Fujitsu has developed a `GaAs on Si' phase locked loop (PLL) frequency synthesiser IC using chip-on-chip technology [24]. The device incorporates a GaAs prescaler as a daughter chip and a CMOS PLL as a mother chip, as shown in Figure 
. The power consumption is 6 mW with a supply voltage of 1 V, at an operating frequency of 800 MHz. The relevance of this example to our research project in that it resembles an option of integrating sensors implemented in GaAs technology on a silicon integrated circuit.