In this letter, compact, high order on- chip wideband bandpass filters (BPFs) are proposed in the gallium arsenide (GaAs)-based integrated passive device (IPD) technology. The design employs a stepped- impedance multimode resonator and achieves a bandwidth of more than five octaves with a high rejection to fourth/ fifth-order of the passband. A very small chip size is realized by implementing Metal-Insulator-Metal (MIM) capacitors and spiral inductors. Meanwhile, the equivalent circuit models and design equations are presented. A fabricated fourth-order prototype showed a low insertion loss of 1.2 dB, a 3-dB fractional bandwidth of 143%, and a upper stopband attenuation of more than 44 dB extending to higher than 30 GHz ( > 10~{f}_{{0}} ). The filter size is 2.16 mm times 0.90 mm, which is only about 0.02lambda _{{0}} times 0.01lambda _{{0}} at {f}_{{0}} =3 GHz. To our knowledge, this is the widest BPF with the smallest circuit size ever published.