exam for Electronic Engineering
Jocelyn yang 
EE1301Session19-SemiconductorTechnology-FETs-Class.pdf
3/5/2018
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EE 1301: MODERN ELECTRONIC TECHNOLOGY
SESSION #19: FIELD EFFECT TRANSISTORS 3/05/2018
Instructor: Joseph Cleveland, Ph.D. Email: [email protected]
Thought for Today
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Bipolar Junction Transistor
(Review)
npn Bipolar Junction Transistor
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Collector
Base
Emitter
The space between pn junctions is very thin so electrons pulled into the base spill into collector region~0.7 V
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pnp Bipolar Junction Transistor
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Collector
Base
Emitter The space between pn junctions is very thin so holes pulled into the base spill into collector region
~0.7 V
npn Transistor Circuit
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Electron flow
Current in the base controls the electron flow through the collector
FWD
REV
~0.7 V
collector
emitter
The transistor appears to be a variable resistor to the battery connected to the collector.
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pnp Transistor Circuit
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e-
e- Ic
IeIb
• Reverse the two p-n sections – Note the battery
polarities
• Base-emitter
FWD bias
• Base-collector
REV bias
~0.7 V
Transistor Switch Circuit
• We now have a way to make a low power, low cost, small “relay”
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Ic
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Common Emitter Amplifier
• When the voltage on the base increases, the collector current increases
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If ,
If . , ,
Ic 10 K
Common Emitter Amplifier …
• When the voltage on the base decreases the collector current decreases
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gets larger
Ic
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Common Emitter Amplifier …
• Let’s apply a musical tone to the input
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A small input signal
produces a large output signal
Ic
MP3 Signal
Inverting Amplifier
Amplification Application
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“Public Address” before electronic active devices
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Amplification Application
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“Public Address” after electronic active devices
~10mV ~100V “Amplifier”
What electronic devices are needed for this function?
BJT Properties
Advantages • Better fidelity
• High gain
• High speed
• Can operate well at high current - power
Disadvantages • Uses more power
• Inverse I/O operation
• Highly variable device- to-device properties
• Difficult to make million- BJT on a single wafer
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Need a different technology for digital computers
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Field Effect Transistors
Field Effect Transistor
• 1925: Julius Edgar Lilienfeld who was an Austro-Hungarian physicist. He received a patent for this in 1930
• The technology did not exist at the time to fabricate it.
• It could not be fabricated until about 1950
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Field Effect Transistor (FET) …
• Advantages over BJTs – Much smaller than an equivalent BJT transistor
– Lower power consumption and power dissipation
– Much smaller transistor variability
– Better for use in integrated circuits and digital computers
• There are two main types of FETs: – JFETS (junction FETs)
– MOSFETs (metal-oxide-semiconductor FETs)
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Junction Field Effect Transistor (JFET)
• The JFET uses the voltage that at the Gate to control the current flowing through a channel.
• The more negative the gate-source voltage, the wider the depletion layer and narrower the channel: less drain current
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Gate
Drain Source
GateDepletion region
N-channel JFET e-
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FET Operation
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Increasing the reverse bias on the Drain- Gate junction is like squeezing the hose:
JFET Switch
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ON: Depletion layers shrink to widen the channel to allow conduction
OFF: Depletion layers expand to pinch off the channel
VDS >>VPinch
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JFET Structure and Symbols
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JFET Amplifier Circuit
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IsI2
so GATE is reverse biased
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MOSFET
• Under the gate is an oxide film that serves as an insulator.
• When there no voltage between the gate and source, the P-type semiconductor separating the source and the drain acts as an insulator.
• Current does not flow between the source and the drain.
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• 1959: MOSFET
Enhancement Mode MOSFET
• When a voltage is applied at the gate it attracts and pulls up free electrons that are just under the gate, resulting in an abundance of these electrons in the area between the source and the drain.
• These electrons form a channel over which current can now flow freely.
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Enhancement Mode MOSFET Switch
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OFF: With VGS 0V, there are very few electrons in the channel to allow conduction
ON: With VGS > 0V, there are many electrons in the channel to allow conduction
MOSFET Symbols
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Using Semiconductor Technology
• How do semiconductor switches to store data and perform calculations?
• How can we use switches to represent numbers and characters?
• How do we assemble switch circuits in order for work to be done?
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End of Session
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