e to be hermetically sealed, rather like a quartz crystal. This packaging problem would disappear if the device could be sealed at the wafer level during manufacture with additional overmoulding to provide long-term protection. Integrated MEMS devices are therefore a plausible candidate for band switching and mode switching within the next three to five years. TX/RX switching (for GSM or other time division multiplexed systems) would be more ambitious due to the duty-cycle requirements but still possible using optimised production techniques. There is also a potential power handling and temperature cycling issue. The high peak voltages implicit in the GSM TX path can lead to the dielectric breakdown of small structures, a problem that occurred with early generations of SAW filters. Because MEMS devices are mechanical, they will be inherently sensitive to temperature changes. This suggests a potential conflict between present ambitions to integrate the RF PA on to an RFIC and to integrate MEMS devices to reduce front-end component count and deliver spectrally flexible phone. The balance between these two options will be an important design consideration. The optimal trade off is very likely to be frequency specific. For example, if the design brief is to produce an ultralow-cost handset, then there are arguments in favour of integrating the RFPA on to the RFIC. However, this will make difficult to integrate MEMS components on to the same device. You can either have frequency flexibility or low cost, but not both together. Telecommunications Economies of Scale 4.5.6 Filtering Using Surface Acoustic Wave (SAW), Bulk Acoustic Wave (BAW) Devices and MEMS Resonators - Implications for Future Radio Systems These devices are covered in detail in the next chapter but are potentially key enabling technologies. It seems inevitable that the regulatory environment will require the industry to produce handsets that are capable of working across ever more numerous multiple bands and that the standards-making process will ensure