Diode and transistor are two commonly used semiconductor devices in electronic circuits. There are many differences between them in structure, working principle, characteristics and application, as follows:

Structure

Diode: A P-type semiconductor and an N-type semiconductor are in close contact to form a PN junction, plus the corresponding electrode leads and tube shell packaging.

Triode: There are three regions, namely the emitter region, the base region and the collector region. The emitter region and the collector region are both N-type semiconductors (for NPN transistors) or P-type semiconductors (for PNP transistors), and the base region is a very thin P-type semiconductor (for NPN transistors) or N-type semiconductors (for PNP transistors). There are three electrode leads corresponding to the three regions, namely the emitter (E), base (B) and collector (C).

Working principle

Diode: Works by using the unidirectional conductivity of the PN junction. When a forward voltage is applied to both ends of the diode, the PN junction becomes narrower and the current can pass smoothly. At this time, the diode is in the on state; when a reverse voltage is applied, the PN junction becomes wider, and only a very small reverse leakage current passes through, and the diode is in the off state.

Transistor: It mainly works based on the current control principle. Taking the NPN transistor as an example, when the emitter junction is forward biased (the base is a positive voltage relative to the emitter) and the collector junction is reverse biased (the collector is a positive voltage relative to the base), the majority of carriers (electrons) in the emitter region are injected into the base region due to diffusion movement. A small part of them is combined with the holes in the base region to form the base current, while most of the electrons can pass through the base region to reach the collector region and are collected by the collector region to form the collector current. The collector current is proportional to the emitter current, and the proportional coefficient is called the current amplification coefficient. By controlling the size of the base current, the size of the collector current and the emitter current can be controlled, thereby realizing the amplification effect on the transistor.

Characteristics

Diode: It has unidirectional conductivity, and its volt-ampere characteristic curve is nonlinear. When forward conduction occurs, the voltage drop is relatively stable. Generally, the forward conduction voltage of a silicon diode is about 0.7V, and that of a germanium diode is about 0.3V. When reverse cutoff occurs, the reverse current is ideally zero, but there is actually a small amount of reverse leakage current, and when the reverse voltage exceeds a certain value, reverse breakdown occurs.

Transistor: It has a current amplification function. Its input characteristic curve is similar to the forward characteristic curve of the diode, but it has different working areas, including the cutoff region, the amplification region, and the saturation region. In the cutoff region, the base current is almost zero, and the collector current is also very small. The transistor is equivalent to a switch disconnected; in the amplification region, the collector current is linearly related to the base current, and the transistor realizes the amplification of the electrical signal; in the saturation region, the collector current no longer increases significantly with the increase of the base current, and the transistor is equivalent to a switch closed.

Performance indicators

Diode: The performance indicators mainly include forward voltage drop, reverse breakdown voltage, and reverse recovery time, which are mainly used to evaluate its performance in rectification and protection circuits.

Transistor: Key performance indicators include DC current gain (β or h_FE), input and output impedance, frequency response and switching speed, which affect its performance in signal amplification and switch control.

Application

Diode: Commonly used in rectifier circuits to convert AC to DC; can also be used in limiting circuits, voltage stabilization circuits (such as voltage stabilization diodes) and protection circuits in various electronic circuits.

Transistor: Mainly used in amplification circuits, such as audio amplifiers, radio frequency amplifiers, etc., to amplify weak electrical signals; can also be used as electronic switches to control the on and off of circuits, and are widely used in digital circuits and various control circuits.

In general, there are many differences between diodes and transistors in structure, function and application. Diodes are widely used in rectification and protection due to their unidirectional conductivity, while transistors are widely used in various electronic circuits due to their amplification and switching functions.