Page 26 - Complete Wireless Design

Wireless Essentials

Wireless Essentials 25

E-MOSFETs are popular in digital ICs as voltage-controlled switches, and
are found as the active element in high-frequency, very high frequency, and
ultrahigh frequency (HF, VHF, and UHF) power amplifiers and drivers. This
is a result of the E-MOSFET parameters’ superiority to those of a typical pow-
er BJT, such as higher input impedance and gain, increased thermal stability,
lower noise, and a higher tolerance for load mismatches. Another advantage
that any MOSFET enjoys over a BJT is the impossibility of thermal runaway,
since MOSFETs are designed to have a positive temperature coefficient at high
drain currents. This means that, as the temperature increases, a MOSFET
will actually decrease its source-to-drain current, instead of increasing its cur-
rent output as a BJT will (see “Thermal Runaway”). This makes thermal run-
away impossible and temperature stabilization components less necessary,
except to stabilize the MOSFET’s Q point. In addition, a MOSFET’s input and
output impedances will change much less for different input drive levels than
a BJT’s, and a MOSFET offers better single-stage stability and 20 percent
more gain. MOSFETs also can survive a very high voltage standing wave ratio
(VSWR), second only to BJTs with an emitter ballast resistor in this respect.
On the negative side, however, MOSFETs are very sensitive to destruction by
static electricity, with almost any electrical spark possibly causing damage to
the gate insulation. And N-channel enhancement MOSFETs, the most com-
mon in RF power applications, as well as depletion-mode power MOSFETs in
Class C and B operation, can begin to exhibit low-frequency oscillations if they
are directly paralleled for increased output. MOSFETs also have inferior low-
order intermodulation distortion (IMD) performance to that of BJTs.

SiGe BiCMOS. SiGe stands for silicon-germanium, while CMOS stands for
complementary metal-oxide-semiconductor. SiGe BiCMOS comprises these two
major technologies: SiGe and the integration of SiGe with CMOS.
SiGe devices, also called SiGe HBTs (silicon-germanium heterojunction bipo-
lar transistors), is a mixture of silicon (Si) and germanium (Ge) within a sin-
gle transistor structure. This produces much higher cutoff frequencies (60 Ghz
for SiGe; 20 GHz for standard silicon), a reduction in noise, and greatly
decreased power dissipation, with the added benefit of increased gain over
that of standard silicon. A primary limitation of current HBTs is that the
breakdown voltage of the device is rather low, decreasing dependability some-
what. This will be rapidly improved, however.
Current SiGe technology allows the high-frequency performance of GaAs at
much lower costs (equivalent to VLSI silicon, or about a quarter of the cost of
GaAs). SiGe also employs much simpler manufacturing techniques (GaAs
manufacturing is intensive, complex, and has lower chip yields than SiGe). In
fact, many companies are claiming that SiGe will eventually completely obso-
lesce GaAs in all frequencies below 60 GHz.
The recent ability to economically combine CMOS with SiGe will permit the
integration of microwave RF front ends with the intermediate-frequency (IF)
and baseband circuitry—as well as the necessary control logic—on a single chip.

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