BJT vs. MOSFET: Powerful Transistors But How Are They Different?

Both BJTs and MOSFETs are semiconductors commonly used in electronic circuits to switch or amplify signals. But that’s where their similarities end. These two devices have different constructions and working mechanisms to handle the amplification and switching tasks.

Let’s compare how these devices work, their types, and applications to determine where you should use BJTs and where to use MOSFETs.

What Is a BJT?

A BJT (Bipolar Junction Transistor) is a current-controlled semiconductor that features three terminals, with the middle one (base) controlling the current flow from the emitter to the collector (or vice versa) to run the amplification or switching.

Construction-wise, this device has three doped regions, with the two outer layers being similar and the inner one having an opposite polarity. This polarity is defined by charge carriers, which can be electrons or holes, and it explains their name.

Bipolar means the device uses two polarities to manage how current flows through two junctions of a semiconductor device (transistor.

The two types of BJTs

Types of BJTs

BJTs come in two types and both have their emitter side heavily doped, collector moderately doped, and base lightly doped.

PNP

PNP BJTs use holes as the majority charge carriers because they have two P-type semiconductors sandwiching an N-type. This arrangement makes its conventional current flow collectively from the circuit into its emitter terminal, or from the emitter to the collector, hence the arrow direction in its circuit symbol.

Basically, with enough negative voltage connected to the base terminal, electrons are injected into the device, while conventional current flows in the opposite direction (emitter to base). This forward-biases this junction and the electrostatic force achieved with the negative voltage applied to the collector makes current flow through the reverse-biased collector-base junction, forming a common-base configuration.

The device can also work as an on or off switch by forward-biasing or reverse-biasing all junctions, respectively.

NPN

NPN BJTs are more common than their PNP counterparts because they are cheaper and provide faster switching (they use electrons as their majority charge carriers, which are more mobile than holes in semiconductors).

They feature one P-type semiconductor sandwiched between two N-type conductors, hence the name. Since they operate in the opposite polarity to PNPs, their conventional current flows from the collector to the emitter, so the arrow in their circuit symbol points to the emitter.

For amplification in a CE (Common Emitter) configuration, the emitter-base junction is forward-biased by applying a positive voltage to the base and a negative voltage to the emitter. This pushes electrons through the junction into the base and they force their way into the collector due to the electrostatic force with the holes from the reverse-biased collector-base junction. With this electron flow, conventional current flows in the opposite direction.

Advantages Of BJTs

• High Amplification: BJTs have high current gains, which you can see when studying their output characteristics curves. This means they can achieve high amplification of low-strength signals.

• Efficient at High Switching Speeds: BJTs are efficient at very high switching frequencies, which makes them ideal for high-power projects.

• Accuracy and Linearity: Linearity makes it easy to accurately determine the output signal strength based on the input strength.

• Robustness and Reliability: BJTs can handle relatively high currents and voltages, making them last longer in electronic circuits.

• Versatility: BJTs can be used in different configurations, such as common emitter, common base, and common collector, to handle the needs of different projects.

• Affordable: These transistors are easy to manufacture.

• Easy to Configure: BJTs require simple biasing, making them easy to configure.

Disadvantages of BJTs

• Generates Heat: BJTs generally emit more heat than MOSFETs because they consume more power (higher loss factors).

• Thermal Instability: BJTs have a higher thermal instability than MOSFETs, so they are more sensitive to temperature swings. 

What Is a MOSFET?

A MOSFET (Metal Oxide Semiconductor Field Effect Transistor) is a voltage-controlled switching or amplification device with three or four terminals. These are source, drain, gate, and body (optional). The gate is suspended on a thin insulating layer of silicon dioxide above the doped semiconductor substrate, and this construction plays a big role in how the device works.

The internal structure of a MOSFET

How MOSFETs Work

These devices operate on the principle of field-effect modulation, where the conductivity of the semiconductor channel between the source and drain in the substrate is controlled by the voltage applied to the gate terminal.

This voltage creates an electric field across the insulating oxide layer to alter the channel’s conductivity, which can allow or block the flow of charge carriers between the source and drain.

Types of MOSFETs

MOSFETs can either be P-channel or N-channel, and this channel refers to the conduction path that is created between the source and drain.

