World Journal of Nano Science and Engineering, 2012, 2, 88-91
http://dx.doi.org/10.4236/wjnse.2012.22011 Published Online June 2012 (http://www.SciRP.org/journal/wjnse)
Design Consideration in the Development of Multi-Fin
FETs for RF Applications
Peijie Feng, Prasanta Ghosh
Department of Electrical Engineering and Computer Science, Syracuse University, Syracuse, USA
Received March 7, 2012; revised April 17, 2012; accepted May 16, 2012
ABSTRACT
In this paper, we propose multi-fin FET design techniques targeted for RF applications. Overlap and underlap design
configuration in a base FinFET are compared first and then multi-fin device (consisting of transistor unit up to 50) is
studied to develop design limitations and to evaluate their effects on the device performance. We have also investigated
the impact of the number of fins (up to 50) in multi-fin structure and resulting RF parameters. Our results show that as
the number of fin increases, underlap design compromises RF performance and short channel effects. The results pro-
vide technical understanding that is necessary to realize new opportunities for RF and analog mixed-signal design with
nanoscale FinFETs.
Keywords: FinFET; Analog; RF; Source/Drain Extension Region Engineering; Simulation; Multi-Fin FET
1. Introduction
According to the International Technology Roadmap for
Semiconductors (ITRS), as transistor dimension scaling
into nanometer regime, the conventional planar bulk
MOSFET technology faces many challenges: e.g., the
close proximity between source and drain worsens leak-
age current; the necessary high doping in the bulk causes
threshold voltage fluctuation, etc. [1]. FinFET, emerging
as a promising device, addresses those Short Channel
Effects (SCEs) and secures the necessary performance in
the sub-32 nm regime due to its scalability, superior
SCEs, and compatibilities to the planar CMOS platform.
Our survey reveals that recent papers are more on Fin-
FET’s digital application and less on analog/RF figures
of merit (FoM) [2]. Several papers present work on source/
drain extension (SDE) region engineering with the goal
of improving a single-fin FET or coupling FinFETs per-
formance (NFinFET ≤ 5) [3-5]. Only few papers are on ana-
log/RF FoM of multi-fin FETs which introduces a large
total channel width to achieve high transconductance,
maintain good noise and mismatch performance [2].
In this paper, with extensive calibrated TCAD simula-
tions, we present results for SCEs and analog/RF FoM of
a base FinFET unit and then a multi-fin FET (NFinFET up
to 50). The effect of SDE engineering on the multi-fin
device RF performance is studied. Simulations along
with theoretical analysis establish the realistic application
potential of underlap design for the multi-Fin FET RF
operation.
2. Simulation Setup and Results
2.1. Base FinFET Unit
Given that base FinFET units within the multi-fin config-
uration are nominal identical to each other, we first
optimize analog/RF FoM of a 22 nm node single-fin FET
which then will be used as a base transistor for a multi-
fin structure [6]. The gate length (Lg) of the n-type FinFET
is set at 25 nm. The fin height (Hfin) is fixed at 50 nm.
The bulk is lightly doped 1015 cm–3 to avoid the dopant
fluctuation. Selective epitaxial growth with heavy doping
are performed for the source/drain region to minimize the
parasitic resistance. SDE region engineering is con-
sidered by the application of overlap and underlap design.
Abrupt junction, which is achievable by solid re-growth
and laser annealing process [5], is designed with a fast
doping decay with lateral straggle (σS/D) at the value of 1
nm/dev at the edge of SDE region whereas the underlap
doping profile in the SDE region is simulated with a
Gaussian model rolling off from a peak value of 1020
cm–3 at the edge of the source/drain. The equivalent
oxide thickness (EOT) of the Hf-based dielectric in the
simulation is 0.7 nm. The work-function of the TiN metal
gate is adjusted to 4.6 eV such that the threshold voltage
Vt of the device is around 0.3 V. The device is investi-
gated with a calibrated TCAD simulation taking into
account quantum effect with Lombardi mobility model [7].
The design of TCAD experiments shown in Table 1
for this study considers the trade-off between current
drivability and SCEs. Overlap design enhances current
C
opyright © 2012 SciRes. WJNSE

