N-Type Semiconductor
An n-type semiconductor is a semiconductor, most often silicon, that has been doped with donor atoms such as phosphorus, arsenic or antimony so that mobile electrons greatly outnumber holes and carry nearly all of the current. The n stands for negative, describing the sign of the dominant carriers and not the charge of the material, and the whole piece of silicon remains electrically neutral.
The extra electron comes from the mismatch in valence between the donor and the silicon it replaces. Silicon has four valence electrons and bonds to four neighbors; phosphorus has five, so when it sits on a silicon lattice site four electrons complete the bonds and the fifth has no bond to join. That electron is held only by the weak Coulomb pull of the phosphorus nucleus, screened by the surrounding crystal, whose dielectric constant of about 11.7 and small effective electron mass make the binding behave like a hydrogen atom stretched across several nanometers. The result is a binding energy of roughly 0.045 electron-volts for phosphorus, about 0.054 for arsenic and about 0.04 for antimony, a small fraction of the 1.12 electron-volt band gap. On a band diagram the donor level therefore sits just below the conduction band.
At room temperature almost every donor has given up its electron, so the electron density is set by the doping rather than by the temperature. Thermal energy near 0.026 electron-volts is comparable to the donor binding, so the electrons are shaken free into the conduction band, leaving each phosphorus atom as a fixed positive ion locked in the lattice. A doping of 10¹⁶ phosphorus atoms per cubic centimeter, about one in five million silicon atoms, yields roughly 10¹⁶ free electrons per cubic centimeter, around a million times more than the intrinsic value near 10¹⁰. Holes still exist, but their density falls to about 10⁴ per cubic centimeter because the product of electron and hole densities stays fixed at the square of the intrinsic density, which is what makes electrons the majority carriers and holes the minority carriers.
The material is neutral because every free electron has a matching positive donor ion left behind. The misconception that n-type silicon is negatively charged confuses the sign of the carriers with the net charge. A wafer of n-type silicon does not shock anyone or attract dust more than a p-type wafer does; the electrons simply are the mobile species while the ions stay fixed. Charge imbalance appears only locally and dynamically, such as in the depletion region of a junction or under a MOS gate, where electrons are pushed away and the exposed positive ions are no longer compensated.
A simple test distinguishes n-type from p-type, and the device uses the distinction constantly. The Hall effect, discovered by Edwin Hall in 1879, deflects moving charges sideways in a magnetic field, and the sign of the resulting transverse voltage reveals whether electrons or holes carry the current. Electrons in silicon also move about three times more easily than holes, with typical low-doping mobilities near 1,400 and 450 square centimeters per volt-second, which is why n-type channels are faster and why NMOS transistors usually outperform PMOS transistors of the same size. Phosphorus is the usual n-type dopant for general use, arsenic is chosen for shallow, abrupt source and drain regions because it diffuses slowly, and antimony is used for buried layers. Large power-device wafers are sometimes made uniform by neutron transmutation, where silicon-30 captures a neutron and decays to phosphorus-31. Every NMOS transistor, every collector of an NPN bipolar device and the n-side of every diode and LED in today's processors and accelerators relies on this one small trick of replacing a few silicon atoms with atoms that have one more valence electron.
Read n-type semiconductor as silicon whose electrons have been set free by donor atoms, leaving an electrically neutral crystal in which negative carriers are plentiful and every one of them is balanced by a fixed positive ion.