P-Channel

P-channel MOSFETs have heavily doped P-type semiconductor regions on the source and drain terminals, with the substrate being an N-type semiconductor. Therefore, current can only flow between the source and drain if there is a channel of holes from one side to the other.

Normally, this transistor is in enhancement mode, where no voltage is applied to the gate, so no current flows through. But when a negative voltage is applied to the gate, it repels the electrons in the N-type substrate, pushing them down. This creates a depletion region that is populated by positive charges from donor atoms.

At the same time, this negative voltage draws holes from the heavily doped P-type source and drain regions, creating a conductive channel between the two terminals. In this case, holes are the charge carriers in the transistor, and this forms the depletion mode (due to the depleted region).

N-Channel

This opposite polarity MOSFET to the P-channel has heavily doped N-type semiconductor regions under the source and drain terminal, with a P-type semiconductor forming the substrate to hold the entire device.

In enhancement mode, this device is off because the holes in the middle block the flow of electrons. But when a positive voltage is applied to the gate, it repels and pushes the holes down into the substrate, creating a depletion region between the heavily doped N-type source and drain regions. This region is populated by the negative charges associated with acceptor atoms.

The positive voltage also attracts the electrons from the source and drain regions to create a conductive channel between the two, making electrons the charge carriers.

An N-channel MOSFET in enhancement mode

Advantages of MOSFETs

• High Energy Efficiency: These transistors have lower power losses than BJTs, using zero energy in their off states and low power when on.

• Low Heat Generation: Low energy losses result in less heat generation.

• Easy to Interface With Digital and Analog Circuits: Since they are voltage-controlled, MOSFETs are easier to integrate with analog and digital circuits.

Disadvantages of MOSFETs

• Low Efficiency at High Switching Speeds: Although they have higher switching speeds than BJTs, charging and discharging their gate capacitances is equivalent to doing the same to a battery, which becomes inefficient at high frequencies.

• High Sensitivity: MOSFETs are highly sensitive to overvoltage and electrostatic discharge.

• Expensive: These transistors have a more complex structure, which makes them costlier to build.

BJTs vs. MOSFETs: A Comprehensive Comparison

When To Use BJTs Over MOSFETs

BJTs are the more attractive option to consider when:

1. You Need Higher Switching Frequencies

MOSFETs naturally have higher switching frequencies than BJTs, but they lose their efficiency at high speeds due to gate capacitance. Therefore, it makes more sense to use BJTs when the frequency is too high.

2. You Need To Make Op-Amps

BJTs have higher current gains than MOSFETs, which makes them better for amplifying weak signals.

3. You Are Worried About Cost

NPN and PNP transistors are more affordable and available in the market than MOSFETs, so using them lowers your project’s costs.

4. You Want a Simple Circuit Design

Creating circuits to provide the precise voltage to drive a MOSFET can be quite complex. But with BJTs, all you need are a resistor and a microcontroller.

When To Use MOSFETs Over BJTs

1. When You Need Low Power

Since they are voltage-controlled, MOSFETs only need their gates charged once. After that, no more current draw is needed to keep the channel open, so they draw significantly less power than BJTs, making them suitable for low-power applications.

In contrast, BJTs need continuous current sourcing (NPN) or sinking (PNP) to keep the device on

2. When the Switching Frequencies Aren’t Too High

As stated earlier, MOSFETs lose their efficiency when the switching frequency is too high. But below that threshold, they are better than BJTs.

3. When You Need To Drive High Power and Current

The best MOSFETs can drive as much as 10X more current than BJTs. For instance, the TIP120 NPN Darlington transistor can only drive 5A, while the IRLB8721 N-Channel MOSFET can drive 62A

Additionally, it is easier to place them in parallel to a circuit to increase the current-carrying capability.

4. When Etching Transistors Into an IC

MOSFETs are easy to scale down or miniaturize, so they are the best option for making chips, especially considering Moore’s Law.

Final Words

BJTs and MOSFETs are simply comparable to iPhones and Android phones. Both achieve the same objective, but do it differently and have some differences in performance that introduce strengths and weaknesses to both parties.

Therefore, you have to consider what your circuit or application needs to pick the most suitable transistor for your project.

Reach out if you need help determining the most suitable semiconductor to use or to order some for your project, and we’ll ship them to you at reasonable prices.