Device Design Considerations for Ion Implanted n-Channel MOSFETs

Design of ion-implanted MOSFET's with very small physical dimensions

“…The ion-implanted case offers both a sufficiently high threshold voltage and a reasonably low substrate sensitivity, particularly for sub V. For sub V, a steep slope occurs because the surface inversion layer in the channel is obtained while the depletion region in the silicon under the gate does not exceed , the step width of the heavier doped implanted region. For sub V, at inversion the depletion region now extends into the lighter doped substrate and the threshold voltage then increases relatively slowly with sub [11]. Thus, with a fixed substrate bias of V, the substrate sensitivity over the operating range of the source voltage (e.g., ground potential to 4 V) is reasonably low and very similar to the slope of the nonimplanted 200-Å design.…”

“…Below threshold, is exponentially dependent on with an inverse semilogarithmic slope, , [10], [11] which for the scaled-down device is given by volts decade log log (4) which is the same as for the original larger device. The parameter is important to dynamic memory circuits because it determines the gate voltage excursion required to go from the low current "off" state to the high current "on" state [11].…”

“…Below threshold, is exponentially dependent on with an inverse semilogarithmic slope, , [10], [11] which for the scaled-down device is given by volts decade log log (4) which is the same as for the original larger device. The parameter is important to dynamic memory circuits because it determines the gate voltage excursion required to go from the low current "off" state to the high current "on" state [11]. In an attempt to also extend the linear scaling relationships to one could reduce the operating temperature in (4) (i.e., ), but this would cause a significant increase in the effective surface mobility [12] and thereby invalidate the current scaling relationship of (3).…”

“…Using the step profile, a model for determining threshold voltage has been developed from piecewise solutions of Poisson's equation with appropriate boundary conditions [11]. The one-dimensional model considers only the vertical dimension and cannot account for horizontal short-channel effects.…”

Investigation of the two-dimensional shape of ion-implanted regions

“…Of even greater importance is the fact that linear scaling is no longer sufficient to ensure proper device operation [7], as secondary effects such as short-channel effects [8], drain-induced barrier lowering [9], punch through [10], substrate reversebias sensitivity [7], subthreshold conduction [11], snap-back [12], and hot carrier effects [13] become the limiting factors in device performance. All of these effects are determined by the details of the electric field distribution determined by the two-dimensional interactions among the doped regions and thus demand control over not only the depth distribution of the implanted ions but their lateral spreading as well.…”