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2. Fill in the blanks with the appropriate words（每小题2分，共40分） (1) A 1) is characterized by having a well-structured periodic placement of atoms. The smallest assembly of atoms that can be repeated to form the entire crystal is called a 2) , with a dimension of 3) . (2) For any semiconductor there is a forbidden energy range in which allowed states cannot exist. Energy regions or energy bands are permitted above and below this energy gap. The upper bands are called the 4) ; the lower bands, the 5) . The separation between the energy of the lowest conduction band and that of the highest valence band is called the 6) or energy gap Eg, which is one of the most-important parameters in semiconductor physics. (3) For an n-type silicon, a substitutional phosphorous atom with five valence electrons has replaced a silicon atom and a negative-charged electron is donated to the lattice in the conduction band. The phosphorous atom is called a 7) . When a boron atom with three valence electrons substitutes for a silicon atom, a positive-charged hole is created in the valence band and an additional electron will be accepted to form four covalent bonds around the boron. This is p-type, and the boron is an 8) . We can calculate the approximate distance of the donor electron from the donor impurity ion, and also the approximate energy required to elevate the donor electron into the conduction band. This energy is referred to as the 9) . (4) Electronic devices rely on transport of electrons (holes) in materials. This transport occurs either under the influence of an electric 10) or carrier concentration 11) . There are two collision or scattering mechanisms that dominate in a semiconductor and affect the carrier 12) : phonon or 13) scattering, and 14) scattering. If the carrier energy is large enough it can transfer energy to the lattice by the emission of an optical phonon. This mechanism is very efficient and limits the maximum drift velocity. The limiting value for the drift velocity is termed the 15) . Besides the 16) , which flows when an electric field is applied and which follows Ohm's law, an additional important component of current can flow if a spatial variation of carrier energies or densities exists within the material. This component of current is called 17) . (5) The bipolar transistor has three separately doped regions and two pn 18) . The three terminal connections are called the emitter, base, and 19) . The width of the 20) region is small compared to the minority carrier diffusion length.
A．I should have paid more attention to my study．
B．I shouldn’t have forgotten my homework．
C．You should have given me more tim
E．
D．I promise that I will finish my homework in time later on．
A．set a criterion as to what are ancient trees．
B．introduce the rich species of ancient trees．
C．compare different kinds of ancient trees．
D．explain the definition of ancient trees．
A．extremely
B．naturally
C．basically
D．especially
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