Time-Domain Readout of 1T–1C DRAM Cells

Abstract

The conventional readout of one-transistor–one-capacitor dynamic random-access memories (1T–1C DRAMs) depends on using a sense amplifier to develop the bitline voltage and settle it to the voltage of the power supply, [Formula: see text], or to 0[Formula: see text]V depending on whether the stored data is “1” or “0,” respectively. However, using the sense amplifier makes the reading process sluggish. In this paper, a capacitive-voltage divider-based readout scheme is proposed. According to this scheme, the developed bitline voltage is converted into a pulse with a certain starting time. Specifically, this pulse appears at a later time in case of “0” storage than that if a “1” is stored, thus the proposed scheme is aptly called “time-domain readout.” The effects of parameter and component mismatches and technology scaling on the proposed scheme are investigated. The proposed scheme is analyzed quantitatively with a suggestion given to widen the time gap between the starting times of the pulses corresponding to the “0” and “1” states. The proposed scheme is verified by simulation adopting the 45 nm CMOS technology with [Formula: see text][Formula: see text]V. According to the simulation results, percentage savings of 68.8%, 56.8%, and 32% in the read-access time, the read-cycle time, and the average power-delay product, respectively, are shown. The proposed scheme requires approximately 40% extra area overhead for the reading circuitry. Also, a noise analysis is performed and it is found that the device noise does not affect the proposed scheme significantly.