Characteristics and Applications of Diodes

Operating Principle of Diode

Crystal diode is a p-n junction formed by p-type semiconductor and n-type semiconductor. Space charge layer is formed on both sides of its interface and self-built electric field is built. When there is no applied voltage, the diffusion current caused by carrier concentration difference on both sides of the p-n junction is equal to the drift current caused by the built-in electric field, so it is in the state of electric equilibrium.

When there is a positive voltage bias, the interaction between the external electric field and the self-built electric field causes the increase of carrier diffusion current and the positive current.

When there is reverse voltage bias, the external electric field and the self-built electric field are further strengthened, and the reverse saturated current I0 independent of the reverse bias voltage is formed in a certain range of reverse voltage.

When the applied reverse voltage is high to a certain extent, the electric field intensity in the space charge layer of p-n junction reaches the critical value, resulting in the multiplying process of carriers, a large number of electron hole pairs and a large number of reverse breakdown currents, which are called diode breakdown phenomena.

Types of Diodes

There are many kinds of diodes. According to the semiconductor materials used, they can be divided into germanium diode (Ge tube) and silicon diode (Si tube). According to its different uses, it can be divided into detector diode, rectifier diode, regulator diode, switch diode and so on. According to the core structure, it can be divided into point contact diode, surface contact diode and planar diode. The point contact diode is a kind of semiconductor wafer with a very thin wire pressed on the surface of the wafer, and connected with a pulse current, so that one end of the contact wire and the wafer are firmly sintered together, forming a "PN junction". Because of point contact, only small currents (no more than tens of milliamperes) are allowed to pass through, which is suitable for high frequency low current circuits, such as radio detection.

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.