Main Difference Between MOSFET and BJT

The main difference between MOSFET and BJT is, MOSFET is a Unipolar Device and BJT is a Bipolar device. MOSFET is a Voltage controlled device and BJT is a current-controlled device.

The Full form of BJT is Bipolar Junction Transistor.

The Full form of MOSFET is Metal Oxide Semiconductor Field Effect Transistor

The working of MOSFET and BJT are almost the same. There is little difference between BJT and MOSFET. So let's see the important differences between BJT and MOSFET.

MOSFET VS BJT

1. As it already said MOSFET is a Unipolar Device because of the current conduction through MOSFET is done by either electrons or holes. In the P-channel MOSFET current is conducted by holes and in the N-channel MOSFET current is conducted by electrons.

But BJT is a Bipolar Device because electrons and holes both participate in current conduction.

2. The other most important difference between BJT and MOSFET is, BJT is a current-controlled device because when the base current is given BJT starts conduction but MOSFET is a voltage-controlled device because when a gate to source voltage is given MOSFET starts conduction.

3. BJT has three terminals named as Emitter, Base, and Collector. But MOSFET has three terminals named as Source, Drain, and Gate. MOSFET is better for switching applications whereas BJT is better for Amplification applications.

4. MOSFET has very high input resistance as it is a Voltage controlled device but BJT has low input resistance as it is a current-controlled device. 

Generally, the input impedance of a BJT lies in the kilohm range, while the input impedance of a MOSFET typically lies in the megohm range. There are two main reasons why MOSFETs have a higher input impedance than BJTs. First, the gate of a MOSFET is a reverse-biased PN junction, which acts like a capacitor. This capacitor has a very high impedance, which is why the input impedance of a MOSFET is so high. Second, the base of a BJT is a PN junction that is forward-biased. This junction has a much lower impedance than a reverse-biased junction, which is why the input impedance of a BJT is lower.

5. The transconductance of a transistor is a measure of how much the output current changes in response to a small change in the input voltage. MOSFET has a lower Transconductance per unit current but BJT has a higher Transductance per unit current.

This is because the base current of a BJT is much smaller than the gate current of a MOSFET. For example, a typical BJT with a collector current of 1 mA may have an input transconductance of 40 mS, while a typical MOSFET with a drain current of 1 mA may have an input transconductance of only 1 mS.

The higher input transconductance of BJTs makes them more suitable for applications where high voltage gain is required. However, MOSFETs have other advantages, such as a higher input impedance and a wider operating range.

6. MOSFET is faster in operation than BJT. So, in high-frequency switching applications, MOSFET provides a better response than BJT.

7. MOSFET is used for high power and voltage control. BJT is used with small power and voltage.

8. The constructional difference between BJT and MOSFET is, BJT has not any metal in its construction but MOSFET has metal oxide as its name suggests.

9. The switching speed of MOSFET is faster than BJT. For this reason, MOSFET can give better performance in digital circuits.

10. MOSFET is mainly used for switching purposes in power circuits whereas BJT is used for amplifying and switching in smaller low-power circuits and devices.

11. Also remember that BJTs are not as temperature-stable as MOSFETs. This is because the base current of a BJT increases with temperature, which can cause the output current to increase. MOSFETs are more temperature-stable because the gate current does not change with temperature.

12. MOSFETs are better than BJTs in some applications, but not at all. For example, Switching speed, Input impedance, Output impedance, Temperature stability, and Power consumption, - for these purposes MOSFET can give you a better output whereas BJT is more suitable for Current gain, Linearity, Robustness, and Cost.

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• [Actual Difference] Between Oscillator and Amplifier
• Transistor applications explained with examples
• [Explained] Applications and Advantages of SCR

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