P. J. FENG, P. GHOSH 89
Table 1. Tcad-predicted SCEs of the 22-nm node DG nFin
FET at room tempe r a ture for SDE en g i neering.
Wfin
(nm)
Lext
(nm)
σS/D
(nm/dec)
DIBL
(mV/V)
S
(mV)
Ioff
(A/µm)
Ion
(A/µm)
17 10 Overlap 146 101.2 6.2e–7 1.2e–3
17 10 5 85 76.1 3.9e–10 9.1e–4
17 20 Overlap 154 99.4 7.8e–7 1.2e–3
17 20 10 72 74.3 2.0e–9 7.6e–4
12 10 Overlap 73 77.9 2.6e–9 1. 1e–3
12 10 5 34 66.2 6.9e–11 8.4e–4
12 20 Overlap 77 76.4 3.2e–9 1. 0e–3
12 20 10 29 65.6 4.4e–11 6.7e–4
drive at the cost of SCEs due to the S/D encroachment
into the channel. Thin Wfin can alleviate the SCEs but it
degrades the drive current since the SDE resistance is
increased. Underlap design specified by spacer length to
lateral doping gradient ratio (Lext/σS/D) keeps a good
compromise between the drain current and SCEs, and
shows great potential in SDE region engineering [3-5]. It
shows a larger intrinsic gain (AVO) than overlap one and a
comparable gm/Ids ratio when Lext/σS/D = 2 (See Figure 1
(a)). As Lext/σS/D goes beyond 2 and Lext increases, Avo
rises but gm/Ids decreases quickly. The degradation is also
found in Figure 1(b), where a longer Lext and a larger
σS/D bring about a poorer gm/Ids ratio for different σS/D
devices with a fixed Lext/σS/D. These degradations are due
to increase of the undoped portion in SDE region that
extends the effective channel length [3], indicating an
increase of 1/gds as demonstrated in the same figure.
Figure 1(c) shows the analog/RF FoM extracted at 100
µA/µm. Both the available S21 (at 10 GHz) and intrinsic
cut off frequency (fT) reaches maximum when Lext/σSD =
2. Considering the fabrication fluctuation and compro-
mise among gm, Avo, fT and SCEs, the optimal value of
Lext/σSD ratio for the base FinFET is set at 2 with a 20 nm
spacer length.
2.2. Multi-Fin FETs
For a single-fin FET, boosting of gm to meet the gain
requirement of RF applications happens at the cost of
SCEs and with a tall Hfin, which also induces difficulties
in manufacturing process. Alternatively, the multi-fin
configuration constructed with a number of fingers
(Nfinger) and multiple fins per fingers (Nfin) shows a
practical means to ease above mentioned concerns. The
total number (NFinFET = Nfinger × Nfin) of FinFETs in such a
structure usually is large, i.e., several hundreds of
transistors [2]. Considering the complexity and realistic
time limit of simulations in TCAD, we have simplified
the structure in [2] by setting Nfinger = 1 as shown in
Figure 2. The spacing between each fin is set as 50 nm
to suit the 22 nm node technology and to achieve
optimum RF characteristics [8]. Results in Figure 3(a)
(a)
(b)
(c)
Figure 1. (a) Variation of gm/Ids and AVO versus Lext; (b)
Variation of gm/Ids and 1/gds; and (c) S21 and ft versus
Lext/σS/D ratio extracted at Ids = 100 µA/µm for various σS/D
value. Device parameters: Wfin = 12 nm, and Vds = 1.0 V.
Figure 2. Schematic diagram of overlap and underlap multi-
fin structure analyzed in this brief.

