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BJT vs FET

This page on BJT vs FET describes difference between BJT and FET. They are developed from different semiconductor materials mainly P-type and N-type. These transistors are used in the design of amplifiers, switches and oscillators etc.


BJT

BJT

BJT stands for Bipolar Junction Transistor. BJT is the current controlled device.
As shown in the figure, there are P-N-P and N-P-N type of BJT transistors. The symbols of these transistors are also depicted in the figure. There are three terminals in a BJT device viz. emitter,base and collector.


•   Trans-conductance gm = ε vs W/d
•   Cutoff frequency fT = gm/(2pCgs)

Following are useful characteristics of BJT:
• Its input impedance is low and output impedance is high.
• Noisy device due to presence of minority carriers.
• It is bipolar device as current flows due to both majority and minority carriers.
• Thermal stability is lesser due to leakage current or reverse saturation current.
• Doping in emitter is highest and in base it is lowest.
• Area of collector is highest and that is base is lowest.


FET

FET

FET stands for Field Effect Transistor. FET is the voltage controlled device.
Figure depicts FET symbols for P channel and N channel type.There are three terminals in a FET device. viz. source, drain and gate. The source is the terminal through which the carriers enter the channel. The drain is the terminal through which carriers leave the channel.The gate is the terminal that modulates the channel conductivity by application of voltage to this terminal.

FET is referred as unipolar transistor. In FET input voltage controls the output current, here input current is usually negligible. This is the great merit of FET when input can not supply much current.


•  Trans-conductance- gm = q Ie/kBT
•  Cutoff frequency- fT = 2Dn/WB2

Following are useful characteristics of FET which mentions difference between BJT and FET features.
• It is a high input impedance device about 100 MOhm and above.
• FET has no offset voltage when used as switch unlike BJT.
• FET is relatively immune to radiation but BJT is very sensitive.
• It is a majority carrier device.
• FET is less noisy compare to BJT. It is more suitable for input stages of low level amplifiers.
• FET provides greater thermal stability compare to BJT.
• FET is a unipolar device.

Following are the main disadvantages(demerits) of FETs.
• FET has relatively small gain bandwidth product compare to BJT.
• FET suffers from greater susceptibility to damage and hence requires careful handling.

Following table mentions difference between BJT(Bipolar Junction Transistor) and FET(Field Effect Transistor) types.


BJT FET
High voltage gain Low voltage gain
Low current gain High current gain
Low input impedance Very high input impedance
Low output impedance High output impedance
Medium Noise Generation Low Noise generation
Medium switching time Fast switching time
Robust Easily damaged
Requires zero input to turn it "OFF" Some need an input to turn it "OFF"
It is a Current controlled device. It is a Voltage controlled device.
Cheap More expensive than BJT.
Easy to bias Difficult to bias

Refer Difference between SCR, DIAC, TRIAC, UJT>>.

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