The "PN junction" area of the surface contact diode is large, which permits the passage of large currents (several to dozens of amperes). It is mainly used in the "rectifier" circuit for converting alternating current into direct current.

Planar diode is a special silicon diode. It can not only pass through large current, but also has stable and reliable performance. It is mostly used in switching, pulse and high frequency circuit.

Conductivity Characteristics of Diodes

The most important characteristic of a diode is its unilateral conductivity. In the circuit, the current can only flow from the positive pole to the negative pole of the diode. Following is a simple experiment to illustrate the forward and reverse characteristics of the diode.

Forward characteristic

In the electronic circuit, the positive pole of the diode is connected to the high potential end, and the negative pole is connected to the low potential end, and the diode will turn on. This connection method is called positive bias. It must be noted that when the forward voltage applied to both ends of the diode is very small, the diode is still not conductive and the forward current flowing through the diode is very weak. Only when the forward voltage reaches a certain value (this value is called "threshold voltage", "germanium tube is about 0.2V, silicon tube is about 0.6V), can the diode be directly conducting. After conduction, the voltage at both ends of the diode remains basically unchanged (germanium tube is about 0.3V, silicon tube is about 0.7V), which is called "forward voltage drop" of the diode.

2. Reverse characteristics

In the electronic circuit, the positive pole of the diode is connected to the low potential end, and the negative pole is connected to the high potential end. At this time, there is almost no current flowing through the diode. At this time, the diode is in the cut-off state, which is called reverse bias. When the diode is in reverse bias, there will still be a weak reverse current flowing through the diode, called leakage current. When the reverse voltage at both ends of the diode increases to a certain value, the reverse current will increase sharply, and the diode will lose its unilateral conductivity. This state is called the breakdown of the diode.