Field effect transistor

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2. • So far we have learnt about transistors which are current controlled
devices
• Their characteristics are controlled by base current
• But in a FET the characteristics are controlled by the input voltage
• This is the biggest difference between transistors and FETs
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5. • JFET stands for Junction Field Effect Transistor
• Here the current conduction is either by electrons or holes
• It is controlled by the electric field between gate and conduction
channel
• JFET has high input impedence and low noise level
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6. • A JFET consist of P-type or N-type silicon bar containing two PN-
junctions at the sides
• The bar forms the conduction channel
• The two PN-junctions are connected internally and a common
terminal called the gate is taken out
• A JFET has three terminals : Gate, Source, Drain
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7. • When a voltage is applied betwen drain and the source and the
voltage on the gate is zero the two PN- junctions form depletion
layers
• The electron flow from the source to drain through the channel
between the depletion layers
• The size of the channel width determines the current conduction
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8.  When a reverse gate voltage is given the depletion layer size will
increase
 This reduces the channel width
 Hence the resistance to the flow of current increases
 On the other hand if the reverse gate voltage is decreased the
current conduction increases
 As the resistance can be controlled by the voltage given at the gate,
JFET is called voltage variable resistor
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10.  MOSFET stands for Metal Oxide Semiconductor Field Effect
Transistor
 MOSFET is an important semiconductor device
 Its input impedence is very much higher than that of a FET
 This is because it has very small gate leakage current
 Its connections are very similar to that of a FET
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11.  MOSFET is very similar to a FET
 But it has only one P region called the “substrate”
 A thin layer of metal oxide (sio2) is deposited over the left side of the channel.
 Since sio2 is an insulator it insulates the gate from the channel
 For this reason the MOSFET is called INSULATED GATE FET ( IGFET )
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12.  A MOSFET has basically three terminals
1) Source
2) Drain
3) Gate
 There are two types of MOSFET
1. N - channel
2. P – channel
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13. • Its working is similar to that of a FET
• But the difference is that in a FET a gate diode is used
• While in a MOSFET a gate capacitor is used
• One plate of this capacitor is the gate and the other being the
channel
• Here the sio2layer is the dielectric
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14. • When the negative voltage is applied to the gate electrons
accumulate on it
• These electrons repel the conduction band electrons in the N –
channel
• Hence less electrons are available for conduction through the
channel
• The greater the negative voltage on the gate the lesser is the current
conduction...
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15. S. NO. JFET MOSFET
1 Gate is not insulated
from channel
Gate is insulated from
gate by a thin layer of
sio2
2 High input impedence
(<10 MΩ)
Very high input
impedence (>10000
MΩ)
3 High drain resistance Low drain resistance
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16. 1. Small size
2. Low power consumption
3. Simplicity of construction
4. Mechanical ruggedness
5. Very high input impedence
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