Fig. 1. Schematic energy band diagram of (a) the primary TL proposed by Feng and Holonyak^[3] (first structure), (b) first proposed GRIN-SCH structure, (second structure), and (c) second proposed GRIN-SCH structure (third structure) under forward bias. Slope of the band diagram in SCH1 and SCH2, which determines the magnitude of the quasi-electric field.

Fig. 2. Calculated minority electron distribution for (a) the first proposed GRIN-SCH structure and (b) the second proposed GRIN-SCH structure. (a) is in good agreement with the calculated carrier population for the GRIN-SCH QW laser^[14], and (b) also complies with the results of graded base HBTs^[4].

Fig. 3. Calculated dc gain βdc(IC/IB). The dc current gain reduces significantly, when the second structure is employed, because of the funnel form of this structure.

Fig. 4. Calculated optical output power P.

Fig. 5. Optical frequency response for the original and proposed GRIN-SCH structures. The −3dB bandwidth of the original TL (first structure) is ∼19.5GHz, and for the second and third structures they are 21 and 20 GHz, respectively.

Fig. 6. Confinement structure dependency: (a) electrical and (b) optical characteristics of the SQW TL.

Table 1. Calculated Physical Parameters for All Three Structures