Circuits and Systems, 2010, 1, 49-53
doi:10.4236/cs.2010.12008 Published Online October 2010 (
Copyright © 2010 SciRes. CS
Designing Parameters for RF CMOS Cells
Viranjay M. Srivastava1, K. S. Yadav2, G. Singh1
1Department of Electronics and Communication Engineering, Jaypee University of Information Technology,
Solan, India
2VLSI Design Group, Central Electronics Engineering Research Institute (CEERI), Pilani, India
Received July 29, 2010; revised August 30, 2010; accepted September 5, 2010
In this paper, we have investigated the design parameters of RF CMOS cells which will be used for switch in
the wireless telecommunication systems. This RF switch is capable to select the data streams from the two
antennas for both the transmitting and receiving processes. The results for the development of a cell-library
which includes the basics of the circuit elements required for the radio frequency sub-systems of the inte-
grated circuits such as V-I characteristics at low-voltages, contact resistance which is present in the switches
and the potential barrier with contacts available in devices has been discussed.
Keywords: CMOS, Cell Library, Contact Resistance, DG MOSFET, DP4T Switch, Potential Barrier, Radio
Frequency, RF Switch, Resistance of MOS, Voltage-Current Curve, VLSI
1. Introduction
Earlier, the radio transceiver switches have been design-
ed using PIN diodes, which consumes more power. As
the modern portable devices demands less-power con-
sumption switches, therefore, the PIN diodes are gradu-
ally replaced by the Metal Oxide Semiconductor Field
Effect Transistors (MOSFET) such as the n-type MOS
and p-type MOS [1,2]. A traditional NMOS switch has
better performances but only for a single operating fre-
quency. For multiple operating frequencies, high signal
distortions are easily observed, which results in an un-
recognizable information signal at the receiver end which
would be measured by using the curve of capacitance
and voltage with VEE Pro software [3,4]. The aggressive
scaling of metal-oxide-semiconductor field effect tran-
sistors (MOSFET) has led to the fabrication of high per-
formance MOSFETs with a cutoff frequency fT of more
than 150 GHz [5]. As a result of this development, the
CMOS is a strong candidate for RF wireless communica-
tions in the GHz frequency range. Accurate device models
are, however, needed to design the advanced analogue RF
circuits and for this regards, various researcher propose
some parameters for cell design as used for RF switch
circuits [6,7].
Continuous scaling of CMOS technology has now
reached a state of evolution, in terms of both frequency
and noise, where it is becoming a severe part for RF ap-
plications in the GHz frequency range. To be able to
transmitting or receiving information through the multi-
ple antennas systems, known as MIMO systems, it be-
comes necessary to design a new RF switch that is capa-
ble of operating with multiple antennas and frequencies
as well as minimizing signal distortions and power con-
sumption [8-10].
The use of analog CMOS circuits at high frequency
have more attention in the last several years, with many
applications focused on the growing commercial market
as RF switch, DP4T RF CMOS switch [11,12]. Modern
consumer products require cost competitive technology
and RF CMOS has proven to be cost-effective and high
volume technology. CMOS is also best suitable to inte-
grate RF with digital circuits making it possible to build a
system on a single chip. Due to these advantages, there
has been growing interest in modeling of RF CMOS
which is especially striking for many applications be-
cause it allows integration of both digital and analog
functionality on the same die, increasing performance at
the same time as keeping system sizes reserved. Applica-
tions for a CMOS switch also covers the areas of micro
power circuits and other wireless applications at fre-
quencies from as low as 100 MHz for low earth orbiting
satellite system to thousand of MHz [13-16]. Various
circuit parameters have been discussed in this paper for
better performance.
Rapid integrated system designs are the use of cell li-
Copyright © 2010 SciRes. CS
braries for various system functions [15,17]. In digital
system design, these standard cells are both at the logic
primitive level (for example NAND and NOR gates) as
well as higher levels of circuit functionality (for example,
ALU, memory). For baseband analog systems, standard
cell libraries are less frequently used. In the design of a
CMOS RF cell library, the cells must be designed to be
flexible in terms of drive requirements, bandwidth and
circuit loading. For RF applications, the most common
drive requirements for off-chip loads are based on 50
impedances. This impedance is a good compromise be-
tween lowest loss and highest power handling for a given
cable size. Also this impedance caught on for RF trans-
mission rather than the well established 75 that had
been used for video transmission. A factor governing the
bandwidth of the RF cells is the nodal capacitance to
ground, primarily the drain and source sidewall capaci-
tances [18,19]. Since these cells are to be used with digi-
tal and baseband analog systems, control by on-chip
digital and analog signals is another factor in the design
The library consists of cells designed, using standard
Micro-Cap 2.0 μm and 0.8 μm CMOS processes. For the
technologies studied, these control voltages varied be-
tween 0.3 V and 5.0 V, with the supply voltage of 1.2 V
is of interest for low power consumption portable system
applications. The cells have been designed for the pur-
pose of radio frequency communication switch devices.
