The dominant technology in the RF switch market has been GaAs. GaAs offers good linearity and isolation with low ON resistance and low OFF capacitance. However, GaAs has disadvantages. Most electronic systems use positive supplies, but most GaAs RF switches are N-channel depletion-mode FETs which require negative gate voltage to turn off. Driving GaAs switches also frequently requires extra interface components. Finally, GaAs has very limited capability to integrate other functions such as logic control and memory.
The UltraCMOS process technology is a patented silicon-on-sapphire (SOS) technology that uses CMOS circuitry on an insulating dielectric sapphire substrate. UltraCMOS technology matches the RF performance of GaAs with a single supply and has on-chip digital logic. Simpler controls reduce part count and total design cost. It also provides additional advantages including very high ESD protection (up to 2000V HBM and more). All this, and UltraCMOS switches are still price-competitive.
GaAs Switches
Figure 1 is a model of a typical RF switch, including GaAs switches. RF switches are arrays of transistors acting like voltage-controlled resistors. Isolation to deselected ports is achieved by turning off series transistors and grounding shunt transistors. Multiple transistors between common nodes increase power handling and improve linearity. GaAs switches most often use N-channel depletion-mode FETs. Pinch-off typically occurs when gate voltage is –2 to –3 V so the gate is normally biased at 0 V.
A problem with GaAs switches occurs in high power operation. High RF power can modulate the gate voltage, which varies channel resistance and generates distortion products. Increasing DC voltage on the gate can reduce this effect, but even modest RF power may require gate voltage of 1 to 2 V to turn the FET on and -3 V and sometimes –5 to –8 V to turn the FET off. Performance for low voltage systems is therefore often significantly degraded.
Another problem is that GaAs switches require coupling capacitors. While inexpensive, capacitors are not free and inventory, board space, and placement may cost even more. External capacitors constrain the minimum operating frequency. Capacitors also have insertion loss, and while usually minimal, in some systems even 0.1 dB is significant.
Some manufacturers instead integrate coupling capacitors in their GaAs switches. This eliminates external components, but monolithic capacitors are typically lower Q than discrete capacitors. Values greater than 10 pF can increase die size and cost. These switches are not suitable for low frequency or wideband applications.
Positive logic control is another problem for GaAs because implementing complementary logic in GaAs consumes significant current. One way to get around this is to float the die relative to DC ground.