OPTOELECTRONICS LETTERS Vol.9 No.1, 1 January 2013 Optimal quantum well width and the effect of quantum well position on the performance of transistor lasers Md. Ahsan Habib 1 * , Subrata Das 2 , Saeed Mahmud Ullah 2 , and Shahida Rafique 2 1. Department of Electronics and Communication Engineering, University of Information Technology and Sciences, Dhaka 1212, Bangladesh 2. Department of Applied Physics, Electronics and Communication Engineering University of Dhaka, Dhaka 1000, Bangladesh (Received 10 August 2012) Tianjin University of Technology and Springer-Verlag Berlin Heidelberg 2013 C Transistor laser (TL) model based on InGaP/GaAs/InGaAs/GaAs is analyzed and presented. It is realized that quantum well (QW) with width of 10 nm may be formed for low base threshold current density J th . The emission wavelength is found to be 1.05 μm, and the indium (In) composition is 0.25 for optimal QW width. It is identified that J th decreases with the movement of QW towards the base-emitter (B-E) interface. Small signal optical response is calculated, and the effect of QW position is studied. The bandwidth is enhanced due to the movement of the QW towards the emitter base junction. Document code: A Article ID: 1673-1905(2013)01-0018-3 DOI 10.1007/s11801-013-2313-2 Transistor has contributed to the development of the elec- tronic integrated circuits industry, but researchers have al- ways been looking for a device which can simultaneously handle electrical and optical outputs. Transistor laser (TL) is a unique three-terminal optoelectronic component, which can generate both electrical and optical signals at the same time at possibly 100 Gbit/s with potential for combining electri- cal and optical integration [1-6] . Theoretical modeling and analysis of the TL characteristics have been done [7-11] with 10 nm-wide QW. In those works of TL, no analysis was made for such a selection of QW width. In this paper, the QW well width is optimized for low threshold current density, and the effects of optimal position of QW in the base region on thresh- old current density and optical response of InGaP/GaAs/ InGaAs(QW)/GaAs heterojunction bipolar transistor laser (HBTL) are studied. The schematic diagram of the TL structure is shown in Fig.1. The TL structure and the fabrication technique have been previously described in details [1, 12] . The results of the current work are presented. For our analysis, we assume the width of the base region is 100 nm [7] . Differential gain is calculated with following equation [13] Fig.1 Schematic diagram of transistor laser model * E-mail: ahsantonmoy2007@yahoo.com tr g n v n g v , (1) where is the optical confinement factor, n g / is the differential gain factor, is photon absorption coefficient in the active region, and v g is the group velocity. We assume that optical confinement factor is given by [14] qw w N , (2) where N is the number of QWs, and is the optical confine- ment per unit width of the QW. The typical value for of 2 10 -3 nm -1[14] is used. Using above equations and from Ref.[7], the threshold current density J th can be obtained as rbo cap qw p g nom qw th 1 1 v v n w N q J , (3)