In the design of cell library for digital and analog, a
swapping between speed and frequency response and
circuit complexity is always encountered. Transistors
making for the purpose of library elements are usually
planned with multiple gate fingers to reduce the capaci-
tances of sidewall. This increases the contact resistance
and reduces the barrier height. The properties for RF
CMOS switch design for the application in communica-
tion and designed results are presented and have been
designed with and optimized for the particular applica-
tion [12].
Each of the cells parameters will be discussed sepa-
rately for the purpose of clarity of presentation and un-
derstanding of the operation of the circuit. The organiza-
tion of the paper is as follows. Voltage-current curve at
low voltages for application in RF switches are discussed
in Section 2, contact resistances present in switch are
discussed in Section 3, the potential barrier with contacts
are discussed in Section 4 and at last conclusion is in
Section 5.
2. V – I Characteristics of RF CMOS
The selection of RF CMOS transistors requires an analy-
sis of performance specifications. Since drain-source
breakdown voltage is the maximum drain to source
voltage before breakdown with the gate grounded [21],
also specifications for RF CMOS transistors include
maximum drain saturation, common-source forward
transconductance, operating frequency, and output power.
RF MOSFET transistors vary in terms of operating mode,
packaging, and packing methods. Devices that operate in
depletion mode can increase or decrease their channels
by an appropriate gate voltage. By contrast, devices that
operate in enhancement mode can only increase their
channels by an appropriate gate voltage. In terms of
packaging, RF MOSFET transistors are available in
small outline (SO), transistor outline (TO), small outline
transistor (SOT), and flat packaging (FPAK). Devices
use either surface mount technology (SMT) or through
hole technology (SMT) and vary in terms of the number
of leads.
This paper proposes a design of RF CMOS cells for
low power consumption and low distortion for applica-
tion of RF switch in communication that operates at 2.4
GHz and 5.0 GHz [20]. n-channel devices were used in
the HF portion of the circuits with p-channel devices
used as current sources. The cells which were designed
here are to drive 50 resistive loads and utilized multi-
ple gate fingers to reduce parasitic capacitance in an eff-
ort to improve the operating frequency.
The gate metal contact forms a MOS contact with the
substrate which exist below the oxide insulator. When a
voltage is applied to the gate terminal, and as it rises
above the threshold of the MOS contact then an inver-
sion layer, a channel is created in the substrate and the
properties of semiconductor will be interchanged be-
tween p-type to n-type properties. The ideal threshold
voltage is determined by,
where ψB, NA, and Co are the surface potential to cause an
inversion layer, the semiconductor doping concentration
in channel/substrate and capacitance of the oxide layer
respectively [22]. The surface potential to cause an in-
version layer, ψS (inv), is given by the equation,
()2=ln A
inv qn
where k, T, q, NA, and ni are Boltzmann constant, abso-
lute temperature, electronic charge, number of doping
molecules and intrinsic concentration respectively. After
the inversion layer formed, a drain voltage is applied to
MOSFET. As in the linear region, drain voltage is un-
dersized also at this inversion layer has a constant resis-
tance because of the linear V-I characteristics. A deple-
tion region forms between the inversion layer in the
channel of the MOSFET, and the drain well as the volt-
Copyright © 2010 SciRes. CS
age difference from the drain to gate increases to more
than VT. This causes the current to reach a maximum
current (saturation current, IDsat,).
For the fabricated device with gate oxide thickness (tox)
650 Å, oxide capacitance (Cox) 5.31*10-8 F/cm2 [23],
channel length (L) 0.8 µm, channel width (W) 400 µm,
mobility 800 cm2/V-s, channel doping (NB) 1015/cm3 so
we found the Vth 0.867 V and found the drain current as
shown in Figure 1 with different drain voltage range.
Similarly, with the gate voltage range (VG) 0.3 V to 2.1 V,
step size 0.3 V drain voltage range (VD) 0 V to 1.5 V, we
found the drain current 3.8 mA for VG 1.5 V, 8.0 mA for
VG 1.8 V, and 13.6 mA for VG 2.1 V.
A RF CMOS has the properties as fixed tuned match-
ing networks, low Q matching networks, ruggedness,
high power output, mounting flange packages, and Sili-
con grease. Power gain (a measure of power amplifica-
tion, is the ratio of output power to input power, dB),
Noise figure (a measure of the amount of noise added
during normal operation, is the ratio of the signal-to-
noise ratio at the input and the signal-to-noise ratio at the
output, dB), High power dissipation (a measure of total
power consumption, W or mW). Some bipolar RF
CMOS transistors are suitable for automotive, commer-
cial or general industrial applications.
3. Contact Resistances
For measuring the contact resistance of a metal-semicon-
ductor junction of MOSFET, deposition of metal on the
semiconductor are required and it patterns, so that vari-
ous identical pads spaced with different distances as
shown in Figure 2.
These patterns include many rows of different pad
sizes. The pads in any one row should be of same size,
whereas the distances between pads are varying. The
measurements required simple voltage and current curves.
