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For microwave systems, there are two types of circuit structures; monolithic microwave integrated circuits (MMIC) and microwave integrated circuits (MIC) [1,2,31. However, these structures have essential problems. The MMIC GaAs die is large because the microwave matching and ater circuits occupy large area resulting in high cost. The MIC is large because the conventional MIC consists of single-layer circuits. The multilayer microwave integratedcircuit, theMuMIC, overcomes these problems. TheMuMIC technology is realized by multilayer substrate of low CO-fied temperature ceramics. MuMIC multi-chip module (MCM) resulting in low cost, small size, and many functions for microwave applications. This technology is used for 1.9GHz digital European cordless telecommunication (DECT) power amplifier. The GaAs MMIC die of the the MuMIC can be less than 1/10 compared to the conventional one by integrating the impedance matching circuits and filters in the layered structure reducing manufaduring cost. The outline of the MuMIC also is smaller than that of the conventional MMIC package. High power-added efficiency of the microwave power amplifier is obtained by taking advantage of the lowresistance inner layer line of the MuMIC and the low on-resistance FETs.The concept of the MuMIC technology is shown in Figure 1. For microwave amplifiers applications, large inductors and capacitors are used to realize matching circuits in the MMIC. Therefore, the conventional GaAs MMIC has a large die as shown in Figure Na). The MuMIC shown in Figure 16) permits large passive circuit elements to be integrated in the multilayer structure of ceramic substrate. Matching and other passive circuits are separatedf" GaAs MMIC die. As a result, the GaAs die of the MuMIC can be reduced to 10% in comparison with the conventional MMIC. The total volume of the MuMIC is smaller than the conventional MMIC, because the MuMIC is package-less and lead-less (Figure l(b)). The MuMIC has flexibility to realize other functions by carrying other functional ICs i.e. HEMT, HBT, Bipolar IC, CMOS LSI, etc.. Another is that the MuMIC structure can integrate as multilayer circuits filters, couplers, and any other high frequency circuits that occupy large circuit area.This MuMIC technology is implemented as follows. Firstly, the conventional multilayer substrate using high-co-fired-temperature ceramics such as Alumina has inner layer lines made of highmelting-point metal such as tungsten or molybdenum. However, the high resistance of this inner layer metal should be reduced to decrease the high-frequency transmission loss or the bias voltage drop. The MuMIC ovemmes these problems by adopting the lowco-fired temperature multilayer ceramic substrate that has the low resistance metal circuit line made of copper or silver for inner layer lines. Secondly, this MuMIC achieves the good isolation characteristics by adopting grounded metal planes in the inner layers as shown in Figure lThe MuMIC technology is applied to the 1.9GHz DECT power amplifier. Figure 2 shows the equivalen...