MRF275L Datasheet, PDF (16/17 Pages) Tyco Electronics – N-CHANNEL BROADBAND RF POWER FET

MRF275L
The RF MOSFET Line
100W, 500MHz, 28V
M/A-COM Products
Released - Rev. 07.07
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.
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.
Gate Termination — The gates of this device are
essentially 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
internalmonolithic zener diode from gate–to–source. If
gate protection is required, an external zener diode is
recommended. Using a resistor to keep the gate–to–
source impedance low also helps damp transients and
serves another important function. Voltage transients on
the drain can be coupled to the gate through the parasitic
gate–drain capacitance. If the gate–to–source imped-
ance and the rate of voltage change on the drain are
both high, then the signal coupled to the gate may be
large enough to exceed the gate–threshold voltage and
turn the device on.
HANDLING CONSIDERATIONS
When shipping, the devices should be transported
only in antistatic bags or conductive foam. Upon removal
from the packaging, careful handling procedures should
be adhered to. Those handling the devices should wear
grounding straps and devices not in the antistatic pack-
aging should be kept in metal tote bins. MOSFETs
should be handled by the case and not by the leads, and
when testing the device, all leads should make good
electrical contact before voltage is applied. As a final
note, when placing the FET into the system it is designed
for, soldering should be done with grounded equipment.
DESIGN CONSIDERATIONS
The MRF275L is a RF power N–channel enhance-
ment mode field–effect transistor (FETs) designed for
HF, VHF and UHF power amplifier applications. M/A-
COM RF MOSFETs feature a vertical structure with a
planar design. M/A-COM Application Note AN211A,
FETs in Theory and Practice, is suggested reading for
those not familiar with the construction and characteris-
tics of FETs. The major advantages of RF power FETs
include high gain, low noise, simple bias systems, rela-
tive immunity from thermal runaway, and the ability to
withstand severely mismatched loads without suffering
damage. Power output can be varied over a wide range
with a low power dc control signal.
DC BIAS
The MRF275L is an enhancement mode FET and,
therefore, does not conduct when drain voltage is ap-
plied. Drain current flows when a positive voltage is ap-
plied to the gate. RF power FETs require forward bias for
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