02ECE108Ch2MOSFETS

02ECE108Ch2MOSFETS - 2. MOSFETs Ece 108 Esener ECE108 1 2....

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2. MOSFETs Ece 108 1 Esener ECE108
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2. 1. MOS DIODE 2 Esener ECE108 2.1.1. Structure SiO 2 V G Al n Si Al: Metal Conductor SiO 2 : Oxide Dielectric Si: S/C Accumulation occurs when majority carriers are attracted to the surface of the S/C by the gate field effect. In this case, the surface of the S/C acts like highly doped material of low resistivity. Depletion occurs when all carriers are chased away from the surface of the S/C by the gate field effect. In this case, the impurities are ionized and the surface is depleted from carriers. Inversion occurs when minority carriers are attracted to the surface by the gate field effect. The surface acts like a low resistivity S/C material with opposite doping to that of the bulk. Like in a p-n junction a depletion layer separates the inversion layer from the bulk. This depletion layer results in a parasitic capacitance. Gate Field Effect: Controls the state of the SiO2/Si interface 3 states are possible:
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2.1.2 CHARGE DISTRIBUTION V G + + + + + + n ++++ ++++ + + + + + + +++++++++ Q B Q i p-type n-type p - - - - - - - - - - - - - - - - - - - - - +++ +++ Accumulation Depletion Inversion + - } ions + - } carriers - - - - - - - - - - - - - - - -
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2.1.3. FACTORS INVOLVED IN THRESHOLD VOLTAGE Esener ECE108 4 The threshold voltage is the amount of gate voltage that must be applied to create a strong inversion. Many factors affect the threshold voltage. These are: • Electrostatic Potential f F The electrostatic potential is a measure of free majority carrier concentration in a S/C. It is given by where φ F is a measure of the doping level • Metal Semiconductor Work Function Because the difference in the free carrier concentration between the S/C and the metal, there exist a potential difference between the two sides of a MOS diode called the metal semiconductor work function. It can be expressed as: φ MS = φ FS φ Fg where S denotes S/C and g denotes gate. For aluminum gate φ Fg = 0.6V; For poly-gate φ Fg = φ Fpoly F f = kT q ln N i n negative n - type positive p - type
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FACTORS INVOLVED IN THRESHOLD VOLTAGE (Cont.) Esener ECE108 5 Oxide Capacitance A MOS diode in inversion is essentially a parallel plate capacitor where the dielectric is silicon dioxide (eox = 4) and the plates are the inversion layer and gate. The oxide
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02ECE108Ch2MOSFETS - 2. MOSFETs Ece 108 Esener ECE108 1 2....

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