Fabrication and Characterization of Submicron InGaAsSb Base Heterojunction Bipolar Transistors

• Main Authors: Sheng Yu Wang, 王聖瑜
• Other Authors: Jen-Inn Chyi
• Format: Others
• Language: en_US
• Published: 2013
• Online Access: http://ndltd.ncl.edu.tw/handle/6p5t7m

博士 === 國立中央大學 === 電機工程學系 === 101 === In order to achieve THz InGaAsSb base heterojunction bipolar transistors we have focused our efforts on three research areas over the past few years, including the growth of high quality InGaAsSb, the design of the device layer structure, and the development of process technologies for sub-micron devices. A method for growing high quality InGaAsSb material by fixing the V/III ratio and growth rate was developed first. By using this method we could control the Sb composition and make the InGaAsSb layer lattice-matched to InP substrate. Additionally, due to the special material properties of InGaAsSb, i.e., its hole mobility and carrier life time show little dependence on their base doping concentration, an InGaAsSb material with a high Sb composition (Sb>25%) and base doping concentration (NB>1×1020 cm-3) would exhibit a sheet resistance comparable to GaAsSb. Utilizing the materials prepared by the aforementioned method, we demonstrated an InP/In0.48Ga0.52As0.89Sb0.11/InGaAs DHBTs with Type I E-B and type II B-C junctions. This was the first time that an InGaAsSb base HBT demonstrated a higher collector current and current gain cut-off frequency (fT) than a conventional SHBT. We also used a Silvoco simulation tool to study the Sb composition effect on a type II B-C structure. It was found that the lowest VBE and VBC turn-on voltage and highest current gain cut-off frequency could be achieved when the Sb composition was around 25% to 30%. In order to further investigate the Sb composition effect while maintaining the benefits of type I B-E and type II B-C junctions, we designed a series of InAlAs/InGaAsSb/InGaAs HBTs. With these HBTs, we found that as the Sb composition increased, there was significant improvement in their current gain and linearity. It also resulted in low base specific contact resistivity, which led to a high maximum oscillation frequency (fmax). Through international collaboration with Professor Ran in UF (University of Florida), an InAlAs/In0.42Ga0.58As0.77Sb0.23/InGaAs HBT with an emitter size of 0.65×8.65 μm2 and base/collector thickness of 40nm and 150 nm, respectively, a fT of 260 GHz and fmax of 485 GHz were achieved. In the area of submicron-meter HBT fabrication, we first investigated emitter and base contact resistivity issues, which are key factors for submicron-meter devices. Heavily doped InAs/InGaAs emitter structures and high Sb content InGaAsSb materials were used to lower the emitter and base contact resistivity. By these methods, emitter and base contact resistivity of 3.4×10-8 Ω-cm2 and 5×10-8 Ω-cm2 were achieved. After solving the contact issue, we used e-beam lithography to define devices with emitter sizes below 300 nm. The fT and fmax of the devices were 272 GHz and 176 GHz, respectively. In addition, in order to improve the yield of self-aligned emitter mesa and base metal, a unique T-shaped emitter and benzocyclobutene (BCB) mesa sidewall technology was invented. This allowed us to avoid the ion bombardment problem during mesa sidewall fabrication. This method was even better for fabricating severely scaled self-aligned emitter HBTs. During the submicron-meter device fabrication procedure, it is found that the current gain in a HBT with an InGaAsSb base layer shows less dependence on the emitter size as compared to the InGaAs base HBTs. Moreover, as the Sb composition increases in the base layer, the emitter periphery surface recombination current density (KSURF) of an InGaAsSb base HBT becomes more and more insignificant. Additionally, the heavily doped InGaAsSb base HBT exhibits a weaker emitter size effect than a lightly doped one. This is considered a very important advantage of the InGaAsSb base HBTs. It is found that heavily doped InGaAsSb bases exhibit a long minority carrier life time, which leads to a decent current gain for an HBT. Current gains as high as 50 have been achieved with a 44nm InGaAsSb base when the base doping goes up to 1×1020 cm-3 in InGaAsSb base HBTs. The dissertation shows that the InGaAsSb base HBTs with high Sb composition and base doping concentration exhibit low turn-on voltage, high current drive capability, high fT and high fmax. Meanwhile, this kind of HBT shows very weak emitter size effects in the scaled device. According to these superior characteristics, the InGaAsSb base HBTs show great potential for achieving THz HBT.