What's the difference between MOSFET and BJT and how to choose?

Introduction

In the world of electronics, transistors are considered the main component and are used in different electronic circuits and projects. The MOSFET or metal oxide semiconductor field effect transistor and  BJT are commonly used types of transistor. The knowledge of their difference and learning how to choose the right component is important for electronic lovers and engineers. So let's get started!

What is BJT?

The BJT called Bipolar Junction Transistor, is the very basic building block of electronic circuits and projects. it belongs to the semiconductors family and is used in different applications like signal amplification and modulation of signals. They are called BJT since the function is based on the use of both holes and electron charge carriers and their motion in the semiconductor.

The three main layers of BJT are the emitter, base, and collector. These three regions of BJT were created with the use of silicon. The emitter layer of this component is highly doped with impurities for increasing the conductivity, the collector is moderately doped and the base that is normally in the center is less doped.  With a combination of these three layers of the transistor, there are two PN junctions are created that help in the working transistor

Two main types of BJT transistor NPN are negative positive and negative combination and PNP or Positive negative positive combination. In NPN transistors majority carriers are electrons and in PNP holes are the majority carriers and electrons are the minority. These differences of charge carriers help to find the direction of current in NPN and PNP transistors

What makes BJT good?

1. Amplification Features

BJT comes with a high value of current gain which means that less change in base current can control larger collector current. This feature is important for the amplification of low-strength signals making it the best option for amplifier projects

2. Linearity and Accuracy:

These transistor comes with linear amplification helps to show the output has the required value according to the input signal. These linear features are important in audio amplifier circuits, that need correct signal generation

3. Robustness and Reliability:

The robust nature of BJT helps to handle high currents and voltages. They are used in high-power projects due to their high durability and their longer working life makes reliable circuits

4. Temperature Stability:

There is stable operation provided by the BJT at different values of temperatures making them dependable devices for different thermal conditions. This stable feature is used when different value temperatures can affect the operation of projects

5. Versatility :

This transistor can be used in different circuits such as common base, common emitter, and common collector circuits. Their versatile nature helps use them in different projects according to circuit needs and provides the flexibility of electronic designs.

6. Precise Switching :

They can easily switch from off to on state and on to off state. High-speed switching features are best for digital circuits where changes between different conditions are important for logic operation and processing

What is mosfet

A MOSFET, or metal-oxide-semiconductor field-effect transistor, uses input volts at gate controls to regulate current flow between two electrodes that source and drain. This transistor is commonly used in integrated circuits and mostly used in mobile phones, computers, and different electronic projects.

This transistor works with the use of an electric field to make conductive channels in a semiconductor material. The vote provided to the gate electrode regulates electric field strength and channel conductivity. If the voltage is high electric field is strong and the channel is highly conductive. If the gate voltage is low electric field is weaker and the channel has less conduction.

It is mainly used in switching circuits such as it used for switching on and off lights and amplification of the signal of the microphone. It is used in digital circuits to store and process different signal details.

It comes with different benefits than other transistors like BJT, MOSFET is high-speed and is highly efficient, and can be easily manufactured. So MOSFET is mostly preferred in modern integrated circuits

What makes MOSFET good?

High Efficiency and Low Power Use:

They mostly function on the fewer power losses making them effective components in electronic circuits. They use almost zero  power in off-state and low power in on-state so they are used in battery operating devices and employed in energy-efficient circuits

High  Switching Speed:

it can quickly change from on to off state providing high-speed switching. This fast-switching feature is used in memory devices, microprocessors, and switching power supplies that need high-speed functions

Low Heat Generation:

Their effective switching helps to produce less heat during operation. These features are best in high-power projects that cause high heat and can damage components and decrease efficiency. It helps to simplify heat management in electronic systems.

Voltage-Controlled Operation

MOSFETs are known as voltage-controlled devices. With the use of a voltage signal to the gate pin, there can be conducive of the source and drains can be controlled. This feature is helpful for easy interfacing with analog and digital circuits providing accuracy and flexible operation

Less input Capacitance:

They come with less input capacitance helping to respond fast to input signals. This feature is best in high-frequency projects like radio frequency circuits that need fast signal processing

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Comparison of BJT vs MOSFET

Which is Better Amplifier BJT or MOSFET?

