EC-5.3 Crystals, Lattices, and How Bands Form
You can explain how discrete atomic levels become continuous bands when atoms are brought together, read a simple band diagram, and say what the gap between two bands means physically.
Before:nothing requiredUnlocks:PE2-5. MEMSEC-6. Semiconductor Devices and Device Models
This is the topic the whole area has been assuming. Bring two atoms close and each shared level splits in two. Bring a mole of them close and each level becomes a dense band of levels, with forbidden ranges between. That picture explains conduction, insulation, doping, junctions and light emission, and it is the single most load-bearing diagram in electronics. The GATE paper names it as the first line of its Electronic Devices section, and until now nothing here taught it.
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The crystal lattice, and what makes a solid crystalline
A crystal repeats one arrangement of atoms in every direction, so knowing a small cell and the repeat rule describes the whole solid. Silicon's structure has each atom bonded to four neighbours, which is why four valence electrons matter so much.
NPTEL: Physics of Materials · CourseWhy bringing atoms together splits every level
Two identical levels cannot both stay identical when the atoms interact, so each one splits into two slightly different ones. With a huge number of atoms the splitting produces a band of levels so close together that it behaves as continuous.
NPTEL: Physics of Materials · CourseThe valence band, the conduction band, and the forbidden gap between
The band holding the bonding electrons is the valence band and the empty one above it is the conduction band, with a range of energies in between that no electron can occupy. The size of that gap is the number that decides what kind of material you have.
Reading a band diagram: energy up the page, position across it
The vertical axis is electron energy and the horizontal axis is position in the device, so a sloped band means a field and a step means a junction. Learning to read this one picture is most of what device physics asks of you.
MIT OpenCourseWare 3.091SC: Introduction to Solid State Chemistry · CourseHoles: why an empty state behaves like a positive particle
When an electron leaves the valence band the vacancy it leaves moves as neighbouring electrons shuffle into it, and the bookkeeping is far easier if you follow the vacancy instead. It is a convenience rather than a particle, and it is the right convenience.
Direct and indirect gaps, and why only some materials emit light
An electron falling across the gap gives up its energy as light only when it can do so without changing momentum as well. Silicon cannot, which is why silicon does not make light and why a light-emitting device is made from something else.
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