Fabrication and Performance Considerations of Charge-Transfer Dynamic Shift Registers

Abstract

Two similar dynamic shift register schemes have recently been proposed, the insulated-gate-field-effect transistor (IGFET) version of the bucket-brigade register and the charge-coupled device (CCD). These charge-transfer dynamic shift registers show great promise for many digital and analog applications because of their small size and simplicity. In this paper the fabrication, performance, and drive characteristics of the registers are considered in detail to bring out common and comparative capabilities, the discussion being supplemented with presently available experimental data. In order to be specific in comparing the two devices, we assume 10-micrometer metallization tolerances and emphasize digital rather than analog operation of the registers. In addition, a refractory gate technology with two levels of metallization has been assumed so that the devices can be compared using similar technologies. With respect to the common capabilities, it is found that the registers have several significant advantages over other existing shift register schemes — a minimum of processing steps leading to areas of under 3 mil <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> per bit, with the possibility of areas down to 1.1 mil <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> per bit using a refractory gate technology; operation up to frequencies of 10 MHz p-channel or 50 MHz n-channel; and power requirements under 5 µW per bit at a clock frequency of 1 MHz, power varying approximately linearly with clock frequency. From a comparative point of view, it is found that the charge-coupled device and the IGFET bucket brigade are so similar that area limitations, voltage, current, and power requirements, and high- and low-frequency operating limitations arise from the same mechanisms and hence are essentially the same within less than a factor of two. There appear to be only two major differences. First, the fabrication requirements are somewhat different. The CCD requires no diffusions in its active region and may be less sensitive to mask realignment when two levels of metallization are available. However,