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The Conventional Approach… GaAs |
The conventional approach to solving the problems of transistor voltage breakdown and impedance matching and has been to use a speciality process such as Gallium Arsenide (GaAs) which has a much higher intrinsic breakdown voltage than CMOS and to use discrete high Q passive components for the match. The active (GaAs) components and the passives are then integrated into a module using a ceramic or organic substrate with multiple layers and complex metal routing. A silicon die must also be included to realize the power controller as the integration capabilities on GaAs are very limited.
This approach delivers a single package PA "module" (shown above) but there are many drawbacks to this conventional approach:
- GaAs is significantly more expensive compared to Silicon CMOS
- GaAs manufacturing capacity is very limited relative to available CMOS capacity, so security of supply and ability to second source is diminished
- Passive devices required by GaAs increase cost and magnify the supply uncertainty
- Complex multi-layer, multi-chip modules and assembly adds to cost and supply concerns
- Heat transfer from GaAs die to substrate to phone board is inefficient
- The assembled components expand at different rates with temperature giving rise to reliability concerns and the potential for field failures
- An additional non-GaAs chip is added for the PA control loop. This second die in a different technology can presents problems with tracking and tolerancing over temperature.
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