A material that permit or allows flow of
electrons are called Conductors (e.g., gold, silver, copper,
etc.).Materials that block flow of electrons are called Insulators (e.g., rubber, glass, Teflon, mica,
etc.).Materials whose conductivity falls between those of conductors and
insulators are called Semiconductors.
Semiconductors are “part-time” conductors, whose conductivity can be
controlled. That’s why we have to use semiconductors in Electronic component’s
called as Diode, Transistor, etc.
PN junction is made up on P-type (contains highly doped holes and minority
electrons) and N-type (contains highly doped electrons and minority holes)
materials. The P & N type materials are combines to form a PN junction is
called as Depletion region.
In N-type side the concentration of electrons are high, so they move towards
the junction is called depletion. In P-type side the concentration of holes are
high, so they are move towards the junction.
Silicon is the most common material used to build semiconductor devices.
Si is the main ingredient of sand and it is estimated that a cubic mile of
seawater contains 15,000 tons of Si. Si is spun and grown into a crystalline
structure and cut into wafers to make electronic devices. An atom in a pure
silicon wafer contains four electrons in outer orbit (called valence Electrons)
.Germanium is another semiconductor material with four valence electrons. In
the crystalline lattice structure of Si, the valence electrons of every Si atom
are locked up in covalent bonds with the valence electrons of four neighboring
Si atoms. In pure form, Si wafer does not contain any free charge carriers. An
applied voltage across pure Si wafer does not yield electron flow through the
wafer. A pure Si wafer is said to act as an insulator. In order to make useful
semiconductor devices, materials such as phosphorus (P) and boron (B) are added
to Si to change Si’s conductivity.
N-Type Silicon Penta-valent impurities such
as phosphorus, arsenic, antimony, and bismuth have 5 valence electrons. When
phosphorus impurity is added to Si, every phosphorus atom’s four valence electrons are locked up in
covalent bond with valence electrons of four neighboring Si atoms. However, the 5th
valence electron of phosphorus atom does not find a binding electron and thus
remains free to float. When a voltage is applied across the silicon-phosphorus mixture,
free electrons migrate toward the positive voltage end. When phosphorus is
added to Si to yield the above effect, we say that Si is doped with phosphorus. The resulting
mixture is called N-type silicon (N: negative charge carrier silicon). The penta valent impurities are referred
to as donor impurities.
P-Type Silicon Trivalent impurities e.g.,
boron, aluminum, indium, and gallium have 3 valence electrons. When boron is added
to Si, every boron atom’s three valence electrons are locked up in covalent bond with valence
electrons of three neighboring Si atoms. However, a vacant spot “hole” is created
within the covalent bond between one boron atom and a neighboring Si atom. The
holes are considered to be positive charge carriers. When a voltage is applied
across the silicon-boron mixture, a hole moves toward the negative voltage end while a
neighboring electron fills in its place. When
boron is added to Si to yield the above effect, we say that Si is doped with boron. The resulting mixture is
called P-type silicon (P: positive charge carrier silicon). The trivalent
impurities are referred to as acceptor impurities. The hole of boron atom
points towards the negative terminal. The electron of neighboring silicon atom
points toward positive terminal. The electron from neighboring silicon atom
falls into the boron atom filling the hole in boron atom and creating a “new”
hole in the silicon atom. It appears as though a hole moves toward the negative
terminal.
A Diode is a 2 lead semiconductor that acts as
a one way gate to electron flow. Diode allows current to pass in only one
direction. A PN-junction
diode is formed by joining together n-type and p-type silicon. In practice, as the n-type Si
crystal is being grown, the process is abruptly altered to grow p-type Si crystal.
Finally, a glass or plastic coating is placed around the joined crystal. The p-side is called
anode and the n-side is called cathode. When
the anode and cathode of a PN-junction diode are connected to external voltage such that the potential at
anode is higher than the potential at cathode, the diode is said to be forward biased. When
potential at anode is smaller than the potential at cathode, the diode is said
to be reverse biased. In a reverse-biased diode current is blocked.
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