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1、2016/12/1512016 Materials Physics (材料物理)Physical Properties and Functional Materials （材料物理性能和功能材料材料物理性能和功能材料）Zhongwu Liu （刘仲武）（刘仲武）Email: Tel: 22236906；QQ：963510550Conduction in Semiconductors (半导体的导电性半导体的导电性)Semiconductor do have a lower conductivity than metals but unique properties make them ver
2、y useful materials. 晶体半导体、非晶半导体及有机半导体。后两者的研究正 在兴起。 Electrical properties of semiconductors are very sensitive to the presence of impurities: 1) Intrinsic semiconductors (本征半导体本征半导体) - electrical conductivity is based on the electronic structure of pure material. 2) Extrinsic semiconductors (杂质半导体杂质半
3、导体) - electrical conductivity is dictated by impurity atoms.2导电性（Electrical Conduction）Charge Carriers（载流子载流子） Two charge carrying mechanisms Electron negative charge Hole equal & opposite positive charge Move at different speeds - drift velocity3Higher temp. promotes more electrons into the conduct
4、ion band may as T Electrons scattered by impurities, grain boundaries, etc.导电性（Electrical Conduction）Conduction in terms of electron & hole migration4+-electronhole pair creation+-no applied electric field Applied electric field Valence electron Si atomApplied electric field electronhole pair migrat
5、ion Conduction in intrinsic semiconductorConcept of electrons and holes:In Si (Eg = 1.1 eV) one out of every 1013atoms contributes an electron to the conduction band at room temperature.导电性（Electrical Conduction）Conduction in intrinsic semiconductor 0K时，Si所有电子处于价带。能 隙Eg=1.1ev。 受到温度、光等热激发，部分价 电子获得Eg的
6、能量成为自由电 子，跃迁到空带，形成导带。价 带内出现空穴。 无外场时，电子、空穴无规则运 动；电场作用下出现自由电子和 空穴的规则运动，形成电流。 自由电子在导带内导带底附近； 空穴在价带顶附近。 本征激发(热)过程中，自由电子 和空穴成对出现，成为载流子。5导电性（Electrical Conduction） 根据费米统计，可以计算出导带中电子浓度以及价带中的空 穴浓度。在某一能带E1和E2之间，单位体积晶体存在的电子 浓度ne可以表示为： 本征半导体中，ne=nh，(或n=p) 费米能级EF为： kT很小，可以粗略的认为费米能级位于本征半导体的价带和 导带正中间的位置（禁带中央）。Car
7、rier and Fermi energy in intrinsic semiconductor66导电性（Electrical Conduction）的电子存在的几率量为狄拉克分布函数，即能费米的电子状态密度；单位体积晶体能量为E-: )(E: )()()(21EfEdEEfEnEEe* ln43)(21eh VCFmmkTEEEFurther reading: p.1702016/12/152Intrinsic Semiconductors7electrons can cross gap at higher Tmaterial Si Ge GaP CdSband gap (eV) 1.11
8、 0.67 2.25 2.40kT/EgapeundopedEnergyfilled bandfilled valence bandempty bandfilled statesGAP?导电性（Electrical Conduction）Number of electrons in conduction band increases exponentially with temperature:K1is a material constant Egis the band gap width k is Boltzmann constant (kB)， 1.3806488 10-23JK-1)2e
9、xp(23 1kTETKpng Since both electrons and holes conduct, the conductivity of an intrinsic semiconductor isElectrons are more mobile than holes, e h In an intrinsic semiconductor, a hole is produced by the promotion of each electron to the conduction band. Therefore, n = p and n (and p) increase expon
10、entially with temperature, whereas eand hdecrease (about linearly) with temperature. as a result, Conductivity of intrinsic semiconductors increases with increasing temperature (different from metals!)Intrinsic Semiconductorsheepen8heheeporen导电性（Electrical Conduction）Number of Charge Carriers9) s/Vm
11、 45. 085. 0)(C106 . 1 ( m)(1021916xenneFor GaAsn = 4.8 x 1024m-3 For Si n = 1.3 x 1016m-3 Intrinsic semiconductor = n|e|(e + n)Ex: GaAselectrical conductivity, (Ohm-m)-150100100010-210-1100101102103104pure (undoped)T(K)导电性（Electrical Conduction） Carrier concentration (载流子浓度)： Drift velocity (自由电子和空穴
12、的平均漂移速度)： Current density (总电流密度)： Resistivity：导电性（Electrical Conduction）)2exp(23 1kTETKpng ii10EvEvppnn；EqnEqpvqpvqnJJJpiiipinipn )(1pniqnJE 1. 本征激发产生成对的自由 电子和空穴，浓度都等于 本征载流子浓度ni； 2. 禁带宽度Eg越大，ni越小； 3. 温度升高，ni增大； 4. ni小，导电能力弱。Conduction in intrinsic semiconductorIntrinsic Semiconductors Pure material
13、 semiconductors: e.g., Si & Ge Group IVA materials11 Compound semiconductors III-V compounds Ex: GaAs & InSb II-VI compounds Ex: CdS & ZnTe The wider the electronegativity (电负性) difference between the elements the wider the energy gap.导电性（Electrical Conduction）1212Electronegativity (电负性电负性)12导电性（Ele
14、ctrical Conduction）鲍林标度根据热化学数鲍林标度根据热化学数 据和分子键能，指定氟据和分子键能，指定氟 的电负性为的电负性为3.98，计算其，计算其 它元素的相对电负性。它元素的相对电负性。电负性：电负性：不同不同 原子形成化学原子形成化学 键时吸引电子键时吸引电子 能力的强弱。能力的强弱。 元素电负性数元素电负性数 值越大，表示值越大，表示 其原子在化合其原子在化合 物中吸引电子物中吸引电子 的能力越强。的能力越强。2016/12/153Extrinsic semiconductor (杂质半导体杂质半导体) Extrinsic semiconductors - elect
15、rical properties (conductivity) is dictated by impurity atoms. Example: Si is considered to be extrinsic at room temperature if the impurity concentration is one atom per 1012 Unlike intrinsic semiconductors, an extrinsic semiconductor may have different concentrations of holes and electrons. It is called p-type if p n and n-type if n p. One can engineer conductivity of extrinsic semiconductors by controlled addition of impurity atoms doping (addition of a very small conce
