Mosfet vs BJT: comparison of mosfet with bjt

MOSFET (metal-oxide-semiconductor field-effect transistor) and BJT (bipolar junction transistor) are two commonly used types of transistors in electronics.

The main difference between MOSFETs and BJTs is the way they control the flow of current.

One key difference between MOSFETs and BJTs is the way they are controlled. MOSFETs are controlled by a voltage applied to the gate terminal, while BJTs are controlled by a current applied to the base terminal. Another difference is the way they operate. MOSFETs are voltage-controlled devices, while BJTs are current-controlled devices. MOSFETs also have a higher input impedance and faster switching speeds than BJTs.

In a MOSFET, the flow of current is controlled by an electric field that is created by applying a voltage to a gate terminal. The MOSFET can be either a depletion-mode or an enhancement-mode transistor. The depletion-mode MOSFET is normally conducting when there is no voltage applied to the gate, while the enhancement-mode MOSFET is non-conducting. When a voltage is applied to the gate of the enhancement-mode MOSFET, it becomes conducting.

In a BJT, the flow of current is controlled by the amount of current that flows through the base terminal. BJTs are three-terminal devices, with the emitter, base, and collector terminals. When a current flows through the base terminal, it controls the current flow between the emitter and collector terminals.

MOSFETs have several advantages over BJTs. They typically have lower power consumption and are faster switching devices than BJTs. MOSFETs are also voltage-controlled devices, which means that they have high input impedance and can be easily interfaced with digital circuits. 

Power MOSFET has several features different from those of BJT.

(i) Power MOSFET has lower switching losses but its on-resistance and conduction losses are more. A BJT has higher switching losses but lower conduction loss. So at high frequency applications, power MOSFET is the obvious choice. But at lower operating frequencies (less than about 10 to 30 kHz), BJT is superior.

(ii) MOSFET is voltage controlled device whereas BJT is current controlled device.

 (iii) MOSFET has positive temperature coefficient for resistance. This makes parallel operation of MOSFETs easy. If a MOSFET shares increased current initially, it heats up faster, its resistance rises and this increased resistance causes this current to shift to other devices in parallel. A BJT has negative temperature coefficient, so current-sharing resistors are necessary during parallel operation of BJTs.

(iv) In MOSFET, secondary breakdown does not occur, because it has positive temperature coefficient. As BJT has negative temperature coefficient, secondary breakdown occurs. In BJT, with decrease in resistance, the current increases. This increased current over the same area results in hot spots and breakdown of the BJT. 

(V) Power MOSFETs in higher voltage ratings have more conduction loss.

(vi) The state of the art MOSFETs are available with ratings upto 500 V, 140 A whereas BJTs are available with ratings up to 1200 V, 800 A.

On the other hand, BJTs have higher gain than MOSFETs and are generally easier to bias. BJTs are also more suitable for low-frequency applications, while MOSFETs are better for high-frequency applications.

Mosfet vs BJT FAQ

Q1.What is the primary difference between a MOSFET and a BJT?

The primary difference between a MOSFET and a BJT lies in their operation and construction. A MOSFET is a voltage-controlled device that uses an oxide layer to insulate the gate from the channel, which is a conductive path between the source and the drain. The voltage applied to the gate controls the amount of current that flows through the channel. In contrast, a BJT is a current-controlled device that uses two p-n junctions to control the flow of current between the emitter and collector. The base current controls the amount of current that flows from the emitter to the collector.

Q2.What is MOSFET vs BJT advantages and disadvantages?

Advantages of MOSFETs:

1. Low power consumption: MOSFETs require very little input current to control the output current, which makes them ideal for low-power applications.
2. High input impedance: MOSFETs have a very high input impedance, which means that they can be easily controlled by a voltage signal.
3. Fast switching speed: MOSFETs can switch on and off very quickly, making them ideal for high-frequency applications.
4. No biasing current: MOSFETs do not require a biasing current, which simplifies the circuit design.

Disadvantages of MOSFETs:

1. Fragile: MOSFETs are more fragile than BJTs, and are easily damaged by electrostatic discharge (ESD).
2. More expensive: MOSFETs are generally more expensive than BJTs, which can be a consideration in some applications.
3. Limited voltage range: MOSFETs have a limited voltage range compared to BJTs, which can limit their use in high-voltage applications.

Advantages of BJTs:

1. High gain: BJTs have a high current gain, which makes them ideal for amplification.
2. Low noise: BJTs have low noise characteristics, which makes them ideal for low-noise applications.
3. Robust: BJTs are more robust than MOSFETs, and can handle higher current and voltage levels without being damaged.

Disadvantages of BJTs:

1. Biasing current: BJTs require a biasing current, which adds complexity to the circuit design.
2. Slower switching speed: BJTs are slower than MOSFETs when it comes to switching on and off, which can limit their use in high-frequency applications.
3. High power consumption: BJTs require more input current than MOSFETs to control the output current, which makes them less efficient in low-power applications.

Q3 Why are MOSFETs preferred over BJT?

Some say BJT, while others say MOSFET. People have different answers because they use them for different applications. For example, BJTs are better in low-current applications, while MOSFETs are better in high-current applications.MOSFETs have a very high input impedance, which means they draw very little current from the input signal source. This makes them suitable for use in circuits where the input signal is very weak, such as in audio amplifiers.MOSFETs can switch on and off very quickly, which makes them suitable for use in high-frequency applications such as power supplies and radio-frequency (RF) circuits.MOSFETs typically require less power to operate than BJTs, which makes them more energy-efficient.MOSFETs can handle higher voltage levels than BJTs, making them suitable for use in high-power applications such as power supplies and motor control circuits.MOSFETs require only a small amount of input current to switch on and off, which makes them easy to drive with simple signal sources such as microcontrollers or digital logic circuits.To choose which transistor better suits your project, properly evaluate the key parameters of your project like budget, the switching speed required, the maximum voltage, and current ratings of the project. Based on these, one can then select the best employee for the task.