An ion source generates dopant ions from precursor gases for use in ion implantation systems. Source types: Freeman source: Hot cathode arc discharge. Versatile, widely used. Bernas source: Similar to Freeman but with different geometry. Common in high-current implanters. Indirectly heated cathode (IHC): Longer cathode life, better stability. Modern standard. Feed gases: BF3 for boron (p-type), AsH3 for arsenic (n-type), PH3 for phosphorus (n-type), GeF4 for germanium (PAI), SiF4 for silicon. Ionization: Gas molecules introduced into arc chamber. Electrons from heated cathode ionize gas by electron impact. Plasma contains multiple ion species. Extraction: Positive ions extracted through slit by negative extraction electrode. High extraction voltage (10-40 kV) forms beam. Ion species: Source produces multiple species (e.g., BF3 gives B+, BF+, BF2+, F+). Mass analyzer selects desired species. Plasma: Dense plasma in source chamber. Arc current and gas flow control plasma density and ion output. Source life: Cathode and arc chamber components degrade over time. Source replacement every few hundred to thousand hours. Beam current: Source brightness determines maximum achievable beam current. Higher current = higher throughput for high-dose implants. Cluster ions: Some sources produce molecular or cluster ions for ultra-low-energy shallow implants.