MRF140 Datasheet, PDF (5/6 Pages) Motorola, Inc – N-CHANNEL MOS LINEAR RF POWER FET

RF POWER MOSFET CONSIDERATIONS
MOSFET CAPACITANCES
The physical structure of a MOSFET results in capacitors
between the terminals. The metal oxide gate structure
determines the capacitors from gate–to–drain (Cgd), and
gate–to–source (Cgs). The PN junction formed during the
fabrication of the RF MOSFET results in a junction capaci-
tance from drain–to–source (Cds).
These capacitances are characterized as input (Ciss),
output (Coss) and reverse transfer (Crss) capacitances on data
sheets. The relationships between the inter–terminal capaci-
tances 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 operat-
ing conditions in RF applications.
Cgd
GATE
Cgs
DRAIN
Cds
SOURCE
Ciss = Cgd + Cgs
Coss = Cgd + Cds
Crss = Cgd
LINEARITY AND GAIN CHARACTERISTICS
In addition to the typical IMD and power gain data
presented, Figure 5 may give the designer additional informa-
tion on the capabilities of this device. The graph represents the
small signal unity current gain frequency at a given drain
current level. This is equivalent to fT for bipolar transistors.
Since this test is performed at a fast sweep speed, heating of
the device does not occur. Thus, in normal use, the higher
temperatures may degrade these characteristics to some
extent.
DRAIN CHARACTERISTICS
One figure of merit for a FET is its static resistance in the
full–on condition. This on–resistance, VDS(on), occurs 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 important design consideration
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 ohms —
resulting in a leakage current of a few nanoamperes.
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 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
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.
EQUIVALENT TRANSISTOR PARAMETER TERMINOLOGY
Collector . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Drain
Emitter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Source
Base . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Gate
V(BR)CES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . V(BR)DSS
VCBO . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . VDGO
IC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ID
ICES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . IDSS
IEBO . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . IGSS
VBE(on) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . VGS(th)
VCE(sat) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . VDS(on)
Cib . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Ciss
Cob . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Coss
hfe . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . gfs
RCE(sat) =
VCE(sat)
IC
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ^{rDS(on) =}
VDS(on)
ID
MOTOROLA RF DEVICE DATA
MRF140
5