MRF275G Datasheet, PDF (10/12 Pages) Motorola, Inc – 150 W, 28 V, 500 MHz N.CHANNEL MOS BROADBAND 100 - 500 MHz RF POWER FET

Figure 16. MRF275G Test Fixture
RF POWER MOSFET CONSIDERATIONS
MOSFET CAPACITANCES
The physical structure of a MOSFET results in capacitors
between the terminals. The metal oxide gate structure deter-
mines the capacitors from gate–to–drain (Cgd), and gate–to–
source (Cgs). The PN junction formed during the fabrication
of the MOSFET results in a junction capacitance from drain–
to–source (Cds).
These capacitances are characterized as input (Ciss), out-
put (Coss) and reverse transfer (Crss) capacitances on data
sheets. The relationships between the inter–terminal capaci-
tances 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 operat-
ing conditions in RF applications.
Cgd
GATE
Cgs
DRAIN
Cds
SOURCE
Ciss = Cgd + Cgs
Coss = Cgd + Cds
Crss = Cgd
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 device. They are
not RF design parameters and no attempt should be made to
use them as such.
LINEARITY AND GAIN CHARACTERISTICS
In addition to the typical IMD and power gain, data pres-
ented in Figure 3 may give the designer additional informa-
tion on the capabilities of this device. The graph represents
the small signal unity current gain frequency at a given drain
current level. This is equivalent to fT for bipolar transistors.
Since this test is performed at a fast sweep speed, heating of
the device does not occur. Thus, in normal use, the higher
temperatures may degrade these characteristics to some ex-
tent.
DRAIN CHARACTERISTICS
One figure of merit for a FET is its static resistance in the
full–on condition. This on–resistance, VDS(on), occurs in the
linear region of the output characteristic and is specified un-
der specific test conditions for gate–source voltage and drain
current. For MOSFETs, VDS(on) has a positive temperature
coefficient and constitutes an important 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 nanoamperes.
MRF275G
10
MOTOROLA RF DEVICE DATA