on-current of the PF diode-type cell string is affected significantly by electric .... body by the thickness (TONO) of an O/N/O gate dielectric stack. (3/6/6 nm). ...... conventional FET cell string. As the device dimensions are. 0.15. 0.20. 0.25. 0.30.
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10-3
10-3
Conventional FET
PF Diode
SL2 -5
e-injection barrier 0.0 -0.5
②
Turn-off Turn-on
④
-1.0
10
10
10-7
10-7
10
0.2
0.4
@WL6 Lg=30nm Ls=30nm Tb=15nm RFOX=40nm
10-11
h-injection barrier
③
VCSL increases
-9
10-13
-1.5 0.0
|IBL| (A)
Energy Level (eV)
① 0.5
-5
-4
-2
0
VCSL increases
VCSL&VBL 0.8V 1.0V 1.2V 1.4V 1.6V 1.8V .2.0V
-9
10
10-11 10-13
4
-4
-2
0
2
VCG (V)
VCG (V)
(a)
(b)
0.6
4
WL7 WL6 WL5 WL4 WL3 WL2 WL1 WL0
Position (µm) (d)
SL1
0.5
Energy Level (eV)
Fig. 1 (a) Cross-sectional view of the PF diode-type cell string with tube type poly-Si body. (b) Timing diagram of the bias scheme for read operation of the PF diode-type cell string. (c) Simulated |IBL| -VCG curve of the PF diode-type cell string. The Lg and Ls in the simulated cell string are the same (30 nm). The RFOX and Tb are 40 nm and 15 nm, respectively. Turn-on voltage is given by Von. In program state, the charge density stored in the storage layer of the WL6 is 1×1019 cm-3. The charges are distributed uniformly in the nitride layer of the WL6. (d) Energy band diagrams cut along the surface of the poly-Si body in the PF diode-type cell string at on/off state.
2
|IBL| (A)
1.0
3
SL2
PF diode
0.0 -0.5
qVEJB
-1.0 -1.5
VCSL = 0.8V ~ 2V
@VWL6 = 5V
-2.0 0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
Position (µm)
(c) WL7 WL6 WL5 WL4 WL3 WL2 WL1 WL0 SL1
Energy Level (eV)
(a)
0.5 0.0 -0.5 -1.0 -1.5 -2.0 -2.5 -3.0
SL2
Conventional FET VCSL = 0.8V ~ 2V
@VWL6 = VCSL + Vth
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
Position (µm)
(b)
(d)
Fig. 2 Band-to-band (BTB) generation rate of the PF diode-type cell string during the read operation (at t6 in Fig. 1 (b)) (a) with and (b) without the pre-charging step.
A. Effect of CSL bias In read operation of the PF diode-type cell string, the CSL is biased positively and the BL is grounded as shown in Fig. 1 (b). As the bias applied on the CSL (VCSL) increases, the BL current (|IBL|) is expected to be increased. To estimate |IBL|-VCG curves quantitatively, we simulate the PF diode-type cell string with various VCSLs. The |IBL|-VCG curves of the PF diode-type cell string are compared with those of conventional FET cell string where the device dimensions are exactly the same as that of the PF diode-type cell string except the doping polarity at the end of the string connected to the CSL.
|IBL| (µA)
III. SIMULATION RESULTS
30 20
PF diode (@VWL6 = 5V) Conventional FET (@VWL6 = VCSL + Vth)
10 0 0.5
1.0
1.5
2.0
VCSL (V)
(e) Fig. 3 Simulated |IBL|-VCG curves as a parameter of VCSL in (a) the PF diode-type cell string and (b) conventional FET cell string. (c) Energy band diagrams along the surface of the poly-Si body in the PF diode-type cell string at a VCG of 5 V. (d) Energy band diagrams along the surface of the poly-Si body in the conventional FET cell string at a VCG of VCSL + Vth. The VCSL is changed from 0.8 V to 2 V with a step of 0.2 V. (e) Comparison of |IBL| of the PF diode-type cell string at a VCG of 5 V with that of the conventional FET cell strings at a VCG of VCSL + Vth.
2168-6734 (c) 2016 IEEE. Translations and content mining are permitted for academic research only. Personal use is also permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information.
This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication. Citation information: DOI 10.1109/JEDS.2016.2593792, IEEE Journal of the Electron Devices Society
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10-7
VCSL=1V Lg=30nm Ls=30nm Tb=15nm RFOX=40nm fixed doping =1×1020cm-3
10-9
10-11
10-13
-4
-2
0
2
peak doping concenctration 1×1020cm-3 5×1019cm-3 1×1019cm-3 5×1018cm-3 1×1018cm-3 5×1017cm-3
10-9
10-11
10-13
4
-4
-2
0
2
VCG (V)
VCG (V)
(a)
(b)
10-5 10-6
4
WL7 WL6 WL5 WL4 WL3 WL2 WL1 WL0 SL1
Energy Level (eV)
0 cm-3
10-9
10-12 qVdep = 0.65eV
p-type doping = 1×1020 cm-3 p-type doping = 5×1017 cm-3
0.5
VCG_WL5 =-2.5 V
WL5
0.0
-1×1019 cm-3
-0.5 -1.0 -1.5 -2.0 0.15
1×1019 cm-3 0 cm-3
WL6 0.20
0.25
0.30
0.35
-2
-1
0
1
2
VCG (V) Fig. 8. Simulated |IBL|-VCG curves of the PF diode-type cell string as a parameter of the state (program or erase) in WL5 cell adjacent to the selected WL6 cell. The inset is energy band diagrams corresponding to the density of the stored charge in the O/N/O stack of the WL5 cell. The charge densities are 0 (square symbols), 1×1019 cm-3 (circle symbols), and -1×1019 cm-3 (triangle symbols).
