11/26/20101LECTURE 6: MOSFETEECS 170A11/26/20101Metal‐Semiconductor/ Metal‐Insulator‐Semiconductor junctions•Previous lectures mainly focused on homo‐junctions made of two same type ofsemiconductors (Si‐Si or Ger‐Ge, etc).•Recent low power intergraded circuits heavily rely on low power semiconductordevices that uses metal‐semiconductor or metal‐insulator‐semiconductorjunctions.•In particular we will focus on physics of MOSFETs.•To start let’s discover the basic properties of metal‐semiconductor and metal‐oxide‐semiconductor junctions.11/26/20102•As we discussed in the 2ndweek of our lecture, semiconductors, metals andinsulators are categorized based on their bandgap energy that determines theirelectrical property.METALEG~0EC, EV, EFECEVSCECEVInsulatorMetal‐Semiconductor junctions (1)•Let’s start our discussion with Metal‐Semiconductor junctions•Since we are dealing with two different materials we need to have a commonreference plane in energy domain. Vacuum level is the commonly usedreference plane in metals and in semiconductors (also in heterojunctions)•Vacuum level (E0) describes the energy required for electron to leave the atomand to become completely free.•The energy differencebetween the vacuum level and the Fermi energy is calledthe work function of the material (φ). It is adistinct parameterfor each metal•Energy difference between the conduction band and the Eat the surfaceis11/26/201030called theelectron affinity(χ) of the SC, and it is a distinct parameter of a SCwhichis constant.E0METALEFMECEVEFMSMetal‐Semiconductor junctions (2)•For instance•Relative magnitude of work functions (φ) in metals and in semiconductors11/26/20104()()3.664.05.154.034.07MeVeVMgSiNiGeGaAsdetermine the property of the junction and how it will operateMETALEFMECEVEFSE0MS
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