Transistors are solid-state devices that replaced the vacuum tubes of the past. It is usually composed of doped silicon or germanium. The basic components of the transistor are comparable to those of a triode vacuum tube and include the emitter, which corresponds to the heated cathode of the triode tube as the source of electrons. Germanium or silicon crystals containing small amounts of certain impurities can conduct electricity even at low temperatures. Such impurities function in the crystal in either of two ways. An impurity element, such as phosphorus, antimony, or arsenic, is called a donor impurity because it contributes excess electrons. This group of elements has five valence electrons, only four of which enter into divalent bonding with the germanium or silicon atoms. Thus, when an electronic field is applied, the remaining electron in donor impurities is free to move through the crystalline material. A germanium or silicon crystal, containing donor-impurity atoms, is called a negative, or n-type, semiconductor to indicate the presence of excess negatively charged electrons. The use of an acceptor impurity produces a positive, or p-type, semiconductor, so called because of the presence of positively charged holes. A single crystal containing both n-type and p-type regions may be prepared by introducing the donor and acceptor impurities into molten germanium or silicon in a crucible at different stages of crystal formation.