MRF1507 Datasheet, PDF (9/12 Pages) Motorola, Inc – LATERAL NCHANNEL BROADBAND RF POWER MOSFET

APPLICATIONS INFORMATION
DESIGN CONSIDERATIONS
The MRF1507 is a common–source, RF power, N–Channel
enhancement mode, Lateral Metal–Oxide Semiconductor
Field–Effect Transistor (MOSFET). Motorola Application Note
AN211A, “FETs in Theory and Practice”, is suggested reading
for those not familiar with the construction and characteristics
of FETs.
This surface mount packaged device was designed primari-
ly for VHF and UHF portable power amplifier applications.
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 Lateral RF power MOSFETs
include high gain, simple bias systems, relative immunity from
thermal runaway, and the ability to withstand severely
mismatched 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 (C_gd), and
gate–to–source (C_gs). The PN junction formed during fabrica-
tion of the RF MOSFET results in a junction capacitance from
drain–to–source (C_ds). These capacitances are characterized
as input (C_iss), output (C_oss) and reverse transfer (C_rss)
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.
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 specified
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.
BVDSS values for this device are higher than normally
required for typical applications. Measurement of BVDSS is not
recommended and may result in possible damage to 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,
V_GS(th).
Gate Voltage Rating — Never exceed the gate voltage
rating. Exceeding the rated V_GS can result in permanent
damage to the oxide layer in the gate region.
Gate Termination — The gates of these devices are
essentially 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 protection
is required, an external zener diode is recommended. Using
a resistor to keep the gate–to–source impedance low also
helps dampen 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 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.
MOTOROLA RF DEVICE DATA
MRF1507 MRF1507T1
9