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Agilent Delivers E-PHEMT PAs With Low-voltage Operation

Agilent is now shipping 2 million E-PHEMT-based power amplifier modules per month, and cell phone consumers are benefiting from the increased battery life enabled by the technology. Dan McNamara describes the development of the devices and future plans for lower-voltage operation and front-end module integration.
In February of this year, component maker Agilent Technologies Inc. shipped its 10 millionth enhancement-mode pseudomorphic high-electron-mobility transistor (E-PHEMT) power amplifier (PA) module. These GSM and CDMA devices are now shipping at a rate of approximately 2 million units per month. While consumers may not have a preference as to the technology of the PA in their mobile handsets, Agilent believes that its choice to develop E-PHEMT technology gives the handset maker a combination of excellent power-added efficiency (PAE), low-voltage operation and high reliability. This can be directly translated into benefits to the end-user, for example increasing battery life or allowing the same battery capacity to power additional handset features such as the popular integrated cameras.

The initial development of E-PHEMT technology began in the 1980s at what was then HP Labs (now Agilent Labs) as a process to manufacture ICs for digital signal processing. Later on, E-PHEMT showed the promise of being able to offer leading-edge performance, high quality and cost-competitive products for RF applications. Once the process demonstrated a high level of performance in PA modules, Agilent invested $100 million to outfit a 6 inch wafer fabrication facility dedicated to E-PHEMT wafer production at its campus in Fort Collins, CO (figure 1). Starting the fab presented many challenges, as there were very few suppliers of equipment. However, it seemed clear that having a 6 inch fab was essential in order to be competitive in the RF semiconductor - particularly PA - market in the long term. The Fort Collins fab has been online and in volume production since 2002.

High epitaxial controlE-PHEMT typically requires a higher degree of control in some aspects of epitaxy than heterojunction bipolar transistors (HBTs). This is due to the multiple thin layers needed for its high-current and low-leakage performance. Agilent has therefore significantly advanced the use of molecular-beam epitaxy reactors to control layer thickness and composition, along with other epitaxial production techniques. E-PHEMT threshold sensitivity also depends on a number of epitaxial growth parameters, and so both in situ monitoring and post-growth wafer characterization methodologies have had to be developed quickly.

Good control of the threshold (Imax) and leakage current depends on process control of the Schottky gate contact. Since an E-PHEMT is a surface-channel device, surface properties are critical, especially because III-V material has no native oxide for protection.

Surface residues not properly cleaned away change the transistor threshold and leakage. For example, too much aqueous cleaning easily erodes the surface layer, resulting in low Imax and low threshold voltage. Plasma bombardment during etching and ashing can decrease channel current and increase leakage. For small digital transistors, threshold uniformity can be achieved with some trade-off of other device parameters. For PAs, the simultaneous achievement of low leakage and high Imax in a specified threshold voltage range is essential. The etching, cleaning and protection of the III-V wafer, and the gate electrode formation sequence, constitute a proprietary process that Agilent considers to be critical to the success of its E-PHEMT technology.

One of the challenges in the development of E-PHEMT was the historically poor leakage current performance of GaAs FET devices. These often required the use of a drain switch, which increased circuit complexity and cost. Agilent s E-PHEMT PAs have a very low drain-source current (dsc) consumption of less than 10 µA at room temperature. This has been achieved with a combination of a buried gate with correct crystal orientation, reducing the second recess depth and optimizing the InGaAs channel with a selective etch process. The result is that E-PHEMT leakage is no longer defect-related, which means that leakage current is not a leading indicator of breakdown, as many incorrectly assume.

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