technology CS Mantech
insertion losses, engineers at WIN have developed a H2W process that employs the company’s latest BiHEMT structure and its HBT4 process, and uses these in combination with a signal gate recess process and an optimized pHEMT epi-structure design. Like the improved standalone HBT previously described, the new process features a thick SiN layer in the place of a thinner polyimide film, plus the opportunity to define multiple gates, which can be used for high-isolation, high-linearity switch applications.
Figure 2.WIN can increase the power gain produced by its HBT by removing the outer base metal fingers,a step that reduces base- collector capacitance and trims base mesa
area.The traditional HBT architecture is on the left; the advanced,higher-power version is on the right
gain at frequencies below 3 GHz, the spectral range used for handsets.
Better BiHEMTs
WIN’s latest BiHEMT process, which unites pHEMTs and HBTs, is designed for a new generation of PAs that have two states: A high power mode; plus a low power mode, the most common operating condition for the handset. The power-added-efficiency of the PA can be improved by optimizing performance in both modes.
The Taiwanese pure-play foundry’s two commercially released BiHEMT processes, which are known as H2W technologies, produce pHEMTs with a significantly higher insertion loss than stand-alone equivalents. This inferiority must be addressed in order to capture the market for two-power-state PAs – a high insertion loss can drag down the amplifier’s power-added-efficiency in low-power-mode operation, and in turn, prevent the H2W process from fulfilling the high-linearity demands in emerging standards, such as 4G and LTE. To trim
Characteristics for the HBT part of the BiHEMT include a typical turn-on voltage of 1.27 V, a DC current gain of 130, and breakdown voltages of 28 V and 14 V between base-and-collector, and emitter-and-collector, respectively. Meanwhile, the pHEMT has an on- resistance of 0.95 Ω.mm, 50 percent less than that produced by the previous H2W process. To evaluate RF switch performance, WIN’s engineers have constructed a full periphery design featuring a single-gate, 9 µm by 125 µm device. Insertion loss was just 0.1 dB, 0.05 dB lower than that for the original H2W process.
pHEMT’s bleak future
A rump session discussing the future of pHEMTs failed to find any silver lining within a looming black cloud. Everyone at that gatering believes that switches in handsets will be increasingly made from some form of advanced silicon, rather than GaAs, and nothing can be done to stop this. Although GaAs switches offer the best levels of performance, they are more expensive, and the current trend towards higher throw switches plays into the hands of silicon-on-insulator devices.
New opportunities for pHEMTs were considered during this discussion, but none seemed tremendously promising. For example, pHEMTs could be used to make WiFi products, but they would have to compete with silicon variants – and in battles between the compounds and silicon, the latter tends to come out on top. It’s a similar story in the automotive radar market, where pHEMTs have been used in reasonable volumes to make transceivers operating at 77 GHz, but are likely to loose market share to SiGe-based chips offering higher levels of integration.
Fortunately, handset manufacturers will not stop making models that use pHEMT switches overnight, so sales from these transistors will slowly decline over several years. In many cases, these chipmakers also have HBT processes, so hopefully any revenue losses incurred can be compensated by growth in power amplifier sales. Over the next few years, growing sales of products made with this class of transistor seem a sure bet.
© 2012 Angel Business Communications. Permission required.
July 2012
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