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This letter presents InP/GaAsSb/InGaAsSb/InP double heterojunction bipolar transistors (DHBTs) with a highly doped base contact layer. In order to reduce the base contact resistivity, a 3-nm-thick highly doped GaAsSb contact layer is inserted between the InP emitter and 17-nm-thick composition-and doping-graded InGaAsSb base. Fabricated DHBTs with a 0.25-µm emitter show a current gain of 32 and a high open-base breakdown voltage BV CEO of 5.2 V. The DHBTs also exhibit f T / f max = 513/637 GHz at a collector current density of 9.5 mA/µm 2 and V CE = 1 V. The f max is higher by 124 GHz than that for InP/InGaAsSb DHBTs without the GaAsSb contact layer. These results indicate that the use of the GaAsSb/InGaAsSb base structure is very effective in improving f max . Index Terms-GaAsSb, double heterojunction bipolar transistors (DHBTs), InP/InGaAsSb, highly doped base contact.

An ultra-high bandwidth (BW) and a low V π InP Mach-Zehnder modulator with an n-i-p-n heterostructure is proposed. The combination of the n-i-p-n heterostructure and the capacitive-loaded travelling-wave electrode provides a modulator with extremely low electrical loss. The device exhibits a 3 dB electro-optic BW of over 67 GHz and a V π of 2.0 V. A 100 Gb/s non-return-to-zero on-off keying modulation with an extinction ratio of over 10 dB is also realised.

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