MRF275G_17 Datasheet, PDF (17/20 Pages) M/A-COM Technology Solutions, Inc. – The RF MOSFET Line 150W, 500MHz, 28V

MRF275G
The RF MOSFET Line
150W, 500MHz, 28V
Rev. V1
RF POWER MOSFET CONSIDERATIONS
MOSFET CAPACITANCES
The physical structure of a MOSFET results in capaci-
tors between the terminals. The metal oxide gate struc-
ture determines the capacitors from gate–to–drain (Cgd),
and gate–to–source (Cgs). The PN junction formed dur-
ing the fabrication of the MOSFET results in a junction
capacitance from drain–to–source (Cds).
These capacitances are characterized as input (Ciss),
output (Coss) and reverse transfer (Crss) capacitances
on datasheets. The relationships between the inter–
terminal capacitances and those given on data sheets
are shown below. The Ciss can be specified in two ways:
1. Drain shorted to source and positive voltage at
the gate.
2. Positive voltage of the drain in respect to source
and zero volts at the gate. In the latter case the
numbers are lower. However, neither method
represents the actual operating conditions in RF
applications.
The Ciss given in the electrical characteristics table
was measured using method 2 above. It should be noted
that Ciss, Coss, Crss are measured at zero drain current
and are provided for general information about the de-
vice. They are not RF design parameters and no attempt
should be made to use them as such.
DRAIN CHARACTERISTICS
One figure of merit for a FET is its static resistance in
the full–on condition. This on–resistance, VDS(on), oc-
curs in the linear region of the output characteristic and is
specified under specific test conditions for gate–source
voltage and drain current. For MOSFETs, VDS(on) has a
positive temperature coefficient and constitutes an impor-
tant design consideration at high temperatures, because
it contributes to the power dissipation within the device.
GATE CHARACTERISTICS
The gate of the MOSFET is a polysilicon material, and
is electrically isolated from the source by a layer of oxide.
The input resistance is very high — on the order of 109
ohms — resulting in a leakage current of a few nanoam-
peres. Gate control is achieved by applying a positive
voltage slightly in excess of the gate–to–source threshold
voltage, VGS(th).
Gate Voltage Rating — Never exceed the gate volt-
age rating (or any of the maximum ratings on the front
page). Exceeding the rated VGS can result in permanent
damage to the oxide layer in the gate region.
17
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