Electrons and holes are mobile charged particles in semiconductors that determine the operation of electrical components like MOS transistors, diodes, switches, and capacitors. Mobile conduction electrons are free to move throughout the silicon crystal. Holes are voids in atomic electron shells that behave like mobile, positively charged particles.
The silicon atom has 4 electrons in its outer shell. In a silicon crystal, each atom is connected to 4 others and shares its outer electrons with those connected atoms. Because of the shared electrons, each atom behaves as if it has 8 electrons in its outer shell, a very stable configuration.
However, at room temperature, an electron occasionally has enough thermal energy to escape and then move freely throughout the crystal. This is called a mobile electron or conduction electron. The empty space left behind is called a hole that results in a net positive charge. The positive charge attracts nearby electrons, which can easily move within an atom's shell. In this way, a hole can move from atom to atom.
Mobile electrons and holes are free to move randomly through the silicon. If they meet, the electron falls into the hole, and both particles cease to exist. This is called recombination. Mobile electrons and holes are constantly created by thermal energy and destroyed by recombination.
A donor atom like phosphorus or arsenic has not 4, but 5 electrons in its outer shell. When put in a silicon crystal, the atom readily releases a mobile electron, and the atom acquires a positive charge.
A donor atom in silicon is something like a sodium atom. A sodium atom has a single electron in its outermost shell. It readily gives up this electron, leaving behind a new stable outer shell of 8 electrons, and acquiring a positive charge.
Silicon with a little bit of arsenic mixed in is called n-type silicon because the material contains negatively charged mobile particles that can carry an electric current. The positively charged donor atoms cannot move or carry a current.
In an analogous manner, an acceptor atom like boron has not 4, but 3 electrons in its outer shell. When put in a silicon crystal, the atom readily accepts an electron from a nearby silicon atom, creating a hole, and the boron atom acquires a negative charge.
An acceptor atom in silicon is something like a chlorine atom. A chlorine atom has 7 electrons in its outermost shell. It readily pulls in an electron, making a stable outer shell of 8 electrons, and acquiring a negative charge.
Silicon with a little bit of boron mixed in is called p-type silicon because the material contains positively charged mobile particles that can carry an electric current. The negatively charged boron atoms cannot move or carry a current.
A 1-cm cube of pure crystalline silicon contains 5 x 10^22 atoms. At room temperature, the cube contains 10^10 electrons and 10^10 holes. Both kinds of particles can carry an electric current. The resistance between two opposite faces of the cube is 5,000 ohms.
Note: Resistance of a 1-cm cube of silicon between two opposite faces, in ohms, is equivalent to the resistivity value of the material in ohm-cm.
Silicon doped with boron to 10^14 atoms per cubic cm results in 10^14 holes per cubic cm. This abundance of carriers results in a lower resistance, 130 ohms.
Silicon doped with arsenic to 10^14 atoms results in 10^14 mobile electrons. Because electrons move more easily through silicon than holes, the resistance is somewhat lower than the boron example, 40 ohms.
When the dopant concentration exceeds 10^19, the material conducts so well, it behaves like a metal is said to be degenerate. In integrated circuits, strongly doped, degenerate silicon is used to make MOS gates and short electrical connections.
The next video describes an analogy between silicon doping and pH chemistry: • Semiconductor doping, donor/acceptor elect...
For mobile carriers to be created, electrons must have enough thermal energy at the prevailing temperature to escape from the covalent bond between adjacent silicon atoms. The energy band model explains the numbers of electrons and holes in intrinsic silicon and why donors and acceptor atoms are fully ionized at room temperature.
For energy band model (conduction band, valence band, Fermi level), see https://www.chu.berkeley.edu/wp-conte...
P-N junctions -- electron/hole diffusion/drift, majority/minority carrier profiles, exponential diode equation: • Semiconductor P-N junction, electron/hole ...
Whole semiconductor device playlist: • Semiconductor physics, p-n junction diodes...
Silicon unit cell, Miller indices: • Silicon & diamond unit cell atomic model, ...
CMOS inverter operation: • CMOS Tech: NMOS and PMOS Transistors in CM...
Информация по комментариям в разработке