July 2007

Rev 3

1/6

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

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.

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