Main Difference – BJT vs. FET
BJT (Bipolar Junction Transistors) and FET (Field Effect Transistors) are two different types of transistors. Transistors are semiconductor devices that can be used as amplifiers or switches in electronic circuits. The main difference between BJT and FET is that BJT is a type of bipolar transistor where the current involves a flow of both majority and minority carriers. In contrast, FET is a type of unipolar transistor where only the majority carriers flow.
What is BJT
A BJT consists of two p-n junctions. Depending on their structure, BJTs are classified into npn and pnp types. In npn BJTs, a small, lightly-doped piece of p-type semiconductor is sandwiched between two heavily-doped n-type semiconductors. Conversely, a pnp BJT is formed by sandwiching an n-type semiconductor between p-type semiconductors. Let us have a look at how an npn BJT works.
The structure of a BJT is shown below. One of the n-type semiconductors is called the emitter (marked with an E), while the other n-type semiconductors is called the collector (marked with a C). The p-type region is called the base (marked with a B).
The structure of an npn BJT
A large voltage is connected in reverse bias across the base and the collector. This causes a large depletion region to form across the base-collector junction, with a strong electric field that prevents the holes from the base from flowing into the collector. Now, if the emitter and the base are connected in forward bias, electrons can flow easily from emitter to the base. Once there, some of the electrons recombine with holes in the base, but since the strong electric field across the base-collector junction attracts electrons, most electrons end up flooding into the collector, creating a large current. Since the (large) current flow through the collector can be controlled by the (small) current through the emitter, the BJT can be used as an amplifier. In addition, if the potential difference across the base-emitter junction is not strong enough, electrons are not able to get into the collector and so a current will not flow through the collector. Due to this reason, a BJT can be used as a switch as well.
The pnp junctions work under a similar principle but, in this case, the base is made of an n- type material and the majority carriers are holes.
What is FET
There are two major types of FETs: Junction Field Effect Transistor (JFET) and Metal Oxide Semiconductor Field Effect Transistor (MOSFET). They have similar working principles, although there are some differences as well. MOSFETs are more commonly used today than JFETS. The way that a MOSFET works was explained on this article so here, we will focus on the operation of a JFET.
Just like BJTs come in npn and pnp types, JFETS also come in the n-channel and p-channel types. To explain how a JFET works, we will look at a p-channel JFET:
A schematic of a p-channel JFET
In this case, “holes” flow from the source terminal (labelled with an S) to the drain terminal (labelled with a D). The gate is connected to a voltage source in reverse bias so that a depletion layer forms across the gate and the channel region where charges flow. When the reverse voltage on the gate is increased, the depletion layer grows. If the reverse voltage becomes large enough, then the depletion layer can grow so large that it can “pinch off” and stop the flow of current from the source to the drain. Therefore, by altering the voltage at the gate, the current from the source to the drain could be controlled.
Difference Btween BJT and FET
Bipolar vs Unipolar
BJTs are bipolar devices, in which there is a flow of both majority and minority carriers.
FETs are unipolar devices, where only the majority carriers flow.
Control
BJTs are current-controlled devices.
FETs are voltage-controlled devices.
Use
FETs are used more often than BJTs in modern electronics.
Transistor Terminals
Terminals of a BJT are called the emitter, base, and collector
The terminals of an FET are called source, grain, and gate.
Impedance
FETs have a higher input impedance compared to BJTs. Therefore, FETs produce larger gains.
Image Courtesy:
“The basic operation of an NPN BJT in Active mode” by Inductiveload (Own drawing, done in Inkscape) [Public Domain], via Wikimedia Commons
“This diagram of a junction gate field effect transistor (JFET)…” by Rparle at en.wikipedia (Transferred from en.wikipedia to Commons by User:Wdwd using CommonsHelper) [CC BY-SA 3.0], via Wikimedia Commons
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