How to understand the different structure of a power semiconductor

Structural basis of power semiconductors of different structures

We are currently in contact with the semiconductor, which form the basis of three structures:

• pn junction
• MOS (Metal - Oxide - Semiconductor)
• MES (Metal - Semiconductor)

To more accurately understand the working principle of semiconductor, we should be familiar with the structure and properties of these three infrastructure.

• pn junction

  About the pn junction, the most important concept is the drift and diffusion. Briefly, p-electrode material due to the different reference electrode material debris n, resulting in different types of carriers inside thereof, a hole for the p-type material, n-type material is an electron. In other without external electric field is applied, the internal n-type material not bound valence electrons bond ( free electrons ) may vary with the concentration of diffusing into the p; At the same time, P-type material, although there is no free electrons, but are bound by a covalent bond the valence electrons are attracted holes, moved to the position near the hole, since the concentration of impurities is much lower than the reference density of the material itself, so the concentration of the valence electrons is much higher than hole concentration (Nonetheless valence electron and can not serve as a carrier , so the interior of the majority carriers or holes p-type material), the surface, if a hole occurs diffusion (if not well understood, the bubbles rise in the water can imagine scenes). This is the phenomenon of diffusion pn junction.
  Considering the drift and diffusion phenomena, it will form a space charge region . The so-called space charge region due to the pn junction area in the vicinity of the interface, diffusion of electrons to the p-n material region, and the holes diffuse into the material of the p region n. Thus, n in the mating surface positively charged, negatively charged P, electric field is formed, an n-point p. Under the influence of an electric field, the majority carriers occurs drift (drift), p e n will go under the electric field, the holes will run to n p. (Of course, in essence, it is a free electron of the valence electron movement) Thus, the drift and diffusion will form an equilibrium state.
  Charge the space charge region is how the distribution of it?
  Look at the distribution of the electric field.
  
  The closer the interface, the stronger the electric field, in order to maintain balance, the stronger diffusion, the majority carriers p, n is the less area, so this is a depletion layer (depletion Layer) .
  When an external electric field is applied, drift and diffusion of the original balance is broken.
  If a forward voltage is applied (positive n minus p), opposite to the direction of electric field of the self, the electron is enhanced when p, n holes to effect diffusion of the comparison is not applied from the p-n forward bias, We need smaller diffusion region, so the space charge region (depletion layer) thinner.

  From the point of view of electron movement, the electrons pass through the space charge region diffuse into the p region. First, the p region is formed at the boundary xp accumulated as an additional minority carriers, the electron density is higher than the internal vicinity xp p region, an electron diffusion current from the p diffusion into the interior of the boundary. These extra electron hole recombination side diffusion region and the p-side, after several diffusion lengths from the entire composite was only off. This area is called the diffusion region. In the case where the forward bias constant, n is a fixed rate regions to inject additional electrons into the p region. Minority carrier diffusion region formed in the stable distribution, thereby forming a stable electron diffusion current.

  Similarly, in the n-region will form a stable hole distribution and stability of the hole diffusion current.

  In the circuit, must have a current path, when the ON current, it is necessary to extract electrons from the p region, electrons are injected into the n-region, it is not necessary to worry about the hole will be fully p region or n region complex is completely electronic exhausted.

  If a negative bias is applied, then the analysis is the same, but, negative bias electric field generated electrons and holes are driven by the p-n area of ​​the region, i.e. minority carriers, and therefore do not form considerable current. Because of their low birth rate is very low in density at the boundary, after application of a negative bias that has minority carrier density near the border to zero. Further, the minority carrier density gradient is not voltage dependent, and therefore the pn junction reverse current is small and hardly changes with voltage.

• MY

  Application of the metal semiconductor inseparable.
  In one aspect the metal as an electrode, on the other hand, some of the metal - semiconductor contact having as a pn junction as unidirectional conductivity . This leads to the Schottky barrier contact concept.

  Before introducing time permits, let's clarify a few physics concepts.

  • Level, band, band gap
    for the isolated atom, the other will not be affected due to the chemical bond, only energy level. Atoms of the crystal will be formed chemically affect other atoms, formed level will split, a band; however, the gap between the bands is not exist electrons, so called a forbidden band.
    Level split into bands, the gap between the band for the band gap.

  • The valence band, the conduction band of
    conductive material depending on the electron energy band of the case is filled. Full band can not transport electrons net charge, i.e., electrons are not filled with a conductive, it can conduct only under the electron band .
    In absolute zero, all of the semiconductor band-gap energy to a boundary characteristics, an energy band in which electrons are all filled, the entire band which empty.
    The energy band is fully occupied all valence electrons of the electron, and thus this band is called the valence band ; empty belt which immediately appears as a small amount of free electrons in the metal at a temperature similar to the non-zero, may participate in conducting electronic, thus the name of the conduction band .
    When the temperature T> 0 when the semiconductor valence band energy of some electrons across the high forbidden band into the conduction band, thus, the valence and conduction band electrons will become full band .
    For metals, even at the absolute zero, the valence band is full band , where all valence electrons of free electrons, it is difficult to a higher band excitation, it is difficult to obtain from a lower energy bands, which metal is a good conductor, conductive easy to change.

  • Fermi level (EF)
    when the system is in a state of thermal equilibrium, the outside world does not work, the system adds an electronic system caused by the free energy of the system is equal to the chemical potential, which is equal to the Fermi level system . (An electronic system to increase the difficulty)

    For metals, the absolute zero, electrons occupy the highest level is the Fermi level.

    For the n-type semiconductor, the conduction band which contains more free electrons (majority carriers), and therefore, the Fermi level near the conduction band, located below the conduction band, higher doping concentration, the closer the guide level bottomed;

    For the p-type semiconductor valence band which contains more free holes (majority carriers), and therefore, the Fermi level closer to the valence band, located above the valence band, the higher the dopant concentration, the closer the level Valence band.
    When forming a pn junction, so that the flow of electrons and holes decrease EFn n region, p region EFp increased, preferably until both are equal to equilibrium.

    Look at the metal-semiconductor contact with the band change time.

    Since different metals and the Fermi level of the semiconductor, both when the close contact, the difference between the Fermi level of each other will cause the transfer of electrons.
    Forming a barrier layer after the transfer of the two metals in contact with the semiconductor, the semiconductor surface because it exhibits the majority carrier blocking barrier metal image transfer, it is referred to as a Schottky barrier contact .

    For example, metal -n-type semiconductor, if satisfied WM <WS, the electrons will be transferred from the semiconductor to the metal, a semiconductor space charge region formed on the surface of a positively charged. The direction of the electric field directed self metal, therefore, the other side of the semiconductor electronic want to move to the side of the metal required to overcome great resistance, and then form a barrier, a barrier layer is called the space charge region.

    When a positive bias is applied, an electric field is weakened self, barrier lowering, electrons may flow from the semiconductor metal, also appeared in the current flowing from the metal semiconductor. Negative bias when the barrier increase, almost no reverse current.

    To form the anti-barrier metal-semiconductor, and an ohmic contact, as a pure electrode used.

• MOS

  For MOS, it needs to be emphasized that, although it seems a MOS internal pn junction, pn junction but only when the action for blocking off a large voltage, the conduction time, the metal - oxide - equivalent to a capacitor of the semiconductor, the semiconductor surface is pushed carriers, inversion of the semiconductor surface, forming a channel power, and does not participate in the pn junction is turned on.