p-n junction
As the name suggests, a semiconductor is a material whose electrical properties categorize it somewhere between conductors (of electric current) and insulators.
Silicon (Si) is the most widely used semiconductor but there are others (germanium, gallium arsenide). In its pure state (also called intrinsic) a block of silicon takes on a crystalline structure with electrical properties close to that of an insulator. The value of the semiconductor lies in the possibility of boosting it, that is to say the possibility of modifying its conductivity by introducing a foreign atom (impurity) into a silicon crystal.
A silicon atom is tetravalent (4 electrons on its valence layer). It completes its valence layer to reach 8 electrons by associating with 4 other Si atoms forming a perfect crystal structure.
n-type doping consists of adding an electron-donor impurity. This is the case if we replace a silicon atom (Si) by a pentavalent atom such as phosphorus (P). Semiconductor physics explains that the fifth electron in the valence layer is loosely bound to its atom. The addition of a phosphorus atom leads to a fixed ion, P+, and a free electron. An n-type semiconductor is therefore neutral, but its conductivity has just increased.
p-type doping consists of adding an electron-acceptor impurity. This is the case if we replace a silicon atom (Si) by a trivalent atom such as boron (B). Semiconductor physics explains that the boron atom seeks to recover an electron. The addition of a B atom leads to a fixed ion B- and a hole. A hole represents an absence of electron. It can be defined as a particle of positive charge -e (e is the charge of the electron). A p-type semiconductor is therefore neutral, but its conductivity has just increased.
- A block of doped silicon is said to be extrinsic.
- An extrinsic n-type semiconductor is an electron donor while a p-type extrinsic semiconductor is a hole donor. A mnemonic for remembering could be: n means donor of negative free charges and p donor of positive free charges.
- The lower-case letter p characterizing doping should not be confused with the upper-case letter P representing a phosphorus atom.
- It is important to distinguish between fixed charges (P+ and B- ions) and free charges (free electrons and valence holes). only free charges are mobile, i.e. able to move through the semiconductor lattice.
- Doping a semiconductor requires an ultra-controlled atmosphere (Clean room). You should know that only one atom of dopant is injected among billions of atoms of Si. The electrical behavior of the semiconductor depends on the purity of the crystal.
