MRF5015 Datasheet, PDF (6/8 Pages) Motorola, Inc – N-CHANNEL BROADBAND RF POWER FET

DESIGN CONSIDERATIONS
The MRF5015 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 construction and char-
acteristics of FETs.
This device was designed primarily for 12.5 volt VHF and
UHF power amplifier applications. The major advantages of
RF power MOSFETs include high gain, simple bias systems,
relative immunity from thermal runaway, and the ability to
withstand severely mismatched loads without suffering dam-
age.
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 fab-
rication of the RF MOSFET results in a junction capacitance
from drain–to–source (Cds). These capacitances are charac-
terized as input (Ciss), output (Coss) and reverse transfer
(Crss) capacitances on data sheets. The relationships be-
tween 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
2. 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
Ciss = Cgd + Cgs
Cds
Coss = Cgd + Cds
Crss = Cgd
Source
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 input resistance is very high, on the order of 109 Ω, re-
sulting 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 rat-
ing. Exceeding the rated VGS can result in permanent dam-
age 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 must 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 networks.
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 MRF5015 is an enhancement mode FET, drain
current flows only when the gate is at a higher potential than
the source. See Figure 5 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 de-
pendent. The MRF5015 was characterized at IDQ = 100 mA,
which is the suggested value of bias current for typical ap-
plications. For special applications such as linear amplifica-
tion, IDQ may have to be selected to optimize the critical
parameters.
The gate is a dc open circuit and draws essentially no cur-
rent. Therefore, the gate bias circuit may generally be just a
simple resistive divider network. Some special applications
may require a more elaborate bias system.
GAIN CONTROL
Power output of the MRF5015 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 4 is an example
of output power variation with gate–source bias voltage with
Pin held constant. This characteristic is very dependent on
frequency and load line.
MRF5015
6
MOTOROLA RF DEVICE DATA