BJT vs FET (Transistors)

BJT vs FET



In this article, we compare and contrast bipolar junction transistors (BJTs) and field effect transistors (FETs).

Though both are transistors and have 3 leads and achieve similar functions, they're fundamentally different in composition. Thus, there are several key differences between the 2 transistors.

The table below pinpoints many of the differences between BJTs and FETs.


BJTs vs FETs
How it operates BJTs FETs
BJTs are current-controlled. They require a biasing current to the base terminal of operation. FETs are voltage-controlled. They only require voltage applied to the gate to turn the FET either on or off. They do not require a biasing current for operation.
Input Impedance BJTs offer smaller input impedances, meaning they draw more current from the power circuit feeding it, which can cause loading of the circuit. FETs offer greater input impedance than BJTs. This means that they practically draw no current and therefore load down the power circuit that's feeding a lot less and minimimally.
Gain
(Transconductance)
BJTs offer greater gain at the output than FETs. The gain (or transconductance) of FETs are smaller than for BJTs.
Size BJTs are larger in size and therefore take up more physical space than  FETs normally. FETs can be manufactured much smaller than BJTs. This is especially important for integrated circuits that are composed up of many transistors.
Popularity BJTs are less popular and less widely used FETS are definitely more popular and widely used in commercial circuits today than BJTs
Cost BJTs are cheaper to manufacture FETs, especially MOSFETs, are more expensive to manufacture



So the above table is a good, brief explanation of some of the differences between bipolar junction transistors (BJTs) and field effect transistors (FETs). Below we'll go over the table in more depth, so that you can get a better in-detailed explanation, if you feel the above lacked. We'll go in order.

So the first thing is how both transistors operate. BJTs are current-controlled devices. This means that BJTs are switched on by a current going through the base of the transistor. This base current then turns the BJT on, allowing for a much greater flow of current from the collector to the emitter of the transistor. FETs, on the other hand, are voltage-controlled. Voltage, not current, either turns the FET on or off. FETs have such high input impedance that they practically draw no current into the gate terminal. Instead they are entirely voltage-controlled.

The second difference is the input impedance. Input impedance is the amount of resistance that a transistor offers on its input terminal. For BJTs, this would be the base terminal; for FETs, this would be the gate terminal. BJTs offer much less resistance to their input terminal than FETs. Because of this much lower resistance, it draws current from the power supply powering the base. This is an effect called loading. Loading is when the power source circuit is affected by a second circuit, in this case the transistor circuit, which is drawing current from it. This small amount of current drawn, which then combines with the much larger current flowing from the other 2 leads can alter dynamics of the power source circuit. So BJTs offer less protection against this loading effect than FETs. FETs have very large input impedances, such as on the order of 1014 Ω, which is several teraohms (something you almost never hear about). With such high input impedance, the FET practically draws no current to its input gate terminal. Therefore, since practically no current is drawn from the power supply circuit, the power supply circuit is not loaded down. It's as if the power supply circuit and the transistor circuit are well isolated and do not interfere with each other. Therefore, better power control is achieved with FETs with less interference of one circuit onto another.

A third difference between BJTs and FETs is the gain (or transconductance). Transconductance is defined as the milliamp per volt ratio of the small change in the current output from an electronic device to the small change of voltage input. In other words, it is the gain of the transistor circuit. This is where BJTs have an advantage. BJTs have greater transconductance, meaning you are able to get more current output per unit power applied. The transconductance of FETs is much lower. So if you use the same amount of power at the input for both a BJT and FET transistor, the BJT transistor will produce more gain. This is why BJTs are more popular for amplifier circuits. They produce gain than a FET can. This is why in the case of simple amplifier circuits, the use of a BJT is preferred and FETs are rarely used. For simple amplifiers, FETs are really only used only when it is desired for there to be extremely high input impedance.

In terms of manufacturing size, FETs can be manufactured to be much smaller than BJTs. This makes them more efficient in commercial circuit design. Being that FETs are smaller, they take up less space on a chip. Thus, the size of a electronic product can be much smaller, which is what electronic design companies want a lot of times. Smaller devices, many times, can be more convenient, consumer-friendly, and FETs allow this. BJTs, on the other hand, require larger sizes generally than FETs.

In terms of expense, FETs, especially MOFSFETs, are more expensive to manufacture than BJTs. FETs normally are at a higher price point, but not significant enough to push away from them. This is just a slight drawback.

For a number of reasons, such as those listed above, FETs are more widely used and more popular than BJTs. FETs can be manufactured smaller and load the power supply less.

So while BJTs are used widely in hobby electronics and many times too in some consumer electronics and have the advantage of being able to produce higher gains than FETS, FETs still offer many advantages for large-scale commercial devices. When it comes to consumer products, FETs are overwhelmingly preferred due to size, high input impedance, as well as other factors. Intel, one of the largest chip makers in the world, uses practically only FET transistors to build its chips which power billions of devices in the world.

Thus, this is a brief overview of FETs vs BJTs.


Related Resources

JFET vs MOSFET (Transistors)

Types of Transistors

Difference between an NPN and a PNP Transistor

Transistor Schematic Symbols

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