Phosphorus Doping

Phosphorus doping is the introduction of phosphorus atoms into silicon to make it n-type, so that current is carried chiefly by electrons. Phosphorus has five valence electrons, one more than silicon's four, so when it substitutes for a silicon atom four of its electrons form bonds and the fifth is only loosely held and is easily freed into the conduction band. Phosphorus is therefore a donor, and it is the most widely used n-type dopant in silicon, particularly where very high concentrations or low-cost batch processing are needed.

The workhorse delivery method is the diffusion furnace using phosphorus oxychloride, abbreviated POCl3. A carrier gas bubbles through liquid POCl3 and carries the vapour into a tube furnace at around 800 to 950 degrees Celsius, where, together with oxygen, it forms a phosphorus-rich glass on the wafer surface, phosphorus pentoxide. Phosphorus then diffuses from this glass into the silicon. Phosphine gas, a very toxic source, serves in other diffusion and deposition processes and in doped epitaxy, and solid and spin-on sources have also been used. After the deposit, the glass is stripped with hydrofluoric acid. Because the furnace loads hundreds of wafers at a time, the method is economical, which is why it persists in solar cell manufacturing, where phosphorus diffusion into a p-type wafer forms the junction.

Phosphorus is the dopant of choice for heavily doped emitters, but the high concentrations cause distinctive effects. In a bipolar transistor the emitter must be doped far more heavily than the base, so that nearly all the current across the emitter-base junction is carried by electrons injected into the base rather than holes flowing back; phosphorus, with a solid solubility of a few times 10 to the 20 per cubic centimetre, can reach this level. At such concentrations, the diffusion profile departs from a simple textbook curve: it develops a flat plateau and a steep kink, and the underlying base can be pushed deeper, a phenomenon called the emitter-push effect. Heavy doping also narrows the bandgap slightly and reduces the injection efficiency below the ideal, so there are limits to how far more phosphorus helps.

A valuable side effect of phosphorus diffusion is gettering, the pulling of harmful metal impurities out of the active parts of a wafer. Fast-diffusing metals such as iron, copper and gold create energy levels in silicon's bandgap that kill carrier lifetime and cause leakage. In heavily phosphorus-doped regions, the metals tend to dissolve more readily and to be trapped, so a phosphorus diffusion can leave the bulk of the wafer cleaner than before. The effect is exploited deliberately, notably in low-cost solar cells made from impure silicon, where the diffusion improves both the junction and the bulk. Gettering is therefore a feature of the process, not merely an accident of it.

Phosphorus is also introduced by ion implantation, and in a quite different way by transmutation. Implants of phosphorus form source and drain regions and n-wells in MOS circuits, although arsenic is often chosen for the shallowest junctions because it is heavier and diffuses more slowly, while antimony is used for buried layers. A separate technique, neutron transmutation doping, exposes pure silicon to neutrons so that some silicon-30 atoms are converted to phosphorus, giving an exceptionally uniform distribution for high-power devices. A frequent misconception is that more phosphorus always means a better emitter: above the solubility limit extra atoms form inactive clusters, and in solar cells a heavily doped surface can raise recombination, so engineers tailor the profile rather than maximise it. Another is that phosphorus is only an electrical dopant, when it also acts in glasses for passivation and as a getter. Safety matters too: phosphine and POCl3 are hazardous and are handled in sealed systems.

Read phosphorus doping as the donor half of silicon technology, an easily diffused, high-concentration source of electrons that builds emitters and junctions and, as a bonus, sweeps stray metals out of the working silicon.