| dc.description.abstract | This dissertation describes the material growth and characterization as well as the device design, fabrication and characterization of InP-based heterojunction bipolar transistors (HBTs) with an InGaAsSb base layer. The use of InGaAsSb as a base layer in HBTs represents an entirely new technological approach as compared to the use of the conventional InP/InGaAs SHBT. Lower turn-on voltage and higher current gain, beta can be achieved with the InGaAsSb–base HBTs due to the reduction of conduction band offset and the increase of valance band offset at the InP/InGaAsSb emitter/base (E/B) junction. Moreover, the InGaAsSb/InGaAs base/collector (B/C) junction constitutes a type-II conduction band lineup. As a result of the postponed Kirk effect and the minimized current blocking effect, the InGaAsSb-base HBT is also superior to the homojunction base/collector or type-I double heterojuction bipolar transistors (DHBTs) when operated at high current densities.
In this study, we have investigated and examined the effects of In and Sb composition, V/III ratio, and growth temperature on the hole mobility and concentration in InGaAsSb as well as device performance. It is found that the Sb composition in the InGaAsSb base layer has a significant impact on the VBE turn–on voltage, the junction ideality and the dc performance of the DHBTs. A record low VBE turn-on voltage of 0.32 V at 1 A/cm2 is demonstrated in this work. Analysis based on a small signal equivalent circuit model shows that the InGaAsSb-base DHBT has a higher average velocity in the collector region and a higher cut-off frequency, resulting from the type-II InGaAsSb/InGaAs B/C junction.
Current gain over sheet resistance, (beta/Rsh), is a figure of merit to examine the dc performance of an HBT. For a higher current gain,beta, InGaAsSb-base DHBTs with an InAlAs emitter layer are grown and characterized. The InAlAs/InGaAsSb E/B junction has a higher conduction band offset and provides higher initial injection energy for the electrons than the InP/InGaAsSb E/B junction, thus making it possible to reduce the base transient time of the DHBTs. Besides, the growth conditions of the InGaAsSb base layer are modified in order to achieve a high current gain and a low sheet resistance simultaneously. A current gain of 52 and a RSH of 778 Ohm/sq are achieved in the InAlAs/InGaAsSb/InGaAs DHBT with a 44 nm-thick In0.24Ga0.76As0.61Sb0.39 base.
Overall, we demonstrate that DHBTs with an InGaAsSb base, which exhibit low VBE turn–on voltage, high beta/Rsh, high current density operation and high cut-off frequency, are highly promising for low-power consumption THz circuits. | en_US |