NEW RF MOSFET Small Signal SPICE Model
Part I
Ickjin Kwon, Minkyu Je, Kwyro Lee, and Hyungcheol
Shin
Department of Electrical Engineering, Korea Advanced Institute of Science
and Technology
Introduction
With the advent of ever increasing operation frequencies,
Silvaco is introducing a new high frequency model for MOSFETs that will
be implemented into the SmartSpice code. This article describing these
new models will be in two parts. The first part describes the mathematical
detail concerning the extraction of the "Y" parameters that contain the
necessary frequency terms and part two, in the July issue, describes the
conversion of the "Y" parameters into the required elements for the Spice
model itself.
Figure 1. The proposed commonsource equivalent circuit of
a MOSFET after deembedding parasitics of onwafer pads and
interconnection lines. Four independent intrinsic capacitances
Cgs, Cgd, Cdg, and Cds are needed for charge
conservation. The definitions of each capacitance are also shown.
When extracting frequency dependent parameters for high frequency MOSFETs
and other active devices from measured data, it is important to isolate
the elements that are due to the device alone, from the "parasitics" that
are inevitably present from bonding pads, packaging and the like. To this
end, the user has to have two types of structure. One structure type contains
all the active elements and the other structure type consists of identical
bond pads and packaging but without the active devices. By measuring both
types of structure, the frequency dependent parameters of the active device
can be isolated by subtracting the effects of the package without the
active devices. This procedure is called "deembedding. The following
article describes the method for extracting an accurate high frequency
Spice model from just such measured data.
1.1 Parameter Extraction Requirement
Measured Sparameters of MOSFET devices and Sparameters of deembedding
patterns (open, short) are needed to extract small signal parameters of
RF MOSFET. Deembedding patterns are used to deembed parasitics of pad
and interconnection line. Gate is port 1 and drain is port 2. Source and
body are grounded. The characteristic impedance is 50 Ohm.
The data is stored in standard sequential ASCII files. One line of data
is a set of data for one frequency and, for example, can be written as

Frequency Mag(S11) Phase(S11) Mag(S21)
Phase(S21) Mag(S12)
Phase(S12) Mag(S22) Phase(S22)
In an SParameter file, a typical line might be

500 0.64 23 12.5 98 0.03 70 0.8 37
In this case, 500 is the frequency in megahertz. The magnitudes of S11, S21, S12 and S22 are 0.64, 12.5, 0.03 and 0.8, and the phases are 23, 98, 70 and 37 degrees, respectively.
Sparameter of devices will have to be measured under DC bias condition of Vgs, Vds. One routine of extraction procedure extracts smallsignal parameters under one DC bias condition of Vgs, Vds. Deembedding patterns (open, short) are measured under zero DC bias condition of Vgs = 0 V, Vds = 0 V.
The symbol definitions for the deembedding procedure are listed in Table 1.
Table 1. The symbol definitions for the deembedding procedure.
1.2 Sparameter Measurement
(1) Sparameters of MOSFET device are represented as
The magnitudes and the phases of S11D, S21D, S12D and S22D are converted to real and imaginary format.
Then, S11D, S21D, S12D and S22D are represented as real and imaginary format as following.
(2) Sparameters of open deembedding pattern are represented by
The magnitudes and the phases of S11O, S21O, S12O and S22O are converted to real and imaginary format.
Then, S11O, S21O, S12O and S22O are represented as real and imaginary format as following.
(3) Measured Sparameters of short deembedding pattern are represented by
The magnitudes and the phases of S11S, S21S, S12S and S22S are converted to real and imaginary format.
Then, S11S, S21S, S12S and S22S are represented as real and imaginary format as following.
1.3 Deembedding Procedure
(1) Measured Sparameters of MOSFET device are converted to Yparameters. (([SD][YD] ) )
Then, MOSFET device Yparameters are represented as real and imaginary parts.
(2) Measured Sparameters of open deembedding pattern are converted to Yparameters. ([S0] [Y0])
Then, open pattern Yparameters are represented as real and imaginary parts.
(3) Subtract open pattern Yparameters from MOSFET device Yparameters. ([YDO] = [YD]  [YO] )
If only open deembedding pattern is used without using shortpattern, following (4)(9) procedures are omitted. In this case, Y11DO, Y21DO, Y12D0, Y22DO are used for parameter extraction.
(4) Measured Sparameters of short deembedding pattern are converted to Yparameters. ([SS][YS] )
Then, short pattern Yparameters are represented as real and imaginary parts.
(5) Subtract open pattern Yparameters from short pattern Yparameters. ([YSO] = [YS]  [YO] )
(6) ([YSO][ZSO] )
(7) ([YDO][ZDO] )
(8) ([ZF] = [ZDO]  [ZSO] )
(9) ([ZF][YF] )
Deembedding procedure using open and short deembedding pattern is finished.
Note that, Y11F, Y21F, Y12F, Y22F are used in the following parameter extraction procedure and Yparameters in the extraction equation is same as Y11F, Y21F, Y12F, Y22F . (i.e. [Y] = [YF] )
If only open pattern is used, Y11DO, Y21DO, Y12DO, Y22DO are used instead of Y11F, Y21F, Y12F, Y22F. (i.e. [Y] = [YDO] )
For the parameter extraction, Yparameters are represented as real and imaginary format as follows.
Conclusion
This concludes the extraction of the "Y" parameters that are required for generating the equivilent circuit elements in the high frequency Spice model for MOSFETs. In the July issue, the conversion of the deembedded "Y"parameters into the frequency dependent circuit elements is described.