MRF5007 Datasheet, PDF (7/10 Pages) Motorola, Inc – N-CHANNEL BROADBAND RF POWER FET

DESIGN CONSIDERATIONS
The MRF5007 is a common–source, RF power, N–Chan-
nel enhancement mode, Metal–Oxide Semiconductor
Field–Effect Transistor (MOSFET). Motorola RF MOSFETs
feature a vertical structure with a planar design. Motorola
Application Note AN211A, “FETs in Theory and Practice,” is
suggested reading for those not familiar with the construc-
tion and characteristics of FETs.
This surface mount packaged device was designed pri-
marily for VHF and UHF portable power amplifier applica-
tions. Manufacturability is improved by utilizing the tape and
reel capability for fully automated pick and placement of
parts. However, care should be taken in the design process
to insure proper heat sinking of the device.
The major advantages of RF power MOSFETs include
high gain, simple bias systems, relative immunity from ther-
mal runaway, and the ability to withstand severely mis-
matched loads without suffering damage.
MOSFET CAPACITANCES
The physical structure of a MOSFET results in capacitors
between all three terminals. The metal oxide gate structure
determines the capacitors from gate–to–drain (Cgd), and
gate–to–source (Cgs). The PN junction formed during fabrica-
tion of the RF MOSFET results in a junction capacitance from
drain–to–source (Cds). These capacitances are characterized
as input (Ciss), output (Coss) and reverse transfer (Crss) ca-
pacitances on data sheets. The relationships between the in-
ter–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 ap-
plications.
Cgd
GATE
Cgs
DRAIN
Cds
SOURCE
Ciss = Cgd + Cgs
Coss = Cgd + Cds
Crss = Cgd
DRAIN CHARACTERISTICS
One critical figure of merit for a FET is its static resistance
in the full–on condition. This on–resistance, RDS(on), occurs
in the linear region of the output characteristic and is speci-
fied at a specific gate–source voltage and drain current. The
drain–source voltage under these conditions is termed
VDS(on). For MOSFETs, VDS(on) has a positive temperature
coefficient at high temperatures because it contributes to the
power dissipation within the device.
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 DC input resistance is very high — on the order of 109 Ω
— resulting in a leakage current of a few nanoamperes.
Gate control is achieved by applying a positive voltage to
the gate greater than 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–cir-
cuited 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
with appropriate RF decoupling.
Using a resistor to keep the gate–to–source impedance
low also helps dampen transients and serves another impor-
tant function. Voltage transients on the drain can be coupled
to the gate through the parasitic gate–drain capacitance. If
the gate–to–source impedance 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.
DC BIAS
Since the MRF5007 is an enhancement mode FET, drain
current flows only when the gate is at a higher potential than
the source. See Figure 7 for a typical plot of drain current
versus gate voltage. RF power FETs operate optimally with
a quiescent drain current (IDQ), whose value is application
dependent. The MRF5007 was characterized at IDQ = 75
mA, which is the suggested value of bias current for typical
applications. For special applications such as linear amplifi-
cation, IDQ may have to be selected to optimize the critical
parameters.
The gate is a dc open circuit and draws no current. There-
fore, the gate bias circuit may generally be just a simple re-
sistive divider network. Some special applications may
require a more elaborate bias system.
GAIN CONTROL
Power output of the MRF5007 may be controlled to some
degree with a low power dc control signal applied to the gate,
thus facilitating applications such as manual gain control,
ALC/AGC and modulation systems. Figure 6 is an example
of output power variation with gate–source bias voltage. This
characteristic is very dependent on frequency and load line.
MOTOROLA RF DEVICE DATA
MRF5007 MRF5007R1
7