VMOSFETs dont succumb to thermal runaway
• Much higher current capacity than traditional
  MOSFETs
• Example IRF-100 N-Channel MOSFET ID Max 16A
  and typical transconductance of 3 mhos

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VMOSFET Package Curves
IRF-100
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Field Effect Transistors

• Vertical MOSFET
• Symbol
• Same as for a plain MOSFET
• Like other MOSFETs these are susceptible to
  static discharges
• Many VMOSFETS have a Zener diode built in to
  protect the against static discharge
• Zeners act as a like an
• open circuit when
• reversed biased with an
• appropriate voltage
• If the gate becomes
• negative, the zener
• will conduct
• Best to have a 1kO on gate

21
Field Effect Transistors

• Vertical MOSFET
• Typical MOSFET Power Interface
• Figure 4-16 on page 81
• Gate signals can come from a digital circuit,
  e.g., microcontrollers, microprocessors, TTL
  circuits
• L1 is a solenoid coil
• D1 and R1 are used to prevent coil kickback
  voltage from growing high enough to damage Q1 or
  other components
• Normally reversed biased when solenoid is
  energized
• Kickback voltage can forward bias D1
• Operation
• W/gate at a logic low (approx 0 VDC) Q1 is off
  and L1 is off
• W/gate at a logic level 1 (5 VDC) Q1 is on and
  L1 is energized

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Field Effect Transistors

• Vertical MOSFET
• Troubleshooting Tips
• If a bad MOSFET is found, check Diode before
  replacing the MOSFET and applying power
• Typical FET Circuits
• Q9 on Next Slide and Figure 4-17 on page 82
• Used as a Noise amplifier
• Biasing
• Current flow through Source resister R142
• Gate voltage held at 0 VDC by R58
• Input signal coupling
• Through C112 and coil L20
• Higher impedance for higher frequency

23
Courier Cruiser CB Circuit

• Typical FET Circuits
• Q9 on Next Slide and Figure 4-17 on page 82
• Output drives Q10 (looks like Q1)

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Field Effect Transistors

• Typical FET Circuits
• Q1 on Next Slide and Figure 4-18 on page 82
• Dual Gate MOSFET used as a amplifier
• Drain current controlled by a combination of the
  two gates
• Biasing
• Positive voltage on G2 provides biasing
• Feed by AGC through R49
• When Input amplitude increases AGC voltage on G2
  decreases
• When Input amplitude decreases AGC voltage on G2
  increases
• Biasing controls Q1 gain
• Q1 Gain decreases when voltage on G1 decreases
• Q1 Gain increases when voltage on G1 increases
• Thus Output signal stays constant with changes
  in Input
• AGC increases gain of Q1 when the input on G1
  becomes weaker

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Courier Cruiser CB Circuit