GaAs technology has finally yielded the long-sought capability ofcombining bipolar and field effect (FET) transistors on the samemonolithic die – affordably and reliably for commercial applications.
This increases the design flexibility in achieving increasedfunctionality through higher levels of circuit integration. It alsoimproves product performance by taking advantage of bipolar's analogcharacteristics in providing highly linear amplification and FET'sadvantages in digital switching.
GaAs remains thetechnology of choice for microwave power amplifiers used in cellphonesand WLANs, offering higher power levels, higher power-addedefficiencies and better SI performance characteristics than CMOS.
GaAs exhibits the linearity and low distortion required for reliablewireless connections. As users demand more capabilities—long batterylife and enhanced capabilities in wireless handsets and/ or morechannels and multituner capabilities in TV – GaAs is evolving to supplyhigher integration and increased functionality.
Despite the encroachment of CMOS in low-end wireless amplifiers,GaAs won't soon be displaced in high-end applications. The new GaAsstructures use a vertically integrated heterojunction bipolartransistor (HBT) and pseudomorphic high-electronmobility FET (pHEMT)epitaxial structure. The HBT and pHEMT device structures are decoupledfrom one another, enabling independent optimization and development ofeach device to achieve the required performance without anycompromises.
To minimize cost and maximize yield, the process flow maximizes thenumber of shared fabrication steps. Extensive DC and RFcharacterization show that HBTand pHEMT devices in thesame wafer have the same performance and reliability as the stand-aloneHBT and pHEMT devices.
|Figure1. Mixed pHEMT/HBT structures enable new levels of design flexibilityand integration in GaAs circuits.|
<>The basics of mixed GaAspHEMT/HBTstructures
Figure 1 above shows the basicconcept of the integrated pHEMT/ HBT structure. The pHEMT layerstructure is epitaxially grown first on the substrate and the HBT layerplaced atop the pHEMT layers. The HBT and pHEMT only share a highlydoped n-type GaAs layer that serves as the cap for pHEMT and as thesubcollector for HBT.
The thickness of this layer should be chosen to achieve the requiredHBT collector resistance while maintaining acceptable pHEMT recesstopography to allow pHEMT gate processing and formation. The optimalsubcollector thickness can be determined through physical devicesimulation and circuit testing.
This approach completely decouples the active device layers of HBTand pHEMT, and opens up new possibilities in device design. Equallyimportant, the fabrication process adds only minimal complexity,allowing the same high yields and device reliability that are achievedwith straight HBT processing.
The new pHEMT/HBT technology finds various practical applications:
1) TV tuners. HBTs foroscillators, varactors for frequency tuning and FETs for highly linearupconverters;
2) WLANs. Highly integratedfront-end modules with power amplification, low-noise amplification, RFpower detection, RF switch functions, simple logic functions and ruggedbiasing schemes;
3) GSM handsets. Poweramplifiers with integrated Vref , RF switch and smart biasnetworks;
4) CDMA handsets. Higherefficiency at lower power output levels.
CDMA handsets demonstrate the advantages of the pHEMT/ HBT process.Studies by the CDMA Development Group reveal that actual output powerlevels are statistically less than +10dBm for greater than 80 percentof the phone's operating time – rather than the +28dBm (for CDMA)maximum output capability of the amplifier.
Users typically need full power less than 5 percent of the time andmost often during the initial connection, while the base stationdetermines the proper power level for the handset's transmissions. Basestations set the power levels for attached handsets to maintain allincoming signals at essentially the same received power level. Thismeans that the station does not have to adjust its gain control up ordown in wide swings that will cause dropped connections.
The challenge created by running a typical power amplifier(PA) at lower power levels is inefficiency. At +28dBm, atraditional amplifier has efficiency of about 40 percent for CDMA and42 percent for W-CDMA. Efficiency drops off precipitously at +16dBm toonly 9 percent. At lower transmit levels, the PA consumes relativelyhigh amounts of current for the power delivered to the antenna.Quiescent current in the standby mode is 50mA, which limits standbytime.
