SILICON-NITRIDE PASSIVATED ULTRA LOW-NOISE INAIAS INGAAS HEMTS WITH N+-INGAAS/N+-INAIAS CAP LAYER

Noise characteristics of InAlAs/InGaAs HEMT's passivated by SiN are investigated to ascertain their suitability for practical applications in circuit such as MMIC's. A 0.18-mu-m-gate-length device with 125-mu-m-gate width and 8-gate fingers showed the lowest minimum noise figure of 0.43 dB at 26 GHz with an associated gain of 8.5 dB of any passivated device ever reported. This value is also comparable to the lowest reported minimum noise figure obtained by bare InAlAs/InGaAs HEMT's in spite of increased parasitic capacitances due to the SiN passivation. This excellent noise performance was achieved by employing non-alloyed ohmic contact, a T-shaped gate geometry and a multi-finger gate pattern. To reduce the contact resistance of the non-alloyed ohmic contact, a novel n+-InGaAs/n+-InAlAs cap layer was used resulting in a very low contact resistance of 0.09-OMEGA-mm and a low sheet resistance for all layers of 145-OMEGA/sq. No increase in these resistances was observed after SiN passivation, and a very low source resistance of 0.16 OMEGA-mm was obtained. An analysis of equivalent circuit parameters revealed that the T-shaped gate and multi-finger gate pattern drastically decrease gate resistance.