Ohmic Contacts to p-Type III-V Semiconductors for the Base of Heterojunction Bipolar Transistors

Abstract

Ohmic contacts to p-type In0.25Ga0.75Sb, InAs, and InAs0.8P0.2 have been investigated for the base of heterojunction bipolar transistors. On all these semiconductors, using Pd and/or Pt as the first layer in the metallization stack provided the lowest contact resistances. Specific contact resistances as low as 3 x 10-7 Ω-cm2 were measured for Au/W/Pd/p-InGaSb. Thin reactive layers of Pd or Pt were used beneath an unreactive W barrier to keep the contacts shallow, but contact resistance and thermal stability were sacrificed when 2 nm of Pd was used instead of 5 nm. The lowest contact resistance we measured on p-InAs was obtained with Pd/Pt/Au, but Pd/W/Au exhibited better thermal stability at 250 °C. For p-type InAsP, Pd/Ru/Au provided a specific contact resistance of 4 x 10-6 Ω-cm2 as deposited. The contact consumed only 4 {plus minus} 2 nm of InAsP after 3 days at 250 °C, and its resistance remained low.