Timeline / Sequence — History of Semiconductor Technology — Notes

Year	Milestone	Significance for NEET Concepts
1874	Karl Ferdinand Braun discovers rectification in crystal contacts	First p-n junction behavior observed
1900s	Crystal radio detectors use galena (PbS) crystals	Natural p-n junction; reverse bias in practice
1930s	Wilson's band theory of solids	Energy band model — foundation for $E_g$ , CB, VB
1947	Shockley, Bardeen & Brattain invent the transistor at Bell Labs	n-p-n and p-n-p junctions in practice
1950s	Doping technology developed; p-n junction diodes manufactured	Extrinsic semiconductors, mass action law applied
1958	Jack Kilby (TI) & Robert Noyce (Fairchild) invent integrated circuits	Multiple logic gates on one chip
1960s	LEDs first demonstrated (red, GaAsP)	Band gap → photon energy → color relationship
1970s	Zener diodes widely used in voltage regulation circuits	Reverse breakdown as design feature
1980s	Green LEDs (GaP), then blue LEDs being researched	Higher $E_g$ → shorter wavelength
1993	Shuji Nakamura demonstrates high-brightness blue LED (GaN)	Nobel Prize 2014; $\lambda = hc/E_g$ validated
2000s	Solar cells become commercially significant	Photovoltaic effect applications
Present	Si still dominates (1.1 eV, 0.7 V) despite 70 years of alternatives	NEET standard values unchanged

Separate p-type (holes majority) and n-type (electrons majority) exist
Materials brought into contact: majority carriers diffuse across junction
Electrons from n-side recombine with holes on p-side near junction
Immobile ions remain: positive donor ions on n-side, negative acceptor ions on p-side
These ions create an internal electric field (n→p direction)
Electric field opposes further diffusion — equilibrium reached
Depletion region width stabilizes; barrier potential established (~0.7 V Si)