Dissertation - FTP Directory Listing - University of Surrey

V G < V FB V FB < V G < V T V T < V GAccumulation Depletion InversionFigure 2.4 Showing different operating modes of the MOS capacitorAccumulation mode: this occurs when the gate voltage V G is less than the flat band voltage V FB (flatband voltage refers to the voltage separating the accumulation and depletion regimes). For a P typephotogate accumulation usually occurs for negative voltages where the negative charge on the gateattracts holes to the oxide-semiconductor interface.Depletion mode: this occurs when the gate voltage is greater than the flatband voltage but less thanthe threshold voltage of the capacitor (the threshold voltage is the gate voltage at which the densityof electrons in the inversion layer is equal to the density of holes in the substrate). Here a negativecharge builds up in the semiconductor. Initially this charge is due to the depletion of thesemiconductor starting from the oxide-semiconductor interface. As the gate voltage is increased thewidth of the depletion region increases.Inversion mode: As the potential across the semiconductor increases another type of chargeemerges at the oxide-semiconductor interface: this charge is due to minority carriers which form an‘inversion’ layer. As the gate voltage is further increased the depletion region width barely increasessince the charge in the inversion layer increases exponentially with the surface potential cancellingincreases in the field.The depletion region forms a potential well in the substrate and when incident photons generate e-hpairs the e are collected in the potential well until the electric field is cancelled out and can hold no8