The traditional approach to power control in a power ampli- fier isto use two states, switching the quiescent current (Icq ) toboth amplifier stages between two levels. The threshold between highand low currents is set at 16dBm.
Because this is the most prevalent method for lowering powerconsumption, it can be considered the baseline against which otherapproaches are compared. The next step in lowering power consumption isto use an external DC/DC converter to switch the Vcc between twolevels.
While this approach lowers the average current consumption, it doesso by adding external components that consume valuable board space andraise costs. Since a clear driver in handset design is to save spaceand reduce costs wherever possible, adding an external DC/DC converterruns counter to the trend. A better solution relies on obtaininggreater efficiency from the power amplifier itself.
PAs based on HBT/pHEMT processes can more than double efficiency at16dBm, from 9 percent to 22 percent, and reduce quiescent currentthreefold, from 50mA to 15mA. Based on typical power distributions ofreal-world handset requirements, the increased efficiencies and lowerquiescent currents cut PA power consumption in half.
In operation, this power-efficient amplifier uses bias-modeswitching to select the amplifier chain and to increase efficiency atlower levels. Above +16dBm, Vmode has 0V applied to select thehigh-power mode. Below +16dBm, Vmode switches to +2.85V to allowhigh-efficiency low-power operation.
At low power, below -10dBm, quiescent currents are reducedthreefold. Average power consumption is also reduced by 50percent—surpassing the reduction of using an external DC/DC converter,but without the additional expense or real estate. It is this reductionin current that raises the efficiency of the PA (See Figure 2, below ).
|Figure2. Power-efficient technology increases efficiency at lower powerlevels without increasing current.|
By adding a three-level approach to operation, PAs reduce quiescentcurrents further to 7mA. Savings in current consumption are up to 75percent.
Figure 3 below compares thetraditional PA with pHEMT/HBT two-level and three-level PAs. It showsthat while the DC/DC converter saves additional power, the additiveeffect is negligible and will not justify the expense in mostapplications.
|Figure3: An internally power-efficient PA can significantly reducecurrent consumption without external components.|
Slashing power usage
The savings offered by PAs are significant, giving a designer theability to extend battery life. Consider typical power-consumptioncharacteristics of a phone in an urban setting. The receive andbaseband sections consume 125mA. The transmit circuit power consumptionwill vary depending on the PA used. The rest of the transmit circuithas the same power consumption.
Power consumption in the transmit circuit includes not only the PA,but other components, such as the power amplifier driver. Here arethree cases for current draw in the talk mode:
Phone withtwo-state PA
PA current draw = 70mA or 29 percent of talk mode current
116mA (Tx) + 125mA (BB+Rx) = 241mA
Phone withtwo-level pHEMT/HBT PA
PA current draw = 34mA or 17 percent of talk mode current
81mA (Tx) + 125mA (BB+Rx) = 206mA
17 percent increase in talk time
Phone withthree-level pHEMT/ HBT PA
PA current draw = 18mA or 9 percent of talk mode current
68mA (Tx) + 125mA (BB+Rx) = 193mA
25 percent increase in talk time
The 70 percent and 86 percent reductions in quiescent current overtraditional two-state PAs also represent significant savings of powerin the standby mode. Monolithic pHEMT/HBT ICs represent a significantstep in advancing the potential for GaAs capabilities by maximizing theadvantages of both bipolar and FET circuits.
The isolation of the two approaches allows each to be optimized andthus reduce possible drawbacks to integration. In addition, because thetechnology uses proven fabrication processes, high yields, ruggedreliability and low costs remove the barriers that have previouslyprevented widespread use of pHEMT/HBT devices. This will result inbetter TV and WLAN connectivity, enhanced cellular capabilities andlonger battery life.
To download a PDF version of this story, go to AdvancingGaAs potentialvia pHEMT/HBT mix.