Ultra Low Power SubThreshold Device Design Using New Ion Implantation Profile
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One of the important aspects of integrated circuit design is doping profile of a transistor along its length, width and depth. Devices for super-threshold circuit usually employ halo and retrograde doping profiles in the channel to eliminate many unwanted effects like DIBL, short channel effect, threshold variation etc. These effects are always become a serious issue whenever circuit operates at higher supply voltage. Subthreshold circuit operates at lower supply voltage and these kind of effects will not be a serious issue. Since subthreshold circuit will operate at much lower supply voltage then devices for subthreshold circuit does not require halo and retrograde doping profiles. This will reduce the number of steps in the fabrication process, the parasitic capacitance and the substrate noise dramatically. This dissertation introduces four new doping profiles for devices to be used in the ultra low-power subthreshold circuits. The proposed scheme addresses doping variations along all the dimensions (length, width and depth) of the device. Therefore, the approaches are three dimensional (3D) in nature. This new doping scheme proposes to employ Gaussian distribution of doping concentration along the length of the channel with highest concentration at the middle of the channel. The doping concentration across the depth of the device from the channel region towards the bulk of the device can follow one of the following four distributions: (a) exponentially decreasing, (b) Gaussian, (c) low to high, and (d) uniform doping. The proposed doping scheme keeps the doping concentration along the width of the device uniform. Therefore, under this scheme we achieve four sets of new 3D doping profiles. This dissertation also introduces a new comprehensive doping scheme for the transistors in subthreshold circuits. The proposed doping scheme would bring doping changes in the source and drain areas along with the substrate and channel region of the transistors. The proposed doping scheme is characterized by the absence of halos at the source and drain end. We propose a Gaussian doping distribution inside the source, drain region and a low-high-low distribution across the depth of the transistor from the channel surface towards the body region. It also has a low-high-low doping distribution along the length of the transistor below the channel region. Results show that a device optimized with proposed doping profiles would offer higher ON current in the subthreshold region than a device with the conventional halo and retrograde doping profiles. Among the four 3D doping profiles for subthreshold device some has better ON current than others. Based on specific requirements one of these four doping profiles can be adopted for different ultra-low-power applications. Our analysis shows better subthreshold swing can be achieved using new doping profile based subthreshold design. Results also show that the optimized device with the proposed comprehensive doping profile would provide higher ON current (Iₒₙ) at smaller body bias condition. The analysis is performed by changing the doping profile, body bias and (Vgs) to observe the off-state current (Iₒff), threshold voltage variation, magnitude of Iₒₙ/Iₒff ratio, transconductance and the output conductance with the proposed doping profiles.
Table of Contents
Introduction -- Subthreshold background -- Implantation profiles -- Threshold voltage calculation -- Comprehensive implantation profile -- Conclusion and future workxvi, 98 pages