The difference between BJT and MOSFET

In the circuit design, if we want to control the current, then the help of the BJT is indispensable. What we commonly call the BJT is called the semiconductor BJT, and its main function is to amplify the tiny signal. There are many similarities between MOSFET and BJT, which makes some novices continue to be unable to understand the difference between the two. Here, China PCB factory SysPCB will introduce some differences between BJT and MOSFET.

What are BJT and MOSFET?

(1) BJT

BJT consists of three terminals: emitter (E), base (B), and collector (C). Its structure is similar to two back-to-back connected pn junction diodes. BJTs are divided into two types: NPN and PNP, with electrons and holes as the main carriers, respectively. BJT is a current controlled device whose output current is a function of input current, typically used in low current applications such as switches and amplifiers.

(2) MOSFET

MOSFET consists of three terminals: Source (S), Gate (G), and Drain (D). MOSFET is a voltage controlled device that controls the current between the source and drain through the gate voltage. MOSFETs are divided into enhancement type and depletion type. The former is in the off state when there is no gate voltage and requires gate voltage to turn on; The latter is already conducting when there is no gate voltage. MOSFETs are widely used in low-power devices and high-frequency circuits, and are favored for their low power consumption and high input impedance.

The difference between BJT and MOSFET:

• Working character: The BJT is controlled by current, and the MOSFET is controlled by voltage.

• Cost issue: BJT are cheap and MOSFET are expensive.

• Power consumption problem: the loss of the BJT is large, and the MOSFET is small.

• Driving ability: MOSFET are often used for power switches and high-current switch circuits.

The following is a list of the selection rules of several MOSFET and BJT for the situations that will appear in some circuit designs:

1. The MOSFET is a voltage control components, and the BJT is a current control components In the case that only less current is allowed to be taken from the signal source, the MOSFET should be selected; and when the signal voltage is low and more current is allowed to be taken from the signal source, the BJT should be selected.

2. The unipolar device mentioned in the power electronics technology refers to the device that conducts electricity by only one type of carrier, and the bipolar device refers to the device that conducts electricity by two types of carriers. The MOSFET uses a majority carrier for conduction, so it is called a unipolar device, while the BJT uses both majority carriers and minority carriers for conduction, are called bipolar devices.

3. The source and drain of some MOSFET can be used interchangeably, and the gate voltage can also be positive or negative, which is more flexible than BJT.

4. The MOSFET can work under the condition of very small current and very low voltage, and its manufacturing process can easily integrate many MOSFET on a silicon wafer, so MOSFET have been widely used in large-scale integrated circuits applications.

5. MOSFET has the advantages of high input impedance and low noise, so it is also widely used in various electronic equipment. In particular, MOSFET are used as the input stage of the entire electronic equipment, and the performance obtained is that ordinary BJT are difficult to achieve.

6. MOSFET are divided into two categories: red junction type and insulated gate type, and their control principles are the same.