This document analyzes the influence of gate geometry (width and length) on integrated accumulation MOS varactors fabricated on a 0.8 μm CMOS technology. Simulation and measurement results show that tuning range and quality factor increase more with gate length than width, while resonance frequency remains constant. Therefore, varactor performance is better for longer gates than wider gates at a given area. The capacitance scales directly with area when increasing width but not length, as measured capacitance decreases with increased length.

1. Influence of gate geometry in integrated MOS
varactors on accumulation mode for RF
E. Amselem, B. Gonzalez, J. Garcia, I. Aldea, M. Marrero, A.G. Iturri, J. del Pino,
S.L. Khemchandani, and A. Hernaindez
Dep. Ingeniería Electrónica y Automática / Instituto Universitario de Microelectrónica Aplicada (IUMA), Universidad de Las Palmas de Gran Canaria, Spain.
VI Conferencia de Dispositivos Electrónicos (Nacional),
Madrid, España,
2007
Abstract
Driven by the many applications that varactors have in RF integrated blocks, this work analyzes the influence of
gate geometry (width and length) on integrated accumulation MOS varactors. For this purpose, a number of varactors
have been designed and fabricated on a 0.8 ,Im CMOS standard technology. The most relevant parameters: quality factor,
tuning range, and capacitance, are simulate and compared.
Fabrication
Measurements & Simulations
Fig. 3. 3D dimensional structure of half
a MI basic cell. The meshing grid
isincluding; w 10,um, / =0.8 ,m. (right)
Fig. 4 Measured and simulated capacitance Figure 6 Measured and extracted quality
vesus gate voltage for a MI basic cell, actor and inductance for three different Cp
including the parasitic capacitances;f= 2.1 values.
GHz.
Fig. 2. Microphotograph of the integrated MOS varactor MI with the guardring.
0
Figure 7 Extracted RSUB from the Figure 8 Measured and extracted quality
measurements actor for three different resistance RSUB
values..
Conclusions
Simulation results predict a direct scaling between
capacitance and area in our structures, but as
compared with measurements this is not precise. It is
true when the area increases due to increase in gate
width, but not when the gate length increases (the
measured capacitance becomes smaller). On the
other hand, the tuning range and quality factor
increases more with the gate length than with the
Fig. 1. Layout of the MI basic cell. gate width, whereas the resonance frequency remains
nearly constant. Therefore, for a given area, the
varactor performance is bigger for gates longer than
wider.
Acknowledgement
This reported work is supported in part by the Spanish MEC under projects TEC-2005-08091 -C03-02 and TEC-2005-06784-C02-02.
INSTITUTO UNIVERSITARIO DE MICROELECTRÓNICA APLICADA (IUMA)
UNIVERSIDAD DE LAS PALMAS DE GRAN CANARIA (ULPGC)