The selection of BJT and MOSFET as amplifiers is based on certain applications and project needs. Every type of transistor comes with certain features that make it best for certain uses. Here are some points explained that will help you to find which one is better for amplifier circuits.

BJT Amplifiers

Advantages:

High Current Gain:

• The value of the current gain of BJT is larger than MOSFET so they prefer circuits that need a weak signal

Linearity:

• It produces the linear amplification that makes it best for audio circuits and analog signal processing that needs good regeneration of input signal

Robustness:

• They are best for high-power circuits due to handling the high current

Disadvantages:

Power Efficiency:

• It is less power efficient than MOSFE since it has high heat production making it best for battery-powered devices.

Heat Dissipation:

• It needed heat sinks and heat management in high-power applications to avoid overheating

MOSFET  Amplifiers

Advantages:

High Efficiency:

• They are highly power efficient due to less power use and less heat production so used in energy-efficient circuit

 Switching Speed:

• It has high-speed switching so used in digital circuits

Low Heat Generation:

• It generated less heat during working and managed thermal operation easily

Disadvantages:

Low Current Gain:

• It has less current gain than BJT so it not be used for circuits that need high-current amplification

Voltage Sensitivity:

• It is sensitive to voltage variations and needs proper voltage control to have stable operation

MOSFET as switch

There are two main modes of operation where MOSFET is operated as a switch in the cut-off or saturation region.

Cut-off region

In the cut-off region, the MOSFET is off and zero current is flowing from the drain to the source. It is done by applying voltage to the gate and less than threshold voltage Vth. The threshold voltage is the minimum vols that can be given to age on the MOSFET

Saturation region

In this region, MOSFET is on, and current passes from drain to source. It is done by providing a voltage that is larger than the threshold voltage.  The on-resistance (RDS(on)) of the MOSFET transistor in saturation regions is low so it can operate with larger current with fewer voltage losses

Using a MOSFET as a switch

The MOSFET in switch configuration is simple and just connects the load with the drain and source pin of the transistor and voltage is provided to the gate pin. If we needed to on the switch provide voltage to the gate which is larger than the threshold voltage. If we needed to off the switch connect the voltage to the gate less than the threshold voltage.

Advantages of MOSFET as a switch

• It has high input resistance so needs less current to operate the gate. So used in digital circuits.
• it provides high-speed switching
• It is effective in terms of low power when gets on.

BJT as a switch

A bipolar junction transistor is used as a switch for current flow control and it is done by either the cutoff or saturation region.

• Cutoff region:
• Here base-emitter junction is not forward-biased and the collector-emitter junction is in a reverse-biased state. So less current passes through the transistor
• Saturation region: 
• Both the base-emitter junction and collector-emitter junction are in the forward-biased state. it helps current to flow in the transistor in a large amount

For using the BJT as a switch we just have to bias it in saturation or cut-off region. It can be done with the use of a voltage-connected base terminal

Such as on the NPN BJT switch we are required to connect positive voltage to the base pin. It will forward-biased base-emitter junction on the transistor. Collector current will pass through the load and transistor in the saturation region.

For off the NPN BJT switch remove the positive supply from the base pin. It will be reverse-biased at the base-emitter junction and off the transistor. The collector current will become zero and not flow and transistor in the cutoff region.

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Mosfet is bipolar or unipolar

MOSFET is a unipolar device, which means current passes through it with the use of one charge carrier hole or electron. BJT is the bipolar components which means there is current flow due to holes and electrons in them

MOSFETs are called voltage-controlled devices, which means the current flowing from them is regulated by the voltage provided to the gate pin. BJT is current controlled device's current passes them is a regulated current at the base pin.  MOSFET is high-speed and more effective than BJT used in digital logic circuits and power applications. While BJT is employed in motor controllers.

NMOSFET vs PMOSFET

Conclusion

MOSFETs and BJT are basic components of electronic devices and projects that come with different features. MOSFET is voltage voltage-controlled device that controls the current between the source and drain pins according to the voltage at the gate pin. Comes with high impedance so used for digital circuits. BJT current-controlled devices control the current between the collector and emitter according to the current at the base

MOSFET is used for circuits that need high operating speed like frequency circuits since they have high input impedance and high-speed switching. BJT prefers current amplification circuits. With that MOSFET uses less power and generates less heat than BJT used for energy-conscious designs. 

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