3.4 SEMICONDUCTOR AND P-N JUNCTION Flashcards
electrical property(властивості) of material
Conductors
materials with many free electrons. These electrons can easily be made to flow through the material
examples: all metals, semi-metals like carbon-graphite, antimony, arsenic
electrical property of material
Insulator
materials that have a very few free electrons
examples: plastic, glass, wood
electrical property of material
Semiconductors
materials that behave like** insulators when pure**, but will conduct when an impurity(домішки) is added and/or in response to light, heat, voltage
examples: elements:silicon, germanium
compounds: gallium arsenide
In an isolated atom
the electron occupy discrete energy levels
band theory
https://i.pinimg.com/originals/4b/ea/3f/4bea3f03e999d4b4d20c8206b52a8c56.jpg
band gap is
is the range in a solid where no electron state can exist.
fermi level is
the maximum permitted energy an electron in a specific structure can possibly have at temperature 0
Conductor’s structure
the highest occupied energy band is only partially fillled (conduction (top) band). There are many empty levels available close to the occupied levels for the electrons to move into.
Electrons can flow easily from one atom to another
Insulator’s structure
the highest occupied band is completely full of electrons (valence band(bottom)). There is NO electrons in the conduction band. And there is a band gap which so large and electons almost never cross it
Semiconductor’s structure
Valence band is completely** full**, but the band gap is small. Due to temerature the electrons can jump the gap
Increasing temperature increasing the number of electrons in conduction band
Bonding in Semiconductor
the most common materials are silicon and germanium, which have 4 valency
Therefore there are a few free electrons aviable to conduct
these semiconductors have a large resistance
A positive hole is
a lack of electron/ free space, which is positively charged, when electron leaves its position in the crystal lattice
BUT
In an intrinsic (внутрішній) semiconductor, the number of holes is equal to the number of electrons which causes the small currentof drifting electrons
Doping is
the conductivity of pure (intrinsic) semiconductors which can be improved by the addition of imparities during manufacture
n-type has
As arsenic nucleus with extra free electon (‘negative’)
p-type has
In indium nucleus with additional hole (‘positive’)
how doping affects band structure
students’ notes p27
https://th.bing.com/th/id/OIP.g5qXs7uBPwQX43w4ePIBaAHaDC?pid=ImgDet&rs=1
p-n junction (it functions as diode)
https://www.mksinst.com/mam/celum/celum_assets/Figure_8-Semiconductor_Handbook_800w.jpg
https://4.bp.blogspot.com/-KW29JLAHhxQ/W-lO79Hxm1I/AAAAAAAABvQ/99nkXracKds-YqjtIIxgUEj3pOT7Rq5GACLcBGAs/w1200-h630-p-k-no-nu/PN-Junction%2BDiode%2BSymbol.png
the DONOR LEVEL is
an isolated energy level in the band gap with free electrons in n-type
where fermi level is raised
the ACCEPTOR LEVEL is
an isolated energy level in the band gap in p-type with free holes
whrere fermi level is lowered to just below this energy
biasing the diode
- Forward-biased (cell connected: positive to p-type, negative to n-type) comforable to use, big current
- Reverse-biased (opposite) very small current, very big, ‘high’ deplation layer where it is difficult to electrons to move ‘uphills’
dapletion layer has
no free charged carries
the PHOTODIODE
a p-n junction in a transparent coating will react to light
Can be used in two modes:
1. Photovoltaic mode
2. Photoconductive mode
https://learningaboutelectronics.com/images/photodiodesymbol.png
Photovoltaic mode
has NO bias voltage
using more incident light (more photons incident per second)
The **voltage **generated is propotional to the light intensity
many photodiodes connected together form the basis of solar cells
PHOTOCONDUCTIVE MODE
has reverse bias
in the dark DIODE DOESN’T conduct
A greater intensity of light will lead to more free charge carries and **less resistance **
Acts as a light dependant resistor (LDR)
