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 (C ), and
gate–to–source (C ). The PN junction formed during the
gs
fabrication of the RF MOSFET results in a junction capaci-
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.
gd
DRAIN CHARACTERISTICS
One figure of merit for a FET is its static resistance in the
tance from drain–to–source (C ).
These capacitances are characterized as input (C ),
ds
iss
full–on condition. This on–resistance, V
, occurs in the
DS(on)
output(C
)andreversetransfer(C )capacitancesondata
rss
oss
linear region of the output characteristic and is specified under
specific test conditions for gate–source voltage and drain
sheets. The relationships between the inter–terminal capaci-
tances and those given on data sheets are shown below. The
current. For MOSFETs, V
has a positive temperature
DS(on)
C
can be specified in two ways:
iss
coefficient and constitutes an important design consideration
at high temperatures, because it contributes to the power
dissipation within the device.
1. Drain shorted to source and positive voltage at the gate.
2. Positivevoltageofthedraininrespecttosourceandzero
volts at the gate. In the latter case the numbers are lower.
However, neither method represents the actual operat-
ing conditions in RF applications.
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
9
input resistance is very high — on the order of 10 ohms —
DRAIN
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,
C
gd
GATE
C
C
C
= C + C
gd gs
iss
C
V
.
= C + C
ds
GS(th)
Gate Voltage Rating — Never exceed the gate voltage
rating. Exceeding the rated V can result in permanent
oss
rss
gd
= C
gd
ds
C
GS
gs
SOURCE
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
monolithiczenerdiodefromgate–to–source. Ifgateprotection
is required, an external zener diode is recommended.
LINEARITY AND GAIN CHARACTERISTICS
In addition to the typical IMD and power gain data
presented, Figure 5 may give the designer additional informa-
tiononthecapabilitiesofthisdevice. Thegraphrepresentsthe
small signal unity current gain frequency at a given drain
current level. This is equivalent to f for bipolar transistors.
T
EQUIVALENT TRANSISTOR PARAMETER TERMINOLOGY
Collector . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Drain
Emitter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Source
Base . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Gate
V
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . V
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . V
(BR)CES
(BR)DSS
DGO
V
CBO
I
C
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . I
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . I
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . I
D
DSS
GSS
I
I
CES
EBO
V
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . V
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . V
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C
BE(on)
GS(th)
DS(on)
iss
V
CE(sat)
C
ib
C
h
ob
oss
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . g
fe
fs
V
DS(on)
V
CE(sat)
r
=
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
DS(on)
R
=
I
D
CE(sat)
I
C
MOTOROLA RF DEVICE DATA
MRF140
5
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