Crystal diode

The basic structure of a crystal diode is formed by combining a P-type semiconductor and an N-type semiconductor to form a PN junction. At the interface of the PN junction, a dipole layer with space charge is formed due to the mutual diffusion of holes in P-type semiconductors and electrons in N-type semiconductors towards each other. This dipole layer prevents the continued diffusion of holes and electrons, causing the PN junction to reach an equilibrium state. When the P-terminal of the PN junction (on the P-type semiconductor side) is connected to the positive pole of the power supply and the other end is connected to the negative pole, both holes and electrons flow towards the dipole layer, causing the dipole layer to become thinner and the current to rise quickly. If the direction of the power supply is reversed, both holes and electrons flow away from the dipole layer, causing the dipole layer to become thicker, and the current is limited to a very small saturation value (called reverse saturation current). Therefore, the PN junction has a single conductivity. In addition, the dipole layer of the PN junction also acts as a capacitor, which varies with the applied voltage. The electric field inside the dipole layer is very strong. When the applied reverse voltage reaches a certain threshold, avalanche breakdown occurs inside the dipole layer, causing the current to suddenly increase by several orders of magnitude. Diodes made using these characteristics of PN junctions in various application fields include rectifier diodes, detector diodes, frequency conversion diodes, varactor diodes, switching diodes, voltage regulator diodes (Zener diodes), avalanche transition diodes (collision avalanche transition diodes), and capture diodes (capture plasma avalanche transition time diodes). In addition, there are tunnel diodes that utilize the special effects of PN junctions, as well as Schottky diodes and Gunn diodes without PN junctions.

Bipolar transistor

It is composed of two PN structures, one of which is called the emission junction and the other is called the collector junction. The thin layer of semiconductor material between two junctions is called the base region. The two electrodes connected to one end of the emission junction and one end of the collector junction are called the emission electrode and the collector electrode, respectively. The electrode connected to the base region is called the base electrode. When applied, the emission junction is in forward bias and the collector is in reverse bias. By emitting the current from the junction, a large number of minority carriers are injected into the base region. These minority carriers diffuse and migrate to the collector junction to form the collector current, and only a very small number of minority carriers recombine within the base region to form the base current. The ratio of collector current to base current is called the co emitter current amplification coefficient. In a common emitter circuit, a small change in base current can control a significant change in collector current, which is the current amplification effect of bipolar transistors. Bipolar transistors can be divided into two types: NPN type and PNP type.

Field-effect transistor

Field effect transistors rely on a thin layer of semiconductor to change its resistance (referred to as field effect) under the influence of a transverse electric field, enabling them to amplify signals. The two electrodes connected at both ends of this thin layer of semiconductor are called the source and drain. The electrodes that control the transverse electric field are called gates.

According to the structure of the gate, field-effect transistors can be divided into three types:

① Junction field-effect transistor (gate with PN structure);

② MOS field-effect transistor (gate composed of metal oxide semiconductor, see metal insulator semiconductor system);

③ MES field-effect transistor (gate formed by metal semiconductor contact); Among them, MOS field-effect transistors are the most widely used. Especially in the development of large-scale integrated circuits, MOS large-scale integrated circuits have special advantages. MES field-effect transistors are generally used on GaAs microwave transistors.

On the basis of MOS devices, a charge coupled device (CCD) has been developed, which uses the stored charges near the semiconductor surface as information to control the potential well near the surface to transfer charges in a certain direction near the surface. This type of device can usually be used as a delay line, memory, etc; Equipped with a photodiode array, it can be used as a camera tube.