半导体技术【英文】

Semiconductor Crystalline Structure
Silicon atoms have 4 electrons in outer shell
– inner electrons are very closely bound to atom
(4.5)
These electrons are shared with neighbor atoms on both sides to “fill” the shell
+
electron flow direction
-
Electron Bands
Electrons circle nucleus in defined shells
–K –L –M –N 2 electrons 8 electrons 18 electrons 32 electrons K L
(4.3)
» no “loose” electrons
Conduction in Crystal Lattices
– 2 in the s subshell – 2 in the p subshell
(4.6)
Semiconductors (Si and Ge) have 4 electrons in their outer shell
(4.1)
Transistors
Implementing logic functions
Electricity
(4.2)
Electricity is the flow of electrons Good conductors (copper) have easily released electrons that drift within the metal Under influence of electric field, electrons flow in a current
– the subshell model doesn’t hold anymore, and the electrons can be thought of as being part of the crystal, not part of the atom – 4 possible electrons in the lower band (valence band) – 4 possible electrons in the upper band (conduction band)
(4.4)
Monocrystal has uniform 3-dimensional structure Atoms occupy fixed positions relative to one another, but are in constant vibration about equilibrium
Semiconductor Devices
Atoms and electricity Semiconductor structure Conduction in semiconductors Doping
– epitaxy – diffusion – ion implantation – MOS – CMOS
– resulting structure is very stable – electrons are fairly tightly bound

– at room temperature, if battery applied, very little electric current flows
Energy Bands in Semiconductors
The space between the bands is the energy gap, or forbidden band
(4.7)
Insulators, Semiconductors, and Metals
(4.8)
This separation of the valence and conduction bands determines the electrical properties of the material Insulators have a large energy gap
Within each shell, electrons are further grouped into subshells
–s –p –d –f 2 electrons 6 electrons 10 electrons 14 electrons
M shell d p s
electrons are assigned to shells and subshells from inside out
As the distance between atoms decreases the discrete subshells spread out into bands As the distance decreases further, the bands overlap and then separate
10 6
2
– Si has 14 electrons: 2 K, 8 L, 4 M
Semiconductor Crystalline Structure
Semiconductors have a regular crystalline structure
– for monocrystal, extends through entire structure – for polycrystal, structure is interrupted at irregular boundaries
Current flow governed by Ohm’s Law V = IR
– magnitude of current depends on magnitude of voltage applied to circuit, and the resistance in the path of the circuit