MRF140 Datasheet, PDF (9/10 Pages) Motorola, Inc – N-CHANNEL MOS LINEAR RF POWER FET

MRF140
The RF MOSFET Line
150W, to 150MHz, 28V
M/A-COM Products
Released - Rev. 07.07
RF POWER MOSFET CONSIDERATIONS
MOSFET CAPACITANCES
The physical structure of a MOSFET results in capacitors
between the terminals. The metal oxide gate structure de-
termines the capacitors from gate–to–drain (Cgd), and
gate–to–source (Cgs). The PN junction formed during the
fabrication of the RF MOSFET results in a junction capaci-
tance from drain–to–source (Cds).
These capacitances are characterized as input (Ciss),
output (Coss) and reverse transfer (Crss) capacitances on
data sheets. 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.
GATE CHARACTERISTICS
The gate of the RF 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.
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 voltage
rating. Exceeding the rated VGS can result in permanent
damage to the oxide layer in the gate region.
Gate Termination — The gates of these devices are es-
sentially capacitors. Circuits that leave the gate open–
circuited or floating should be avoided. These conditions
can result in turn–on of the devices due to voltage build–up
on the input capacitor due to leakage currents or pickup.
Gate Protection — These devices do not have an internal
monolithic zener diode from gate–to–source. If gate protec-
tion is required, an external zener diode is recommended.
LINEARITY AND GAIN CHARACTERISTICS
In addition to the typical IMD and power gain data pre-
sented, Figure 5 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 tran-
sistors. 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 extent.
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