P01
Write Voltage and Read Reference Current Generator for MLC-PCM
Considering with Temperature Characteristics
Koh Johguchi, Toru Egami, Kousuke Miyaji and Ken Takeuchi
Department of Electrical, Electronic, and Communication Engineering, Chuo University, Tokyo, JapanI.

Introduction

Phase change memory (PCM) is one of the candidates for the storage class memory (SCM) due to non-volatile,
high speed and low energy characteristics [1], as an alternative for NAND flash memory or hard-disk drives (HDDs).
Since PCM gives multi-level-cell (MLC) functionality [2] and simple 4F2 cell [3], the bit cost can be reduced. Inaddition, the access speed of PCM is about 500-times faster than NAND flash [1].
Ge2Sb2Te5 (GST) is a mature material for PCM. As temperature rises, SET/RESET voltages (VSET/VRESET) become
low/high because the device temperature is close to the crystallization temperature of GST. On the other hand, theresistance of PCM also has temperature dependence due to its semiconductor nature [4, 5]. Additionally, from ourexperimental results, these temperature coefficients can change not only by the temperature at read operation(TREAD), but also by the temperature at write operation (TWRITE) [6, 7]. Especially in the MLC case, this dependency
may lead to read errors if the fixed reference currents are used for each level. Therefore, the write voltage andreference currents (Iref) should be calculated considering these problems.
II. Proposed write voltage and read reference current generator and experimental results
The variable temperature coefficient (TC) read reference generator has been presented in [6]. This generator canoutput both positive and negative linear TC currents. Thus, this generator is used for VSET and VRESET generation.
Fig. 1 shows the fitting results. By using positive and negative coefficient, the proposed voltage generator achieves
accurate reproduction of the temperature dependencies of VSET and VRESET.
On the other hand, the read current has non-linear but exponential TC. Hence, for the Iref generation, only one
configuration of the variable TC read reference generator cannot precisely reproduce the PCM’s TC. Therefore, inthe proposal, the temperature range is divided into three regions: -5ºC-25ºC, 25ºC-55ºC, 55ºC-85ºC as shown inFig. 2. Using different setting for each temperature region, the optimum read reference can be generated. Fig. 3
shows the block diagram of the proposed reference current generation for a MLC-PCM. TWRITE data is updated
and stored in SLC region of PCM when the corresponding user data is written in MLC-PCM. Fig. 4 demonstrates
the read reference generation for various temperatures by SPICE simulation. Using the optimum configuration on
the current temperature region, the reference current can accurately reproduce the PCM characteristics.

3VRESET

2VSET

1Read current

VSET, VRESET [V]4

From meas.

Upper level

A3T – D3Lower level

From vol. gen.

0-5

2555

85

TWRITE [ºC]
Fig. 1 Measured temperature dependence andfitting result of SET and RESET voltage
iTREAD
Fig. 2 Concept of the proposed exponential reference
generator.

TWRITE table<10 bit

PCM page size > 512B

WL# Page#01029 ºC
82 ºC

(1)
-2 ºC

Npage

MLC-PCM region

(4)

IREAD

Variable TCIref generator(3)

IrefComparator

Read out

SLC-PCM region
TWRITE = 29 ºC(2)

Configure tablefor current source(pre-recorded)
TREAD = 50 ºC
Temperature(Current temp.)
sensor
(1)

Fig. 3 Block diagram of the proposed read circuit for MLC-PCM.

85

85

15µ
1.50E-05“11”

Iread [A]10µ1.00E-05

Iref15µ5.00E-06

“10”
2555

2555
TREAD [ºC]85

85

TREAD range -5ºC-25ºC 25ºC-55ºC 55ºC-85ºC
282831
P_PTAT
161624
N_CONST(a) Iref1 (k = 1/20)

0.00E+000-5-525552555
TREAD [ºC]8585

TREAD range -5ºC-25ºC 25ºC-55ºC 55ºC-85ºC
111117
P_PTAT
9916
N_CONST

4.0µ
4.00E-06
3.50E-06
3.0µ
3.00E-06
2.50E-06
2.0µ2.00E-06
1.50E-06
1.0µ
1.00E-06
5.00E-07
0.00E+000-5-5200n
2.00E-07“00”

0.00E+000-5-52555

2555
TREAD [ºC]8585

TREAD range -5ºC-25ºC 25ºC-55ºC 55ºC-85ºC
3812
P_PTAT
1712
N_CONST

1.2µ
1.20E-06

“10”

“01”

1.0µ
1.00E-06
800n
8.00E-07

Iref2“01”25552555
TREAD [ºC]8585

TREAD range -5ºC-25ºC 25ºC-55ºC 55ºC-85ºC
252931
P_PTAT
222526
N_CONST(b) Iref2 (k = 1/60)

Iread [A]55

Iread [A]25

Iref3300n
3.00E-07

100n“01” 1.00E-07

2.00E-07
0.2µ2555
TREAD [ºC]“01”

Iread [A]

Iref2

0.4µ
4.00E-07

5.00E-07
0.5µ

TREAD range -5ºC-25ºC 25ºC-55ºC 55ºC-85ºC
348
P_PTAT
016
N_CONST

TWRITE = 85ºC

“10” 4.00E-07
400n

Iread [A]1.00E-06
1.0µ0.00E+000-5-5

1.0µ
1.00E-06

Iref1 8.00E-07
0.8µ
0.6µ
“10” 6.00E-07

Iread [A]TWRITE = -5 ºC“11”

1.50E-06
1.5µ0.00E+000-5-5500n
5.00E-07

1.2µ1.20E-06

2.0µ2.00E-06

Iref3200n
2.00E-07

“00”

600n
6.00E-07
400n
4.00E-07

0.00E+000-5-52555

2555
TREAD [ºC]8585

TREAD range -5ºC-25ºC 25ºC-55ºC 55ºC-85ºC
71228
P_PTAT
61131
N_CONST(c) Iref2 (k = 1/100)

Fig. 4 Demonstration results of read reference current generation for 2 bit-MLC-PCM.

III. Conclusions
The temperature characteristics of PCM are presented with the experimental results. Since the optimum SET andRESET voltages changes by the temperature, the voltage supply circuit must track this characteristic. In addition, themeasurement results show that the read current depends on not only the current but also the write temperatures.
Dividing into three temperature regions and using the variable temperature coefficient current generator, theproposed generator can track both the linear write voltage and the exponential read current changes by thetemperature.
Acknowledgement
This work was partially supported by “Ultra-Low Voltage Device Project" of LEAP founded by METI and NEDOReferences[1] R. Bez, Dig. Tech. Papers, .IEDM, pp. 89-92, 2009. [2] A. Cabrini, et.al., Proc. ICECS, pp. 186-189, 2008. [3]Y. Sasago, et. al., Dig. Tech. Papers, VLSIT, pp. 25-26, 2009. [3] S. Raoux and M. Wuttig eds., “Phase changematerials: Science and applications,” Springer, 1st edition, 2008. [4] A. Cabrini, et. al., Proc. ESSCIRC, pp. 419-422,
2011. [5] K. Miyaji, et. al., Proc. A-SSCC, pp. 313-316, 2012, [6] K. Johguchi, et. al., Proc. IMW, pp. 104-107,
2013.