What are the differences between UJT and FET

Differences between UJT and FET.

ujt (transistor at a junction): it is a transistor with a single junction and three terminals: a transmitter (e) and two bases (b1 and b2).

UJT Unijunction Transistor

bjt (bipolar junction transistor): this type of transistor consists of two junctions and three terminals, namely the emitter e, the base b and the collector. there are two types of bjt, i) pnp and ii) npn.

in addition, bjt is a bipolar device and it is a device controlled by the current, it is a unipolar device and controlled by the voltage.

Since only the majority carriers are responsible for the current flowing. but in all cases, the carriers of the majority minority are responsible for the current current.

Ujt is a uni junction transistor where, like bjt, a transistor with two junctions.

BJT Bijunction Transistor

ujt: This is a 3-lead electronic semiconductor device with a single junction that acts exclusively as a power switch.

fet: it is a transistor that uses an electric field to control the electrical behavior of the device.

difference between ujt fet

UJT (UniJunction Transistor) is a transistor with only one junction and three terminals: an emitter (E) and two bases (B1 and B2).

BJT (Bipolar Junction Transistor) type of transistor consists of two junctions and three terminals, namely Emitter “E”, Base “B” and Collector”C”.


  • there is only one channel p in ujt whereas fet has 2.
  • the channel p of ujt is more strongly doped than the channel p in fet.


  • ujt works in direct bias, whereas fet works in inverse bias.
  • ujt has a low input impedance but fet a high input impedance.

The principle of operation ujt is that charge carriers (electrons or holes) are injected into the conductive area (channel) between the outside terminals and this requires current.

In a fet device, the carriers are attracted (or pushed back) into the channel only by the electric field of the gate electrode, there is therefore no significant input current (certain leakage currents, picoamperes, etc.).

In short, ujt is supplied with current while fet is supplied with voltage. The curious fact is that, since (in the case where the fets are not insulated gate transistors), the internal construction is quite similar, with a correct bias of the junction (forward or in blocking), you can use the same device in fet mode or ujt (to a certain extent, because the construction of the device will be optimized for one or the other mode).

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