AN1228 Datasheet, PDF (1/6 Pages) STMicroelectronics – How to relate LMOS device parameters to RF performance

AN1228
Application note
How to relate LMOS device parameters to RF
performance
Introduction
This second installment of a two-part paper series on LDMOS technology (see
Understanding LDMOS Device Fundamentals, AN1226) will explain LDMOS circuit-level
performance through MOS intrinsic device characteristics. Understanding current laterally
diffused Metal-Oxide-Semiconductor (LDMOS) technology is necessary to optimally use
these devices in high-power RF circuitry. RF circuit designers must come to an
understanding of the relationship between circuit performance and device characteristics
beyond first-order approximations. These higher-order device relationships can offer insight
into many common device parameters and their interdependencies and, more importantly,
enable the design engineer to monitor the semiconductor manufacturing process more
effectively.
In general, for LDMOS devices and MOS field-effect transistors (MOSFETs) the channel is
of primary importance. The channel is the inversion layer created within the body of the
device that electrically connects the source and drain, as described in the first part of this
series. The channel dimensions and its doping determine the forward transconductance
(g_fs) and contribute to the body-related capacitances that ultimately influence RF power gain
and frequency response. The body-doping profile is critical for device ruggedness and
reliability. Since the introduction of LDMOS devices for high-voltage commercial RF
applications, device dimensions have evolved from supermicron to submicron in only a few
short years.
This progress is indicative of future LDMOS generations and it should be noted that the
reduction in device size below one micron has not necessarily followed traditional scaling
laws.
Specification sheets for RF MOSFETs include many parameters that will be explained in the
context of circuit design and performance criteria. The order in which these device
parameters are presented here is not indicative of relative importance.
July 2007
Rev 3
1/6
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