Minority carrier, that is, minority carrier, is the concept of semiconductor physics. It is relative to Polyglot for. 
P-type semiconductor semiconductor material There are both electrons and holes in carrier 。 If a certain carrier occupies a minority in semiconductor materials and plays a secondary role in conduction, it is called minority carrier. For example, in N-type semiconductor The hole is a minority carrier, and the electron is a majority carrier; stay P-type semiconductor The hole is the majority carrier, and the electron is the minority carrier. [1] Formation of many and few carriers: taking silicon as an example, pentavalent element There are five atoms of valence electron , when it replaces lattice Four prices in silicon Every pentavalent element atom The four valence electrons in the covalent bond The remaining one is not bound by covalent bonds, and the heat energy obtained at room temperature is enough to make it break away from the attraction of atomic nucleus and become free electron 。 Since the electron is not a valence electron in the covalent bond, no hole will be generated at the same time. For each pentavalent element atom, although it releases a free electron, it becomes an electron charge Quantitative Positive ion However, it is bound in the lattice and cannot conduct electricity like a carrier. In this way Intrinsic excitation concentration comparison, N-type semiconductor Intermediate free electron concentration However, the chance of hole recombination due to the encounter of free electrons increases, and its concentration is smaller. Minority carrier concentration is mainly determined by intrinsic excitation, so it is greatly affected by temperature.
Relation curve between minority carrier lifetime and resistance Determinants of minority carrier lifetime
The main factors that affect the minority carrier lifetime in different semiconductors are carrier Recombination mechanism of（ Direct recombination 、 Indirect recombination 、 Surface recombination , Auger composite, etc.) and related issues. For Si, Ge, etc Indirect transition Because conduction band Bottom and Price band The top is not at the same point in the Brillouin region, so the conduction band electrons and the valence band hole The direct recombination of is difficult (it can only be realized with the help of phonons, etc. - because the carrier recombination Momentum conservation ）, the main factor determining the minority carrier lifetime is through Composite center The indirect recombination process of. Thus, the recombination centers (types and quantities) caused by harmful impurities and defects in semiconductors have a great impact on the minority carrier lifetime of these semiconductors. Therefore, in order to increase the minority carrier life, harmful impurities and defects should be removed; On the contrary, if you want to shorten a few Carrier lifetime , some impurities or defects that can produce composite centers can be added (such as doping Au, Pt, or using high-energy Particle beam Bombardment, etc.). For GaAs, etc Direct transition Because conduction band Bottom and Price band The top is at the same point in the Brillouin region, so the main factor determining the minority carrier lifetime is the conduction band electrons and the valence band hole Of Direct recombination Process. Therefore, the minority carrier lifetime of such semiconductors is generally short. Of course, harmful impurities and defects will further promote recombination and shorten life.
Influence of minority carrier lifetime on semiconductor devices
For bipolar semiconductor devices that mainly rely on minority carrier transport (diffusion), minority carrier lifetime is an important parameter that directly affects device performance. At this time, a relevant parameter often used is the minority carrier diffusion length L (equal to the square root of the product of diffusion coefficient and life), which represents the average distance that minority carriers can travel while diffusing and recombination. The longer the minority carrier life, the larger the diffusion length.
For BJT, in order to ensure that the recombination of minority carriers in the base region is as little as possible (to obtain a large current amplification factor), the base region width must be shortened below the diffusion length of minority carriers. Therefore, the longer the minority carrier lifetime of the base region is, the better.
semiconductor The unbalanced carrier lifetime in is a basic characteristic parameter of semiconductors. Its length will directly affect the performance of semiconductor devices that rely on a few carriers. Such devices include bipolar devices and p-n junction optoelectronic devices. However, unipolar devices (such as MOSFETs) containing p-n junction in structure will also be affected by carrier lifetime. The non-equilibrium carrier lifetime mainly refers to the non-equilibrium minority carrier lifetime. The main factors affecting the minority carrier lifetime are the semiconductor band structure and the recombination mechanism of non-equilibrium carriers; For indirect band gap semiconductors such as Si, Ge and GaP, impurities and defects in semiconductors are generally the main factors that determine the lifetime.
The characteristics of electronic devices that have obvious dependence on minority carrier lifetime mainly include switching characteristics, conduction characteristics and blocking characteristics of bipolar devices; For photocells Photodetector The characteristics directly related to minority carrier lifetime of optoelectronic devices such as photogenerated current, photogenerated electromotive force, etc. Main methods for controlling minority carrier lifetime
Generally, there are two aspects to consider:
One is to control the carrier lifetime in the process so that no change occurs. Here, attention should be paid to the control of cleanliness and operation process to avoid the introduction of harmful impurities and reduce the secondary defects induced by the process.
Second, it can be controlled by intentionally doping some deep level impurities or causing some crystal defects, because many deep level impurities and crystal defects will form a composite center. In Si devices, the deep level impurities commonly used as the recombination center are Au and Pt. The measure commonly used to introduce crystal defects is electron irradiation. The introduction methods of Au, Pt and electron irradiation are different. Generally, we can see that:
① For highly doped (low resistance) semiconductor materials, the τ H/τ L ratios of Au doped and Pt doped materials are both large; However, for low doped (high resistance) semiconductor materials, only Au doped τ H/τ L ratio is large. Therefore, from the perspective of both reducing the conduction voltage drop and increasing the switching frequency, the effect of doping Au is better.
② From the ratio of minority carrier generation life to large injection life (τ s/τ H), the ratio of Pt doped and electron irradiation is large. Therefore, both Pt doped and electron irradiation can maintain the Reverse leakage current Smaller. ③ For Pt doped Si, the τ H/τ L ratio varies greatly with the doping concentration, so Pt is not ideal as the composite center of power devices;
④ For electron irradiated Si, the τ H/τ L ratio basically does not change with the doping concentration, so electron irradiation can provide an ideal recombination center for power devices;
⑤ For Au doped Si, the τ H/τ L ratio does not change with the doping concentration at all. Therefore, Au is also an ideal composite center for power devices. [1]
Although the number of minority carriers is small, the current generated by them is not necessarily small. The main reason is that they can produce a large concentration gradient, which can transport a large current. For example, SCR with a working current of hundreds of amperes is a device with a few carriers, and all BJTs are devices with a few carriers. On the contrary, the current of most carrier operated devices is not necessarily large.
A few carriers can store (accumulate), which has a great impact on the switching speed of devices; The capacitive effect of most carriers (barrier capacitance) is often the factor affecting the highest operating frequency of devices.
