5

MRF151

MOTOROLA RF DEVICE DATA

Figure 10. Series Equivalent Impedance

150

30

7.5

4

2

ZOL*

Zin

15

f = 175 MHz

100

30

15

7.5

Zo = 10 Ω

VDD = 50 V

IDQ = 250 mA

Pout = 150 W

ZOL* = Conjugate of the optimum load impedance

f = 175 MHz

150

100

4

2

NOTE: Gate Shunted by 25 Ohms.

RF POWER MOSFET CONSIDERATIONS

MOSFET CAPACITANCES

The physical structure of a MOSFET results in capacitors

between the terminals. The metal anode gate structure de-

termines the capacitors from gate–to–drain (Cgd), and gate–

to–source (Cgs). The PN junction formed during the

fabrication of the MOSFET results in a junction capacitance

from drain–to–source (Cds).

These capacitances are characterized as input (Ciss), out-

put (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

SOURCE

Cgs

DRAIN

Cds

Ciss = Cgd = Cgs

Coss = Cgd = Cds

Crss = Cgd

LINEARITY AND GAIN CHARACTERISTICS

In addition to the typical IMD and power gain data pres-

ented, Figure 6 may give the designer additional information

on the capabilities of this device. The graph represents the

small signal unity current gain frequency at a given drain cur-

rent 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 ex-

tent.

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 un-

der 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 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 gate of this device is essentially

capacitor. Circuits that leave the gate open–circuited or float-

ing should be avoided. These conditions can result in turn–

on of the device due to voltage build–up on the input

capacitor due to leakage currents or pickup.