Oxford University Publishing Microelectronic Circuits by Adel S. Sedra and Kenneth C. Smith (0195323033) 5.1.3. Creating a Channel for Current Flow threshold voltage ( V t ) – is the minimum value of v GS required to form a conducting channel between drain and source typically between 0.3 and 0.6 V dc field ‐ effect – when positive v GS is applied, an electric field develops between the gate electrode and induced n ‐ channel – the conductivity of this channel is affected by the strength of field SiO 2 layer acts as dielectric effective / overdrive voltage – is the difference between v GS applied and V t . oxide capacitance ( C ox ) – is the capacitance of the parallel plate capacitor per unit gate area ( F / m 2 ) (eq5.1) OV GS t v v V 2 2 is permittivity of SiO 3.45 11 / is thickness of SiO layer 2 (eq5.3) in / ox ox ox ox ox F m t C t F m E V tn is used for n ‐ type MOSFET, V tp is used for p ‐ channel
Oxford University Publishing Microelectronic Circuits by Adel S. Sedra and Kenneth C. Smith (0195323033) 5.1.3. Creating a Channel for Current Flow Q: What is main requirement for n ‐ channel to form? A: The voltage across the “oxide” layer must exceed V t . For example, when v DS = 0… the voltage at every point along channel is zero the voltage across the oxide layer is uniform and equal to v GS Q:
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