P. J. FENG, P. GHOSH
90
(a)
(b)
(c)
Figure 3. (a) Variation of gm and 1/gds extracted at Ids = 100
µA/µm as a function of NFinFET at 10 Hz; and ft and fmax of (b)
Overlap and (c) Underlap structure extracted at Vgs = 0.6 V.
Device parameters: Wfin = 12 nm, and Vds = 1.0 V.
show a good linearity of the intrinsic gm versus NFinFET
for both underlap and overlap design models. It should
be noted that the Avo for multi-fin FETs does not benefit
from arrays of transistors. It is limited to the base FinFET
analog FoM because of the equation that Avo =
NFinFET·gm/ NFinFET·gds = gm/gds. This can be verified by the
degra- dation of 1/gds as shown in Fi g ure 3( a).
The RF FoM ft and fmax are simulated using the TCAD
mixed-mode module with considerations of parasitic
resistances and capacitances associated with gate pads,
S/D contacts and coupling effects. Those extrinsic com-
ponents are identified as the bottleneck of the RF perfor-
mance and the values depending on process techniques.
In the simulated structure, the parasitic gate capacitance
is mainly due to the fringe capacitance Cf which is set at
0.18 fF/µm as per ITRS[1]. For a multi-fin FET, Cfmulti is
almost linearly proportional to NFinFET [2] and therefore
for simulation simplification purpose we can conclude that
fmultiFinFET f
·CNC

. (1)
The normalized series resistance RSD is taken the value
of 250 Ω·um [1] and the S/D contact pad of single-fin
transistor is designed as 0.04 × 0.06 um2. As the fin num-
ber increased, the spacing between each fin will be cov-
ered with contact and we can model the multi-fin struc-
ture S/D extrinsic series resistance as:

SDmulti SDFinFET
67 1RR N 



 (2)
The parasitic gate resistance for a single fin is set as
four times of the RSD, due to the limited contact area, to
be 1000 Ω·um, including a 50 Ω·um for the gate pad
component [2]. Therefore, for multi-fin, the gate extrinsic
resistance will be:
gmulti FinFET
50 950RN

 (3)
Including the parasitic components, the simulated RF
FoM is shown in Figures 3(b) and 3(c). For NFinFET = 2,
the overlap structure shows ft of 260 GHz and fmax of 219
GHz, which is consistent with the reported result [8]. As
NFinFET increases, ft does not vary a lot according to the
equation that


tmggintrinsic f
2π·fg CC




 (4)
where the transconductance and the capacitances share
the same factor NFinFET. However, fmax starts to degrade
because the large component Rgmulti is non-linearly in-
versely proportional to NFinFET and this leads to the de-
crease of fmax based on the equation in [9]. The fmax in
multi-fin FETs with overlap design drops below 200
GHz as NFinFET goes only beyond 10. In contrast, the un-
derlap design, shows a comparable ft and a much higher
fmax, compared to the corresponding overlap design. The
higher fmax value is due to the significant reduction in gds
(Figure 3(a)) [8]. As NFinFET increases, although under-
lap structure shows degradation in fmax as overlap one, it
still maintain the maximum frequency and cutoff fre-
quency above 200 GHz even when Nfin reaches 50.
3. Conclusion
It is shown that the multi-fin FET will be particularly
useful at sub-32 nm regime for the development of
devices for RF applications. Even with a large number of
NFinFET, using underlap design in SDE region with an
optimal value of Lext/σS/D ratio, good SCE is achieved and
its RF FoM ft and fmax are better than the one with overlap
design. Furthermore, the cost of multi-fin device is
expected to be lower than other heterojunction devices

P. J. FENG, P. GHOSH
91
and III-V compound devices because of its compatibility
with the CMOS planar process technology.
4. Acknowledgements
The authors would like to thank Synopsys, Inc., for pro-
viding the Sentaurus TCAD tool set for device design
and simulation.
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