doping semiconductor

**Doping** — intentionally introducing impurity atoms into a semiconductor crystal to control its electrical conductivity. **N-Type Doping** - Add Group V elements (phosphorus, arsenic, antimony) to silicon - Each dopant atom has 5 valence electrons — 4 bond with Si, 1 is free - Free electrons are majority carriers - Typical concentration: $10^{15}$ to $10^{20}$ atoms/cm$^3$ **P-Type Doping** - Add Group III elements (boron, gallium, indium) to silicon - Each dopant atom has 3 valence electrons — creates a "hole" (missing electron) - Holes are majority carriers **Methods** - **Ion Implantation**: Accelerate dopant ions into wafer. Precise depth/dose control. Dominant method - **Diffusion**: Expose wafer to dopant gas at high temperature. Simpler but less precise **Key Concepts** - Intrinsic carrier concentration of Si: $1.5 \times 10^{10}$ cm$^{-3}$ at room temperature - Even light doping ($10^{15}$) increases conductivity by 100,000x - Compensation: Adding both N and P dopants — net type determined by higher concentration