A simplified planar silicon diode is illustrated in Fig. 2.0.3. Using this process for silicon diodes produces two differently doped layers of silicon, which form a ‘PN junction’. Undoped or ‘intrinsic’ silicon has a lattice structure of atoms, each having four valence electrons, but P type silicon and N type silicon are doped by adding a relatively very small amount of material having either an atomic structure with three valence electrons (e.g. Boron or Aluminium) to make P type, or five valence electrons (e.g. Arsenic or Phosphorus) to make N type silicon. These doped versions of silicon are known as ‘extrinsic’ silicon. The P type silicon now has a shortage of valence electrons in its structure, which can also be considered to be a surplus of ‘holes’ or positive charge carriers, whereas the N type layer is doped with atoms having five electrons in its valence shell and therefore has a surplus of electrons which are negative charge carriers.
A diode conducts strongly in the direction of the black arrow, and essentially 000 current can flow in the opposite direction. A diode made of silicon has a forward voltage between 0.60−0.75V.
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