-1.0
qVdep = 0.35eV n-type doping = 1×1020 cm-3 n-type doping = 5×1017 cm-3
-0.5
0.0
@ WL6
Position (µm)
0.0
-1.0
10
10-11 p-type doping split
0.0
-8
10-10
SL2
0.5
-0.5
1×1019 cm-3
-1×1019 cm-3
10-7
|IBL| (A)
|IBL| (A)
10-7
|IBL| (A)
Various n-type doping concentration
10-5
than at least 1×1018 cm-3 is needed to give reasonable |IBL|. It was assumed in this work that no charges are stored in the storage layer of WLs except the selected WL6 in the cell string. The state (program or erased) of the WL5 adjacent to the WL6 may give an effect on the |IBL|-VCG characteristic since the hole barrier is formed in the channel of the WL5 cell. Now we investigate the effect with the state of the WL5 cell.
Energy Level (eV)
Varoius p-type doping concentration
10-5
6
n-type doping split 0.1
0.2
0.3
0.4
0.5
0.6
0.7
Position (µm)
(c) Fig. 7 Simulated |IBL|-VCG curves of the PF diode-type cell string as parameters of (a) p-type peak doping concentration in the region connected to the CSL and (b) n-type peak doping concentration in the region connected to the BL. The peak doping concentration ranges from 5×1017 cm-3 to 1×1020 cm-3. (c) Energy band diagrams of the PF diode-type cell string at on-state with p- and n-type doping concentrations of 1×1020 cm-3 and 5×1017 cm-3. Here, the VCSL is 1 V.
Fig. 7 (a) and (b) show simulated |IBL|-VCG curves of the PF diod e-type cell string with various p-type and n-type peak doping concentrations in the region connected to the CSL and BL, respectively. The peak doping is changed from 5×1017 cm-3 to 1×1020 cm-3. As the peak doping concentration decreases, the doped poly-Si region near the SL transistors is depleted, resulting in the increased depletion barrier height. As a result, the |IBL| decreases significantly. Fig. 7 (c) shows energy band diagrams cut along the poly-Si body near the interface of the PF diode-type cell string at VCG_WL6 = 5 V and VCSL = 1V. It can be clearly seen that the barrier height (see the barriers on the right in upper window and the left in lower window) for a given concentration of 5×1017 cm-3 is higher than that for 1×1020 cm-3. According to the data in Fig. 7, the doping concentration higher
Fig. 8 shows |IBL|-VCG curves as a parameter of the charge density stored in the storage layer of the WL5 (WLk-1) cell which is adjacent to the selected WL6 (WLk) cell. By changing the charge density stored in the WL5 cell from -1×1019 cm-3 to 1×1019 cm-3, the Von is increased by ~150 mV. The physics responsible for the Von shift is explained using energy band diagram as follows. With the charge stored in the nitride layer of the WL5 cell, the width and height of the hole barrier in the poly-Si body under the WL5 are changed slightly as shown in the inset. Note the energy band diagrams are obtained when the WL6 cell is turned off by applying -2.5 V. By storing a charge density of 1×1019 cm-3 in the charge storage layer of the WL5 cell, the depth and width of the potential well (hole barrier) formed in electrically floated poly-Si body of the WL5 cell become deep slightly and wide, respectively. As a result, the triggering of the PF is slightly suppressed and the Von is increased a little. IV. CONCLUSION In this paper, we have investigated the effects of design parameters on key performances (|IBL|, Von and SS characteristic) of the PF diode-type cell string by using calibrated TCAD simulation tool. Considered design parameters include bias applied to the CSL (VCSL), device dimensions (Lg, Ls, Tb and RFOX) and peak doping concentration of the cell string. With increasing VCSL, |IBL| of the PF diode-type cell string increases more rapidly than that of conventional FET cell string. As the device dimensions are
2168-6734 (c) 2016 IEEE. Translations and content mining are permitted for academic research only. Personal use is also permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information.
This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication. Citation information: DOI 10.1109/JEDS.2016.2593792, IEEE Journal of the Electron Devices Society
> REPLACE THIS LINE WITH YOUR PAPER IDENTIFICATION NUMBER (DOUBLE-CLICK HERE TO EDIT) < changed, the conventional FET cell string shows significant change in SS, but the PF diode-type cell string maintains still steep SS. The behavior of the |IBL| and Von of the PF cell string was analyzed with the dimension. The doping concentration at both ends of the cell string needs to be at least 1×1018 cm-3 to keep high |IBL|. REFERENCES [1]
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