Suppressed Self-Heating in Multi-Finger InP-Based DHBTs with Au Subcollector Fabricated on SiC Substrate by Surface-Activated Bonding

Abstract

InP-based double-heterojunction bipolar transistors (DHBTs) have demonstrated excellent high-frequency performance suitable for submillimeter-wave ICs and wideband-communication’s ICs. For these ICs, increasing the operation current density of the transistor is effective for improving the high-frequency performance. Multi-finger transistor design is very useful in reducing the IC chip size because it reduces the required number of transistors. However, it results in the increased power dissipation per unit area, leading to an increase in junction temperature and thus to the degradation of current gain of DHBTs. In order to suppress these undesirable effects, we propose the multi-finger DHBT on a highly thermal-conductive SiC substrate fabricated by surface-activated bonding (SAB). The measured thermal resistance, R th, shows the significant impact of the proposed DHBT structure on suppressing device self-heating thanks to the highly thermal-conductive SiC substrate and a simultaneously introduced highly thermal-conductive Au subcollector. Figure 1 shows a cross-sectional view of a fabricated multi-finger DHBT. The DHBT structure is formed on a SiC substrate (thermal conductivity κ = 490 W/m/K) through an Au subcollector (κ = 320 W/m/K). For single-finger devices, we have demonstrated the marked improvement in heat dissipation [1]. Epitaxial layer structures are designed for high-current-density operation (> 10 mA/μm2). The emitter is 20-nm-thick InP. The InGaAsSb base is 20-nm thick and highly C-doped at 7 x 1019 cm-3. The collector is 60-nm-thick InP. For the contact to the Au subcollector, 10-nm-thick, highly Si-doped InGaAs is employed. Emitter fingers and base electrodes are formed on one Au subcollector. We prepared twelve types of multi-finger DHBTs with combinations of four to six emitter fingers and emitter-finger spacing of 0.9, 1.4, and 3.4 μm. The device fabrication started with the epitaxial growth of DHBT layers on a 3-inch InP substrate by metal-organic chemical vapor deposition as shown in Fig. 2. After Au had been deposited as an adhesive and subcollector material both on the DHBT layers and a 3-inch SiC substrate, they were bonded by SAB. This bonding technique can prevent thermal degradation of the epitaxial layers because of its low bonding temperature (~150oC) [2]. After the removal of the InP substrate, DHBTs with emitter widths of 0.25 μm and 0.5 μm were fabricated using the self-aligned process [3]. Figure 3 shows a scanning electron microscope image of a fabricated four-finger DHBT with an emitter finger size of 0.5 μm x 4.0 μm. DHBTs were successfully fabricated on the SiC without any serious problems associated with the bonding process. Figure 4 plots the R th for the fabricated DHBTs as a function of the emitter-finger spacing and finger number. For comparison, the R th for the conventional DHBTs without an Au subcollector fabricated on an InP substrate is also plotted. Here, the R th was estimated by following Ref. [4] and normalized by the total emitter junction area. For the conventional DHBTs on the InP, decreasing the emitter-finger spacing or increasing the emitter-finger number markedly increases the R th due to increased heat density. On the other hand, such abrupt increases are not observed for the Au-subcollector DHBTs on the SiC. These results clearly show the combination of the Au subcollector and the SiC substrate efficiently transfers the generated heat from the inside of the device to the substrate. The proposed DHBTs exhibit 68% reduction in the R th, compared with the conventional ones when the emitter-finger spacing is reduced to 0.9 μm. Consequently, the multi-finger DHBTs on the SiC possibly operate even at much higher current density. Figure 5 shows collector I-V curves for the four-finger DHBT with a 0.25-μm-wide emitter. As expected, the DHBT on the SiC successfully suppresses the current-gain lowering in the high-V C and high-I C bias regions (~20 mW/μm2). In summary, we fabricated multi-finger InP DHBTs with an Au subcollector on a SiC using the SAB technique. The DHBTs show that the R th is almost independent of the emitter-finger spacing or emitter-finger number. In addition, the fabricated DHBTs exhibit a 68% reduction of R th compared with the conventional ones. The proposed multi-finger DHBT with the SAB technique is very useful in increasing the operation speed and decreasing the chip size of ICs.