Doping & Impurities
How adding tiny amounts of impurities transforms silicon's electrical properties
N-Type and P-Type Silicon
N-Type and P-Type Silicon
Doping is the process of intentionally adding impurity atoms to pure silicon to control its conductivity. The two types:
- N-type (negative): Add atoms with 5 valence electrons (phosphorus, arsenic). The extra electron is free to conduct. These dopants are called donors.
- P-type (positive): Add atoms with 3 valence electrons (boron, gallium). The missing electron creates a hole — a positive charge carrier. These dopants are called acceptors.
Doping concentrations are remarkably small: typically 1 dopant atom per million silicon atoms (10¹⁶ cm⁻³) to 1 per thousand (10¹⁹ cm⁻³), yet they change conductivity by factors of 10,000 or more.
Analogy: Stadium Seating
Imagine a stadium where every seat (bond) is occupied. N-type doping adds people with an extra friend who has to stand (free electron). P-type doping removes some people, creating empty seats (holes) that others can move into.
How Doping Is Done
How Doping Is Done
In modern fabs, doping is primarily done by two methods:
- Ion implantation: Dopant atoms are ionized and accelerated as a beam into the silicon at precise energies and doses. This allows exact control of depth and concentration. Most common method for advanced chips.
- Diffusion: Wafers are heated in a gas containing dopant atoms, which diffuse into the silicon from the surface. Older technique, still used for some steps.
Key Concept: Junction Formation
The boundary where N-type and P-type regions meet is called a PN junction. This junction is the fundamental building block of diodes and transistors — it's where the magic of semiconductor devices happens.
Knowledge Check
Knowledge Check
1 / 2What type of dopant creates N-type silicon?