What do you know about bipolar transistor?

What do you know about bipolar transistor?



Transistor generally refers to semiconductor components, including diode, bipolar transistor, field effect transistor, thyristor, etc.

Transistor has detection, rectification, amplification, switching, voltage regulation, signal modulation and other functions.


Bipolar transistor is a current control element.

It has the function of current amplification.

When the bipolar transistor works, the majority of carriers and the minority of carriers are involved in operation, that’s why it’s called bipolar transistor.

There are two PN junctions very close to each other on a semiconductor substrate (silicon or germanium), dividing the whole semiconductor into three parts.

The middle part is the base region, and the two sides are the emitter region and the collector region.

There is PNP type and NPN type.


NPN type

N-type adds phosphorus into silicon to produce free electron conduction.

P-type adds boron into the silicon to create hole conduction.

NPN type is composed of two N-type and one P-type, the arrangement is NPN.

The PN junction between the emitter and the base is called the emitter junction.

The PN junction between the collector and the base is called the collector junction.

Three leads are called the emitter, base and collector.


The state of bipolar transistor

Cut-off state

When the voltage applied to the emitter is less than the turn-on voltage of the PN junction, there is no current in the base.

The bipolar transistor loses the current amplification function now.


Amplified state

When the voltage applied to the emitter is more than the turn-on voltage, the emitter junction is biased forward, and the collector junction is biased reverse.

At this time, the base current plays a control role on the collector current, so that the bipolar transistor has a current amplification effect.


Saturation conduction state

When the base current increases to a certain extent the collector current will stop increasing.

The voltage between the collector and the emitter is very small, the bipolar transistor loses the current amplification effect.

It is called saturation conduction state.


Various types of bipolar transistor

Switching transistor

Switching transistor works between cut-off state and saturation state, equivalent to circuit cut off and on.

It is widely used in a variety of switching circuits, such as  switching power circuit, drive circuit, high frequency oscillation circuit, analog to digital conversion circuit, pulse circuit and output circuit, etc.


High power transistor

The dissipation power is usually more than 1 watt.

It’s widely used in high, medium and low frequency power amplifier, switching circuit, voltage regulator circuit, analog computer power output circuit.


High frequency transistor

High frequency transistor is used in VHF, UHF, CATV, wireless remote control, RF module and other high frequency broadband low noise amplifiers.

It is generally used in low voltage, small signal, small current and low noise, with a maximum power of 2.25 watts and a maximum collector current of 500 mA.


The Darlington transistor

Darlington transistor is two bipoar transistor connected together.

The polarity is only recognized as the front transistor.

The amplification is the product of two transistor amplification.

The arrangement are NPN+NPN, PNP+PNP, NPN+PNP, PNP+NPN.



Phototriode, also called photosensitive transistor.

The phototransistor can control the collector current according to the intensity of the light.

There are only two leads as the collector and emitter.

The base is a  light receiving window.


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