AN1228 datasheet(4/6 Pages) STMICROELECTRONICS

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Defining ruggedness

AN1228

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the doping of the body and the metallurgical gate-source overlap. This capacitance is critical

since it is the largest component of the input capacitance and constrains device switching

speed which is comparable to limiting the maximum frequency of operation. Cds is the

capacitance formed between the drain and ground plane where the referenced ground

plane is formed by the LDMOS source, body, epitaxial layer and substrate. The charge

formed by application of a voltage to the drain is dependent on the area of the LDD and the

heavily doped drain, the concentration of the epitaxial layer and, to a lesser extent, the body

doping. This capacitance is critical since it is the largest component of the output

capacitance and influences device efficiency.

Device datasheets identify these primary capacitances in the form of Crss, Ciss, and Coss.

Capacitance Crss is simply the gate-drain capacitance, Cgd, whereas Ciss is the parallel

combination of Cgs and Cgd.

Capacitance Coss is the parallel combination of Cds and Cgd. The forward transconductance

(gfs) identifies the differential drain current for a differential gate voltage.

There are three major regions of the function gfs versus Vgs. As Vgs increases from low to

mid-range values gfs expands until a linear gfs region is reached. Beyond this region, as high

Vgs voltages are applied, gfs compresses. For Class AB operation the peak device current

should remain below the gfs compression region for maximum linearity. The gfs specification

is usually measured in the gfs linear region as shown on device datasheets.

Defining ruggedness

The ruggedness or the load-mismatch tolerance of LDMOS technology can be defined in

two ways. The first is that after being subjected to extreme load conditions there is not any

degradation in device performance or output power. A more stringent criterion would be that

there is not any degradation in the device parameters such as a shift in threshold voltage, an

increase in leakage current or a subtle increase in Rds(on). Changes in these parameters

can be an indication of long-term reliability problems.

The overall ruggedness of a device when tested in extreme load conditions is related to the

amount of localized thermal stress, the ability to sustain high levels of drain-source current

in BVDSS maximum current capability and the intensity of avalanching occurring under or

near the gate structure.

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