Figure 1. Gate voltage range (VG) = 0.3 V to 1.2 V, step size
= 0.1 V, drain voltage range (VD) = 0 to 0.5 V.
For calculating the resistance, we apply a voltage be-
tween some pairs of adjoining pads in of a row, and
measure the current flow. From this, calculate the resis-
tance between those two pads. As shown in Figure 2, the
total resistance between any two pads is the series com-
bination of following three resistors; metal to semicon-
ductor, through the semiconductor and back into metal.
Since ohmic contacts are the same for both polarities, so
R = 2Rpad + Rsemi as shown in Figure 3.
When the distance between two pads tends to zero,
then resistance through the semiconductor (Rsemi) goes to
zero and only resistance between metal to semiconductor
and back into metal will be left (2Rpad). Now we have
Rpad, multiply that value by the area of the metal pads (in
cm2). For author’s circuit it is taken as 10-5 -cm2. Con-
sider that in the modern processes, the vias that contact
the silicon have a contact area of about 0.1 μm2 or 10-8
cm2. If contact resistance is 10-5 cm2, that amounts to
1.0 k resistor to get into the silicon (plus another to get
out). A good contact resistance is on the order of 10-7
The Schottky contact resistance between the silicide
layer and polysilicon is the most likely cause of the ex-
cessive gate resistance and as measured using above
techniques. The contact resistance values, which are in
good agreement with the result as shown in Figure 3, for
monolithic silicon, were shown to contribute substantially
to the gate resistance. Downscaling of CMOS technolo-
gies will make the problem more pronounced, since the
interface contact resistance is inversely proportional to
the total gate area as in term of length and width of a gate.
The reduction of resistance should lead to improved RF
Figure 2. Example of the metal pattern of a RF MOSFET.
05 10 15 20 25 3035 40 45 5
Length between pads(um)
Calculated Resistance(ohm)
Figure 3. Resistance variation with pad lengths.
Copyright © 2010 SciRes. CS
properties in MOSFETs [6,24,25].
4. Potential Barriers
Potential barrier exist between metal and semiconductor
layer when they are in close contact, this stops the ma-
jority of charge carriers to pass from one layer to the
other layer. Only a few charge carriers have an adequate
amount of energy to pass though the barrier and cross to
the other side material. After applying a bias voltage to
the junction, it has following two effects, first it can cre-
ate the barrier come into view lower from the semicon-
ductor side, or second, it can make so higher. But the
bias does not change the barrier height from the metal
side. This creates a Schottky Barrier also known as rec-
tifying contact, where junction conducts only for one
bias polarity, not the other. This rectifying contact makes
good diodes and can even be used to make a kind of
A direct method of potential barrier height determina-
tion is presented in this section. The best agreement
between the barrier heights determined this method using
a dependence of the Si-SiO2 interface barrier height on
the thickness of the aluminium gate has been observed
[15]. Since the barrier height is the property of a material,
so we try to use these materials for the CMOS in appli-
cation of RF, whose barrier height is small. Here is a
possibility to create an alloy between metal and semi-
conductor junction, at the time of annealing, which low-
ers the barrier height [26]. The probability of tunneling
becomes high for extremely thin barriers (in the tens of
nanometers). By the heavy doping process one can make
the very thin barrier which is approximately doing con-
centration of 1019 dopant atoms/cm3 or more.
As the barrier height is closer to zero, ohmic contact
increases. For this one can concludes the following result
as shown in Table 1, for a positive barrier height.
Aluminium (Al), for doping of polysilicon 1015/cm3,
n-type, p-Silicon, semiconductor barrier height will 0.92
eV and metal-semiconductor junction depletion layer
width is 0.284 μm wide.
Ploysilicon, doping of polysilicon 1015/cm3, n-type, p-
Silicon, semiconductor barrier height will 0.85 eV and
metal-semiconductor junction depletion layer width is 0
So for the polysilicon, we achieve the barrier height is
closer to zero, which increases the ohmic contact.
Table 1. Direction of current flow.
Type Current flow into (easily)
n semiconductor
p metal
5. Results and Conclusions
After calculation of the currents, resistance and potential
barrier, we conclude that the drain current increases with
increase of the gate voltage or control voltage. Also, the
MOS device parameters can be used with VEE Pro-
gramming [27]. For the purpose of RF switch, where
control voltage should be low and then current flow will
be less and in terms of contact resistance, it will in-
creases with increase in number of gate fingers. So in
application of RF switch authors have tried to lower the
gate finger. The semiconductor barrier height and deple-
tion layer width is more for aluminum compare to
polysilicon metal-semiconductor junction which is 0.284
μm wide. Since the operating frequencies of the RF
switches are in the order of GHz, therefore, it is useful
for wireless local area network (WLAN) and other IEEE
802.11 applications.
6. Acknowledgements
The authors would like to thank Prof. A. B. Bhattach-
haraya, Jaypee Institute of Information Technology, In-
dia and Mr. Peeyush Tripathi, Intel Corporation, Banga-
lore, India for insightful discussions.
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