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1、Two Terminal MOS Structure0506 The metal work function is mand the electron affinity is.The parameter iis the oxide electron affinity and,for SiO2,i=0.9 V.Work Function DifferencesEnergy levels in a MOS system prior to contactm:a modified metal work functionthe potential required to inject an electr
2、on from the metal into the conduction band of the oxide.is defined as a modified electron affinity.Vox0is the potential drop across the oxide for zero applied gate voltage The potential s0 is the surface potential.Metal Oxide SemiconductorEnergy-band diagram of MOS in thermalequilibrium after contac
3、t:zero gate voltage.Work Function Differences Equation(11.9)can be rewritten as We can define a potential msas If we sum the energies from the Fermi level on the metal side to the Fermi level on the semiconductor side,we have123123123(11.9)Work Function Differences Metal gate:Degenerately doped poly
4、siliconEnergy-band diagram of a MOS capacitor with an n+polysilicon gate and a p-type substrate.Energy-band diagram for the case of a p+polysilicon gate and the p-type silicon substrate.We will initially assume that EF=Ecfor the n+case and EF=Evfor the p+case.Work Function Differences For the n+poly
5、silicon gate,the M-S work function difference:and for the p+polysilicon gate,we have However,for degenerately doped n+polysilicon and n+polysilicon,the Fermi level can be above Ecandbelow Ev,respectively,by 0.1 to 0.2 V.The experimentalmvalues will then be slightly different fromthe values calculate
6、d by using Equations(11.12)and(11.13).（11.12）（11.13）Work Function Differences An n-type MOS capacitor：Anegative voltage is applied to the gate.The metalsemiconductor work function difference:Work Function Differences The flat-band voltage:the applied gate voltage such that there is no band bending i
7、n the semiconductor;Zero net space charge.V across oxide could 0:work function difference and possible trapped chargeFlat-Band VoltageEnergy-band diagram of a MOS capacitor at flat band.In fact,a net fixed charge density,usually positive,may exist in the insulator.Thepositive charge has been identif
8、ied with broken or dangling covalent bonds nearthe oxidesemiconductor interface.Charge distribution in a MOS capacitor at flat band.Flat-Band Voltage The magnitude of this oxide charge seems to be a strong function of the oxidizing conditions such as oxidizing ambient and temperature.The net fixed c
9、harge:located fairly close to the O-S interface.An equivalent trapped charge per unit area,Qss.For the moment,we will ignore any other oxide-type charges that may exist in the device.Flat-Band Voltage For zero applied gate voltage If a gate voltage is applied,the potential drop across the oxide and
10、the surface potential will change.Using Equation(11.15),we have（11.15）（11.16）Charge distribution in a MOS capacitor at flat band.（11.17）Flat-Band Voltage The charge density on the metal is Qm,from charge neutrality Flat-Band Voltage：S=0.We can relate Qmto the voltage across the oxide byCoxis the oxide capacitance per unit area.Substituting Equation(11.18)into Equation(11.19),we have（11.18）（11.19）（11.20）（11.21）Flat-Band Voltage