power electronics

05/01/23 Prabhu MIke 1

• 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


• 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


• 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


• 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


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


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


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 )


A MOSFET has basically three terminals1) Source2) Drain3) Gate

There are two types of MOSFET1. N - channel2. P – channel


• 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 sio2 layer is the dielectric


• 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...


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 resistance05/01/23 Prabhu MIke 15

1. Small size2. Low power consumption3. Simplicity of construction4. Mechanical ruggedness5. Very high input impedence


power electronics

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