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This paper describes a newly developed Si/GaAs hybrid multiband power amplifier module (MB-PAM) that supports major quad wideband code division multiple access (WCDMA) bands (Bands 5, 8, 2, and 1) for handset applications. With four dies (two GaAs-HBT dies, one GaAs-HEMT die, and one CMOS die) and several surface-mount devices, the hybrid MB-PAM accommodates two amplifier chains and two single-pole double-throw HEMT band-select switches on a 5 mm\,\times\, 5.5 mm laminate, covering 824–915 and 1850–1980 MHz. Each amplifier chain has two switchable signal paths corresponding to dual (high and low) power modes [high-power mode (HPM) and low-power mode (LPM)] for saving battery current in practical handset use. One of the main features of this MB-PAM is the integration of driver stages, RF switches, and their bias control circuits on the CMOS die for pursuing cost reduction. Only the two output power stages are fabricated in a GaA-HBT process. Measurements conducted under the condition of a 3.4-V supply voltage and a WCDMA (Third Generation Partnership Project Release 99) modulated signal are as follows. Owing to optimized linear design and broadband output matching design, the hybrid MB-PAM achieves a power-added efficiency (PAE) as high as 39%–40% at 28 dBm of output power ( Pout) over 824–915 MHz in the HPM while maintaining a \pm \5-MHz-offset adjacent channel leakage power ratio (ACLR1) to less than -\39 dBc. In the LPM, PAE of 15% at a Pout of 17 dBm is obtained while ACLR1 of less than -\40 dBc is maintained. For 1850–1980 MHz, the MB-PAM delivers 35% of PAE with ACLR1 of less than -\40 dBc at 28 dBm of Pout in the HPM and 15% PAE at 17.5 dBm of Pout in the LPM. [ABSTRACT FROM PUBLISHER]