What is a bipolar transistor, its structure, classification and basic principle

Bipolar Junction Transistor (BJT) is a semiconductor device and one of the most used electronic devices at present. It consists of three regions, namely P-type, N-type, and P-type regions, abbreviated as PNP-type or N-type transistors. BJT can act as a current amplifier, switch, BAS21 oscillator, etc., and is widely used in electronic circuits.

Const of BJT

BJT consists of three regions, namely P-type, N-type, and P-type regions (PNP-type transistor) or Ntype, P-type, and N-type regions (NPN-type transistor). The junction between the P-type and N-type regions is called the PN junction and the two ends of the PN junction are connected to the P-type and N-type regions, respectively. There is a potential barrier on the surface of the PN junction. the PN junction is in forward bias, the barrier becomes smaller, and electrons and holes can flow; when the PN junction is in reverse bias, the barrier becomes larger, and electrons holes cannot flow.

Classification of BJT

BJT can be divided into PNP-type transistors and NPN-type transistors. In PNP-type transistor, the P-type region is the base, the N-type region is the emitter, and the P-type region is the collector; in NPN-type transistor, the N-type region is the base, the P-type region is the emitter, and the N-type region is the collector.

Basic Principle of BJT

The basic principle of BJT is current control. In a PNP-type transistor, when the collector is at positive voltage, base is at negative voltage, and the emitter is at negative voltage, the PNP-type transistor is in the off state, and no current flows; when the collector is positive voltage, the base is at negative voltage, and the emitter is at positive voltage, the PNP-type transistor is in the amplification state, and current flows.

In an NPN-type transistor, when the collector is at negative voltage, the base is at positive voltage, and the emitter is at positive voltage, the NPN- transistor is in the off state, and no current flows; when the collector is at positive voltage, the base is at positive voltage, and the emitter is at negative voltage, NPN-type transistor is in the amplification state, and current flows.

Technical Points of BJT

The technical points of BJT include current amplification, input resistance, output resistance, maximum withstand voltage, maximum withstand current, and maximum operating temperature, etc.

The current amplification factor refers to the ratio of the input current the output current, which is usually represented by β. The input resistance refers to the ratio of the change in input voltage to the change in input current, which is usually represented byρ. The output resistance refers to the ratio of the change in output voltage to the change in output current, which is usually represented by r.

The maximum withstand voltage refers the maximum reverse voltage or maximum forward voltage that BJT can withstand, which is usually represented by VCEO and VCBO. The maximum withstand current refers to the maximum current thatJT can withstand, which is usually represented by ICmax. The maximum operating temperature refers to the highest temperature at which BJT can operate normally, which is usually represented Tjmax.

Design Flow of BJT

The design flow of BJT includes circuit analysis, selection, circuit design, circuit test, etc.

uit analysis refers to the analysis of the circuit to understand the nature and characteristics of the circuit. Selection refers to the selection of the appropriate BJT model according to the circuit. Circuit design refers to the design of the circuit that meets the requirements according to the circuit requirements. Circuit test refers to the testing of the designed circuit to verify the performance and characteristics the circuit.