Industrial Electronics

The difference between MOSFETs and BJTs

04 October 2024
Figure 1: Types of MOSFETs. Source: EFY Group

Transistors are semiconductor devices that act as electronic switches or amplifiers as they can control the flow of electric current through a circuit. Amplification is crucial in applications like audio systems and communication devices. Switching is fundamental to digital electronics, where information is represented as binary (0s and 1s). Transistors can be combined to create logic gates (AND, OR, NOT) as well, which form the basis of digital circuits and computers. The development of smaller and more efficient transistors has led to the miniaturization of electronic devices, from smartphones to laptops. Metal-oxide-semiconductor field-effect transistors (MOSFETs) and bipolar junction transistors (BJTs) are two popular types of transistors that will be discussed in this article.

What is a MOSFET?

MOSFETs are a type of field-effect transistor (FET) widely used in modern electronic circuits. It consists of a channel (usually made of silicon) between a source and a drain terminal. A gate insulator (often made of silicon dioxide) separates the gate terminal from the channel. A positive voltage provided to the gate generates an electric field that draws electrons into the channel, creating a conductive tunnel; this voltage controls the current flow from the source to the drain. When the gate voltage is negative, the channel is depleted of electrons, and current flow is blocked. Due to this difference in operation, MOSFETs have been characterized into two types: enhancement-mode E-MOSFETs and depletion-mode D-MOSFETs. E-MOSFETs require a positive gate voltage to turn them on. D-MOSFETs are always partially on and require a negative gate voltage to turn them off.

E-MOSFETs are typically used in digital circuits because they can be easily turned on and off by applying a voltage to the gate. This makes them ideal for building logic gates and other digital components. D-MOSFETs, on the other hand, are used in analog circuits and power electronics. D-MOSFETs can be used to create linear amplifiers and other analog circuits. They have a more predictable behavior than E-MOSFETs in the linear region of operation. They can be used in power switching applications, such as motor drives and inverters. They can be used to create a "pull-down" or "pull-up" function in these circuits. Pull-down function refers to a signal line that is pulled low (to ground) when there is no active input and pull-up function ensures that a signal line is pulled high (to Vcc) when there is no active input.

What are BJTs?

BJTs are another type of transistor commonly used in electronic circuits. Unlike MOSFETs, BJTs rely on the flow of both majority and minority charge carriers to operate. They have two basic types: NPN and PNP. In NPN, the p-type semiconductor is sandwiched between two n-type layers, whereas in PNP, the n-type semiconductor is sandwiched between two p-type layers. In NPN, electrons are injected into the base area in response to a positive voltage supplied to it. From the emitter to the collector, a current is generated when these electrons combine with holes in the base region. In PNP, the operation of a PNP BJT is similar, but the roles of the majority and minority carriers are reversed.

Figure 2: The BJT is a semiconductor device which can be used for switching or amplification. Source: AspenCore, Inc.Figure 2: The BJT is a semiconductor device which can be used for switching or amplification. Source: AspenCore, Inc.

If a circuit is powered by a positive voltage (e.g., 5 V), NPN transistors are generally more convenient. This is because the base-emitter junction of an NPN transistor is forward-biased when the base voltage is higher than the emitter voltage. But when the circuit is powered by a negative voltage, PNP transistors are more suitable, since the base-emitter junction of these devices are forward-biased when the base voltage is lower than the emitter voltage. Similarly, if a circuit requires a pull-down function (i.e., to pull a signal line low), an NPN transistor is typically used. This is because the collector of an NPN transistor can be connected to ground to pull the signal line low. If a circuit requires a pull-up function (i.e., to pull a signal line high), a PNP transistor is typically used, as its collector of can be connected to the positive power supply to pull the signal line high.

Difference between MOSFET and BJT

Feature

MOSFETs

BJTs

Operation

Controlled by electric field at gateControlled by current through base-emitter junction

Input Impedance

HighLow

Power Consumption

LowHigher

Switching Speed

Generally slowerGenerally faster (in some applications)

Fabrication Complexity

EasierMore complex

Sensitivity to Radiation

More sensitiveLess sensitive

Common Applications

Digital logic, analog circuits, power electronicsAnalog circuits, power electronics, discrete circuits

Conclusion

BJTs are versatile components that have been used in electronic circuits for many years. While they have some advantages over MOSFETs, such as higher current gain and faster switching speeds, their use is declining in some applications due to the advantages offered by MOSFETs. MOSFETs are generally preferred for digital and analog circuits due to their high input impedance and low power consumption, while BJTs are often used in applications requiring high current gain or faster switching speeds.

To contact the author of this article, email GlobalSpecEditors@globalspec.com


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