Donor Atom
A donor atom is an impurity atom that has one more valence electron than the host atom it replaces, such as phosphorus, arsenic or antimony in silicon, and that readily releases that extra electron into the conduction band. Once it does, the donor is a fixed positive ion locked into the crystal, while the electron it released becomes a mobile negative carrier.
The defining property of a donor is a weakly bound extra electron, and the binding energy is small enough that heat alone releases it. In silicon the four host bonds use up four of phosphorus's five valence electrons, and the fifth orbits the positive core much like the electron in a hydrogen atom. Two features of the crystal soften that attraction: the relative dielectric constant of about 11.7 screens the Coulomb force, and the electron's effective mass is lower than that of a free electron. The ionization energy therefore drops from the 13.6 electron-volts of hydrogen to about 0.045 electron-volts for phosphorus, about 0.054 for arsenic and about 0.04 for antimony. These are shallow donors, with levels a small step below the conduction band, in contrast with deep-level impurities such as gold or iron, which sit near the middle of the gap and trap carriers instead of releasing them.
At room temperature essentially every donor is ionized for ordinary doping levels, so the electron density follows the doping instead of the temperature. Thermal energy near 0.026 electron-volts is within a factor of two of the binding energy, and the conduction band offers a huge supply of available states, so the equilibrium strongly favors the freed electron. For phosphorus concentrations up to roughly 10¹⁷ atoms per cubic centimeter, nearly all donors are ionized between about 100 K and several hundred kelvin, the plateau known as the extrinsic range in which silicon devices work. At very low temperatures electrons freeze back onto their donors and conductivity drops sharply, while at high temperature thermally generated electron-hole pairs overwhelm the doping, which sets an upper operating limit that is lower for silicon than for wide-gap materials.
Donor atoms are physically different from one another in ways that matter for manufacturing, even though they play the same electrical role. Phosphorus diffuses relatively fast and is common for deep wells and for general n-type doping; arsenic is larger and diffuses slowly, so it is the standard for shallow, abrupt source and drain regions that are put in by ion implantation; antimony diffuses very slowly and is used for heavily doped buried layers in bipolar and BiCMOS processes. A silicon-30 atom that captures a neutron decays into phosphorus-31 with a half-life of about 2.6 hours, a route to unusually uniform phosphorus doping in large-diameter wafers for power devices. In germanium, the donors are even shallower, around 0.01 electron-volts, which helps explain why germanium conducts so readily and why early devices were so sensitive to warmth.
The ion left behind is just as important as the electron that left, because it is the source of electric fields in every junction. When electrons near a pn junction diffuse across to the p side, the positive donor ions they leave behind cannot follow, and the exposed charge builds a field that opposes further diffusion and sets the built-in potential. The same fixed positive charge is what a MOS gate must repel electrons from to turn a transistor off, and it is what makes depletion layers behave like capacitors. A misconception is that a donor atom is a charged atom added to the crystal; in fact a neutral phosphorus atom is added, it becomes charged only when it gives up the electron, and the crystal as a whole stays neutral because electron and ion appear as a pair.
Donors also impose limits when too many are added, and the limit appears in modern chips as a real engineering constraint. Above about 10¹⁸ to 10²⁰ atoms per cubic centimeter the individual donor levels merge into a band and the semiconductor becomes degenerate, behaving more like a metal; transistor source and drain regions are doped to such levels to keep series resistance low. But solubility limits cap how many phosphorus or arsenic atoms sit electrically active on lattice sites, and clusters of dopants can stay inactive, a growing problem as transistor dimensions shrink toward a few nanometers and every dopant counts. Compensation also matters: acceptors in the same region cancel donors one for one, so the net doping, not the total, controls the electrical behavior.
Read donor atom as the supplier of one loosely held electron that, once released, leaves behind a fixed positive ion, the two together keeping the crystal neutral.