1304 IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 59, NO. 5, MAY 2012
A Novel 1T-1D DRAM Cell for
Embedded Application
Cheng-Wei Cao, Song-Gan Zang, Xi Lin, Qing-Qing Sun, Charles Xing, Peng-Fei Wang, and David Wei Zhang
Abstract—A novel one transistor and one diode (1T-1D) dy-
namic random access memory cell for embedded applications
is proposed. This memory cell consists of a floating-gate (FG)
MOSFET and a gated diode. The anode of the diode is connected
to the FG, so that the threshold voltage of the FG MOSFET can
be modulated by the current through the diode. In this paper,
basic device operations, speed, retention, and disturb performance
are investigated using Silvaco technology computer-aided design
simulation tools. The process compatibility is also investigated by
integrating the memory array and specific sense amplifier.
Index Terms—1T-1D, embedded dynamic random access mem-
ory (DRAM), system-on-chip, tunneling field-effect transistor
(TFET).
I. INTRODUCTION
T
HE BASIC one transistor and one diode (1T-1C) dynamic
random access memory (DRAM) cell consists of one
access transistor and a storage capacitor. The capacitance of the
storage capacitor is not scalable i n such a memory cell, while
the access transistor must maintain a drive current of about 25
µA and an extremely low leakage current of about 1 fA [1]. As
the DRAM technology is scaled down beyond the 70-nm node,
recessed channel transistor is applied in order to fulfill the leak-
age current criteria [2]. However, the manufacturing complexity
soars when integrating such a memory cell containing high
aspect-ratio storage capacitor and U-shape transistor. Recently,
alternative capacitorless DRAM concepts have been proposed
to simplify the manufacturing processes. For example, the
floating-body cell and the floating-body/gate cell have been
proposed and investigated [3]–[6]. A novel DRAM cell with
amplified capacitor is also proposed to improve the integration
compatibility with the standard logic processes [7].
The gated diode with enhanced gate control is also called
tunneling field-effect transistor (TFET). The TFET has been
Manuscript received September 26, 2011; revised January 31, 2012;
accepted January 31, 2012. Date of publication March 8, 2012; date of current
version April 25, 2012. This work was supported in part by the Program for
Professor of Special Appointment (Eastern Scholar) at Shanghai Institutions
of Higher Learning and in part the National Nature Science Foundation of
China under Grant 61176074. The review of this paper was arranged by Editor
Y. Momiyama.
C.-W. Cao, X. Lin, Q.-Q. Sun, P.-F. Wang, and D. W. Wang are with the
State Key Laboratory of Application-Specified Integrated Circuit and System,
Department of Microelectronics, Fudan University, Shanghai 200433, China
(e-mail: pfw@fudan.edu.cn).
S.-G. Zang is with the Advanced Micro Devices Shanghai Research and
Development Center, Shanghai 201203, China.
C. Xing is with the Semiconductor Manufacturing International Corporation,
Shanghai 201203, China.
Color versions of one or more of the figures in this paper are available online
at http://ieeexplore.ieee.org.
Digital Object Identifier 10.1109/TED.2012.2187060
intensively investigated in the past few years [8]–[11]. Although
most of the researches are focused on the application of the
TFET in logic circuits, the applications of the TFET in memory
cell have been experimentally demonstrated [6]. A U-shape
floating junction gate memory cell using TFET as
WRITE
transistor has been proposed for ultradense DRAM application
[12]. However, the process complexity is high to fabricate
U-shape channel device and recessed source/drain. In order
to simplify the processes and improve the compatibility with
logic processes, a planar 1T-1D DRAM cell is proposed and
investigated in this paper.
II. D
EVICE STRUCTURE AND CELL OPERATION
In Fig. 1, the equivalent circuit, the cross-sectional view, and
the layout of the planar channel 1T-1D DRAM cell are shown.
This device consists of one floating-gate (FG) NMOS and one
gated diode. The control gate (CG) of t he FG NMOS extends
over the diode. The CG is n
+
doped, and the FG is p
+
doped.
The out-diffusion region of the p
+
FG, the drain, and the CG
of the FG NMOS together form a TFET. The CG of the FG
NMOS works as the gate of the TFET as well. The p-type FG
out-diffusion part is surrounded by an n-type well. That means
that the FG is connected to the floating anode of the p-n junction
diode. Therefore, the threshold voltage of the FG MOSFET can
be modulated by the current through the p-n junction diode
of the embedded TFET. The TFET has low drive current and
low off-current. It has low channel-length dependence because
the bottleneck of the current path is the tunneling junction.
The drive current of a Si-based TFET is around 1 µA/µm
[9]. Because of its reverse-biased p-i-n structure, the
OFF-state
leakage current of a TFET can be extremely low [10], [11]. The
low leakage current is beneficial for the retention performance.
As shown in Fig. 1(b), a TFET is embedded into the n-type
MOSFET, and a compact cell is formed. The corresponding
top–down layout is demonstrated in Fig. 1(c). Memory cells
with a unit cell size of 13.2 F
2
are designed with the 90-nm
ground rule.
As shown in Fig. 1(b), the structure of this 1T-1D cell is
similar to the electrically erasable programmable read-only
memory (EEPROM) [13]. However, in the proposed 1T-1D cell,
the tunnel oxide of the usual EEPROM is omitted, and the
FG contacts the counterdoped substrate directly. This device
configuration simplifies the U-shape channel and improves
the compatibility with the available dual-poly products such
as embedded dual-poly memory devices. In the latter part of
this paper, the process compatibility is investigated by inte-
grating a 1T-1D DRAM array using EEPROM processes. In
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