Transistor is a semiconductor device, which is one of the foundations of modern electronic technology. It can be used to amplify electrical signals, switch circuits, and act as an oscillator. Transistors are made of semiconductor materials, usually silicon (Si) or germanium (Ge). The shape of a transistor is usually a small chip that can be installed on a circuit board. The following article will briefly introduce the working principle and three working states of transistors. Let's take a look together!

A transistor has three regions: a P-type semiconductor region, an N-type semiconductor region, and a P-N junction region. The electrons in the material of the P-type semiconductor region are reduced, while the number of electrons in the material of the N-type semiconductor region increases. When the P-type and N-type regions come into contact, a P-N junction is formed. In the region of the P-N junction, electrons flow from the N-type region to the P-type region, while holes flow from the P-type region to the N-type region. This flow is called drift flow.

The operating principle of transistors is based on the electrical properties of P-N junctions. When a positive voltage is applied to the P-type region and a negative voltage is applied to the N-type region, a reverse bias is formed at the P-N junction, and the transistor is turned off. When a negative voltage is applied to the P-type region and a positive voltage is applied to the N-type region, a positive bias is formed at the P-N junction, and the transistor is in a conduction state.

The three working modes of transistors are: common base, common emitter, and common collector. These operating modes are named according to the circuit connections of the three regions in the transistor, which control the current and voltage of the transistor respectively to achieve different circuit functions.

Common Base (CB) Operation Mode

In a common base operation, the base of the transistor is used as the input terminal, the emitter is used as the output terminal, and the collector is used as the common terminal. The input signal is applied to the base electrode and the output signal is obtained from the emitter electrode. In this mode of operation, the voltage gain of the circuit is small, but the current gain is large. Therefore, it is commonly used in high-frequency amplification circuits and circuit stabilizers.

Common emitter (CE) operation mode

In a common emitter operation, the emitter of a transistor is used as the input terminal, the collector is used as the output terminal, and the base is used as the common terminal. An input signal is applied to the emitter and an output signal is obtained from the collector. In this mode of operation, the voltage gain and current gain of the circuit are both large, so it is commonly used in amplifier and switching circuits.

Common collector (CC) working mode

In a common collector operation, the collector of a transistor is used as the input terminal, the emitter is used as the output terminal, and the base is used as the common terminal. An input signal is applied to the collector and an output signal is obtained from the emitter. In this mode of operation, the voltage gain of the circuit is large, but the current gain is small. Therefore, it is commonly used in circuit buffers and circuit stabilizers.

These three modes of operation have different characteristics and application situations, so it is necessary to select appropriate modes of operation in actual circuits to achieve the required circuit functions.