An important reason for using epitaxy is the flexibility（灵活性）of impurity control in the epitaxial （灵活性） film（薄膜）. The dopant in the film may be n or p （薄膜） type and independent of the substrate（衬底） （衬底） doping. Therefore, epitaxial growth can be used to form a lightly doped layer on a heavily doped layer, or on a pn junction between the epitaxial film and the substrate. Three different methods are available（有 （ 用的） 用的）to produce epitaxial films: vapor-phase epitaxy (VPE), liquid-phase epitaxy (LPE), and molecularbeam epitaxy (MBE).
Drift Current and Diffusion Current

Drift Current and Diffusion Current
The drift current is the transport of carrier when an electric field is applied. Another important current component can exist if there is a spatial variation of carrier concentration in the semiconductor material, that is, the carriers tend to move from a region of high concentration to a region of low concentration. This current component is called diffusion current.
Donors and Acceptors
The silicon becomes n-type because of the addition of the negative charge carrier, and the arsenic atom is called a donor. Similar, when a boron atom with three valence electrons substitute for a silicon atom, additional electron is accepted to form four covalent bonds around the boron, and a positively charged “hole” is created in the valence band. This is a p-type semiconductor, and the boron is an acceptor.
微电子技术 专业英语
邱成军 编 哈尔滨工业大学出版社
Donors and Acceptors
Donors and Acceptors
When a semiconductor is doped with impurities, the semiconductor becomes extrinsic and impurity energy levels are introduced. A silicon atom is replaced (or substituted) by an arsenic atom with five valence electrons. The arsenic atom forms covalent bond with its four neighboring silicon atoms. The fifth electron becomes a conduction electron that is “donated” to the conduction band.
A semiconductor is a material that has a resistivity value between that of a conductor and an insulator 绝缘体） （绝缘体）. The conductivity of a semiconductor material can be varied under an external（外部的） （外部的） electrical field. Devices made from semiconductor materials are the foundation（基础） of modern （基础） electronics, including radio, computers, telephones, and many other devices（器件、设备）. （器件、设备） Semiconductor devices include the transistor, many kinds of diodes，digital and analog integrated ， circuits.