FABRICATION AND CHARACTERISATION OF HETEROJUNCTION BIPOLAR TRANSISTORS FOR HIGH CURRENT DRIVE CAPABILITY

Abstract

In this work, the simulation, fabrication and DC characterisation of InP /In0.53Ga0.47As/InP double heterojunction bipolar transistors (DHBTs) with compositionally step-graded collector structure have been accomplished. In addition, an investigation of (Pd, Ti, Au)-based ohmic contact systems, namely Ti/Pd/ Au and Pd/Ti/Pd/ Au, to n- and p-doped In0.53Ga0.47As has been performed. The energy balance model provided in MEDICI may be inadequate to represent the nonstationary carrier transport phenomenon present in high speed submicron HBTs. A fabrication process for InP /In0.53Ga0.47As As/InP DHBTs has been developed and the DC characterisation results have shown that surface passivation with polyimide leads to a reduction in the base current. The high collector leakage current and low breakdown voltage observed could result from the presence of heterointerface defects at the base-collector junction and poor dopant confinements within the collector region, respectively. (Pd, Ti, Au)-based contacts has demonstrated the feasibility of replacing (Pt, Ti, Au)-based contacts to n- and p In0.53Ga0.47As. A thin contacting Pd layer, optimally 100A, is crucial to the formation of a low resistance contact to p+ - In0.53Ga0.47As with Pd(100A)/Ti(400A)/Pd(400A)/ Au(2000A) exhibiting a specific contact resistances (Pc) of 7.59x10-7?-cm2• On the contrary, a Ti contacting layer is desirable in contacts to n+- In0.53Ga0.47As with Ti(200A)/ Pd(200A)/ Au(2000A) demonstrating a pc of 2.54x10-7?cm2. Pd(100A)/Ti(400A)/ Pd(400A)/ Au(2000A) emerged as the more metallurgically stable contact and may be used for contacting the thin base region of a HBT. It can also serve as a common contact to both n- and p- In0.53Ga0.47As. In terms of thermal stability, Pd(100A)/Ti(200A)/ Pd(500A)/ Au(2000A) demonstrated the longest projected mean time to 50% increase in pc at 150°C which is of the order of 104hrs.