A pure semiconductor without doping and without lattice defects is called an intrinsic semiconductor. More colloquially, a completely pure semiconductor is called an intrinsic semiconductor or Type I semiconductor. Silicon and germanium are both quadrivalent elements with four valence electrons in the outermost layer of their nuclei. They are all \"single crystals\" made of the same kind of atom and belong to the intrinsic semiconductor.
At absolute zero temperature, the valence band of the semiconductor is full band (see band theory), and after photoelectric injection or thermal excitation, some electrons in the valence band will cross the forbidden band\/band gap into the empty band with higher energy. The presence of electrons in the empty band becomes the conduction band, and the absence of an electron in the valence band forms a positively charged vacancy, which is called a hole. The combination of electrons in the conduction band and holes in the valence band is called an electron-hole pair. The electrons and holes generated above can move freely and become free carriers, which produce directional motion under the action of external electric field and form macroscopic current, respectively called electron conduction and hole conduction. This type of hybrid conduction due to the generation of electron-hole pairs is called intrinsic conduction.