12
RF Device Data
Freescale Semiconductor
MRF1517NT1
APPLICATIONS INFORMATION
DESIGN CONSIDERATIONS
This device is a common - source, RF power, N-Channel
enhancement mode, Lateral M
etal-Oxide Semiconductor
F
ield-Effect Transistor (MOSFET). Freescale Application
Note AN211A, “FETs in Theory and Practice”, is suggested
reading for those not familiar with the construction and char-
acteristics 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 Lateral RF power MOSFETs in-
clude high gain, simple bias systems, relative immunity from
thermal 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 (C
gd
), and
gate - to-source (C
gs
). The PN junction formed during fab-
rication of the RF MOSFET results in a junction capacitance
from drain-to -source (C
ds
). These capacitances are charac-
terized as input (C
iss
), output (C
oss
) and reverse transfer
(C
rss
) capacitances on data sheets. The relationships be-
tween the inter -terminal capacitances and those given on
data sheets are shown below. The C
iss
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.
Drain
C
ds
Source
Gate
C
gd
C
gs
C
iss
= C
gd
+ C
gs
C
oss
= C
gd
+ C
ds
C
rss
= C
gd
DRAIN CHARACTERISTICS
One critical figure of merit for a FET is its static resistance
in the full- on condition. This on-resistance, R
DS(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
V
DS(on)
. For MOSFETs, V
DS(on)
has a positive temperature
coefficient at high temperatures because it contributes to the
power dissipation within the device.
BV
DSS
values for this device are higher than normally re-
quired for typical applications. Measurement of BV
DSS
is not
recommended and may result in possible damage to the de-
vice.
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 10
9
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 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.
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.
DC BIAS
Since this device is an enhancement mode FET, drain cur-
rent flows only when the gate is at a higher potential than the
source. RF power FETs operate optimally with a quiescent
drain current (I
DQ
), whose value is application dependent.
This device was characterized at I
DQ
= 150 mA, which is the
suggested value of bias current for typical applications. For
special applications such as linear amplification, I
DQ
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 this device may be controlled to some de-
gree with a low power dc control signal applied to the gate,
thus facilitating applications such as manual gain control,
ALC/AGC and modulation systems. This characteristic is
very dependent on frequency and load line.
