What is the difference between BJT and FET?

Difference between bjt and fet

Main Difference between BJT and FET is that BJT is Current Controlled device and FET is voltage control device. They are developed from different semiconductor materials, mainly type P and type N. These transistors are used in the design of amplifiers, switches and oscillators, etc.

BJT – Bipolar Junction Transistor

BJT is the Bipolar Junction Transistor. BJT is the current controlled device.

As shown in the figure, there are types of BJT P-N-P and N-P-N transistors. The symbols of these transistors are also described in the figure. There are three terminals in a BJT device. transmitter, base and collector.

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

Features of BJT:

  • The input impedance is small and the output impedance is high.
  • Noise device due to the presence of minority carriers.
  • It is a bipolar device as current flows due to major carriers and minorities.
  • Thermal stability is lower due to leakage current or reverse saturation current.
  • The doping in the transmitter is the highest and the bottom is the lowest.
  • The collector surface is the highest and the bottom is the smallest.

FET – Field Effect Transistor

FET is the Transistor with field effects. FET is the controlled voltage device.

The figure describes the FET symbols for the P channel and the channel type N. There are three terminals in a FET device. source, drain and gate. The source is the terminal through which carriers enter the channel. Drain is the terminal through which carriers leave the channel. The gate is the terminal that modulates the conductivity of the channel by applying voltage to this terminal.

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

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

Features of FET

  • It is a high impedance input device of about 100 MOhm and above.
  • FET has no offset voltage when used as a switch as opposed to BJT.
  • FET is relatively immune to radiation, but BJT is very sensitive.
  • It is a major transport device.
  • FET is less noisy than BJT. It is more suitable for low-level amplifier input stages.
  • FET offers higher thermal stability compared to BJT.
  • FET is a unipolar device.

Main drawbacks of FET:

  • FET has a relatively low bandwidth product compared to BJT.
  • FET suffers from a greater sensitivity to damage and therefore requires careful handling.

The Upper table mentions the difference between BJT (Bipolar Transistor Transistor) and Field Effect Transistor (FET) types.

Difference between bjt and fet in tabular form

Bipolar junction transistors (BJTs) Field effect transistors (FETs).
BJTs are controlled by the current. They need a bias current to the base terminal to operate. FETs are voltage controlled. All you need is a voltage applied to the door to enable or disable the FET. You do not need a pre-stream for the operation.
BJTs have lower input impedances, which means that they draw more power into the power circuit, which can lead to load on the circuit. FETs offer higher input impedance than BJTs. This means that they consume virtually no energy and therefore do not overload the circuit that feeds them.
BJTs have a greater output gain than FETs. The gain (or transconductance ) of the FETs is lower than that of the BJTs.
BJTs are larger and therefore require more space than FETs normally. FETs can be much smaller than BJTs. This is particularly important for integrated circuits composed of many transistors.
BJTs are less popular and less frequently used FETs are clearly more popular nowadays and widely used in commercial channels than BJTs
BJTs are cheaper to manufacture FETs, especially MOSFETs, are more expensive to manufacture

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