Fundamentals of semiconductors, characteristics of semiconductors

Objects such as germanium, silicon, selenium, gallium arsenide, and many metal oxides and metal sulfides, whose conductivity is between conductors and insulators, are called semiconductors.

Semiconductors have some special properties. For example, the relationship between the resistivity and temperature of semiconductors can be used to make thermistors (thermistors) for automatic control; Its photosensitive characteristics can be used to make photosensitive elements for automatic control, such as photocells, photocells and photoresistors.

Semiconductors also have one of the most important properties, and if trace impurities are properly incorporated into pure semiconductor substances, their conductivity will increase millions of times. This characteristic can be used to manufacture a variety of semiconductor devices for different purposes, such as semiconductor diodes, transistors, etc.

When one side of a semiconductor is made into a p-type region and the other side is made into an n-type region, a thin layer with special properties is formed near the junction, which is generally called a p-n junction. The upper part of the figure shows the diffusion of carriers at the interface between p-type semiconductors and n-type semiconductors (indicated by black arrows). The middle part shows the formation process of the p-n junction, indicating that the diffusion of the carriers is greater than the drift (indicated by a blue arrow, and a red arrow indicates the direction of the built-in electric field).

The lower part is the formation of the PN junction. Represents the dynamic equilibrium of diffusion and drift.

Semiconductors have three main characteristics:

1.Thermal characteristics.

The resistivity of semiconductors changes significantly with temperature. For example, for pure germanium, for every 10 degrees of humidity, its resistivity decreases to 1 2. Subtle changes in temperature can be reflected in the obvious changes in semiconductor resistivity.

Taking advantage of the thermally sensitive properties of semiconductors, thermistors can be used as temperature sensing elements for use in temperature measurement and control systems.

It is worth noting that all kinds of semiconductor devices have thermal characteristics, which affect the stability of their operation when the ambient temperature changes.

2.Photosensitive properties.

The resistivity of semiconductors is sensitive to changes in light. When there is light, the resistivity is very small; When there is no light, the resistivity is great. For example, the commonly used cadmium sulfide photoresistor has a resistance of tens of megaohms when there is no light, and when exposed to light.

The resistance suddenly drops to tens of thousands of ohms, and the resistance value changes by a thousand times. Using the photosensitive characteristics of semiconductors, various types of optoelectronic devices are produced, such as photodiodes, phototransistors and silicon photocells. It is widely used in automatic control and radio technology.

3.Doping characteristics.

In a pure semiconductor, the doping of a very small amount of impurity elements will cause its resistivity to change greatly. For example. Doped in pure silicon.

The resistivity of boron will decrease from 214,000 · cm, that is, the conductive energy of silicon will be increased by more than 500,000 times. It is by incorporating some specific impurity elements to artificially and precisely control the conductivity of semiconductors to manufacture different types of semiconductor devices. It is no exaggeration to say that almost all semiconductor devices are made of semiconductor materials doped with specific impurities.

1. At very low temperatures, the valence band of the semiconductor is a full band (see band theory), after being thermally excited, part of the electrons in the valence band will cross the forbidden band into the space band with higher energy, and the presence of electrons in the band becomes the conduction band, and the lack of an electron in the valence band forms a positively charged vacancy, which is called a hole. Hole conduction is not an actual motion, but an equivalence.

2. When electrons conduct electricity, the holes of equal charge will move in the opposite direction. They produce directional motion under the action of an external electric field to form macroscopic currents, which are called electron conduction and hole conduction, respectively. This hybrid conduction, which is formed due to the generation of electron-hole pairs, is called intrinsic conduction.

The electrons in the conduction band fall into the holes, and the electron-hole pairs disappear, which is called recombination.

3. The energy released during recombination becomes electromagnetic radiation (luminescence) or thermal vibration energy (heating) of the crystal lattice. At a certain temperature, the generation and recombination of electron-hole pairs coexist and reach dynamic equilibrium, at which point the semiconductor has a certain carrier density and thus a certain resistivity. As the temperature increases, more electron-hole pairs will be produced, the carrier density will increase, and the resistivity will decrease.

The role of semiconductors is to become a component material for information processing. At present, the core unit of many electronic products in the world, such as computers, mobile **, and digital voice recorders, uses the conductivity change of semiconductors to process information. Common semiconductor materials include silicon, germanium, gallium arsenide, etc., and silicon is the most influential in commercial applications among various semiconductor materials.

Semiconductors can also be made of components, integrated circuits, etc., which are important basic products in the electronics industry.

The production and scientific research of semiconductor materials, devices and integrated circuits have become an important part of the electronics industry.

Integrated circuits are one of the most active areas in the development of semiconductor technology, and have developed to the stage of large-scale integration. Tens of thousands of transistors can be made on a few square millimeters of silicon wafers, and a micro-information processor can be made on a silicon wafer, or other more complex circuit functions can be completed.

The development direction of integrated circuits is to achieve higher integration and micropower consumption, and to enable information processing speed to reach the microsecond level.

Microwave DevicesSemiconductorsMicrowave devices include receiving, controlling, and transmitting devices. Receiver devices below the millimeter wave band are widely used. In the centimeter band, the power of transmitting devices has reached several watts, and people are developing new devices and new technologies to obtain greater output power.

The development of electronic devices, semiconductor luminescence, imaging devices, and laser devices has made optoelectronic devices an important field. Their application range is mainly: optical communication, digital display, image reception, optical integration, etc.

The principle of semiconductor chips, now let's take a look.

The working principle of the chip is: the circuit is carefully manufactured on the surface of the semiconductor chip for calculation and processing. Integrated circuits have two main advantages over discrete transistors: cost and performance.

The low cost is due to the fact that the chip takes all the components through photolithography and prints them as a single unit, rather than making only one crystal pure tube at a time. The high performance is due to the fact that the components switch on and off quickly, consuming less energy because the components are small and close to each other.

In 2006, chip areas ranged from a few square millimeters to 350 mm, and each millimeter could reach one million transistors. Digital integrated circuits can contain anything, with logic gates, flip-flops, multitaskers, and other circuits ranging from a few thousand to millions in a few square millimeters. The small size of these circuits allows for higher speeds, lower power consumption (see Low-Power Design) and reduced manufacturing costs compared to board-level integration.

These digital ICs, represented by microprocessors, digital signal processors, and microcontrollers, operate in binary and process 1 and 0 signals.