# Modulation Characteristics of High-Speed Transistor Lasers
## 3 Modulation Characteristics of TL
### Optimization of Each Layer in the Base Region
#### 3.2.2 Increasing the Potential of the SCH Layer Facilitates Carrier Transport  
To further enhance the transport rate of carriers in the base region, changing the material mole fraction of the base region to reach a gradient that tilts the energy band can provide additional potential energy or an electric field that assists the carrier transport, which forms the SCH layer. In 2016, [PERSON] et al. designed a GaAs based single QW-SCH structure [51]. The device changes the mole fraction of indium in the upper and lower base regions according to the gradient; the material of the top base region is In\({}_{\xi}\)Ga\({}_{1.\xi}\)As (\(\hat{\varepsilon}\); \(\cdot\)0\(\rightarrow\)0.029); the material of the bottom base region is In\({}_{\xi}\)Ga\({}_{1.\xi}\)As (\(\hat{\varepsilon}\); 0.038\(\rightarrow\)0.05). Figure 8a is the original TL band structure. The rate of carrier movement from the emitter to the QW is accelerated owing to the band tilt, which reduces the recombination lifetime and base transit time of the carrier. This was the first application of the gradient-doped SCH structure to TL. The simulation results showed that the SCH layer can effectively reduce the base transit time due to the additional drift electric field and can increase the theoretical bandwidth of the TL from 69 GHz to 71 GHz. Figure 8: (**a**) First TL band structure proposed by [PERSON] et al., i.e., Structure (1) [52]. (**b**) First introduced GRIN-SCH structure, i.e., Structure (2) [52]. (**c**) Second introduced GRIN-SCH structure, i.e., Structure (3) [52]. (**d**) Frequency response curve of TL corresponding to Structure (1), (2), and (3) [52]. (**e**) Optical bandwidth and output power of each structure [52]. In 2017, [PERSON] et al. introduced the graded index (GRIN) SCH into TL; the band structure is shown in Figure 8b [52]. Compared with structure (3), structure (2) optimizes the mole fraction of the bottom base material (i.e., the mole fraction of indium changes from 0.02\(\rightarrow\)0 to 0\(\rightarrow\)0.02) and the mole fraction of the top base material (i.e., the mole fraction of indium changes from 0.07\(\rightarrow\)0.02 to 0.05\(\rightarrow\)0), which effectively improves the carrier capture ability of the QW. Considering the drift charge of the SCH layer, the simulated frequency response curve after modifying the charge continuity equation is shown in Figure 8d. Due to the change in the SCH2 energy band, the bandwidth of structure (2) is increased by 1.5 GHz compared to structure (3). As shown in Figure 8e, structure (2) also has a higher output power because the band structure of SCH2 provides an additional negative electric field for the carriers heading to the base region. Subsequently, this effectively improves the carrier confinement capability of the QW [53]. It was also reported that the threshold current of structure (2) is 6.5 mA, which is lower than that of structures (1) and (3). The report also stated that the current gain of Structure (3) is higher than that of Structure (1), which indicates that more carriers enter the collector rather than the QW. However, Structure (2) has the lowest current gain because the \"funnel-shaped\" energy band of Structure (2) suppresses carrier injection into the collector. The maximum output light intensity of Structure (2) in Figure 8e can also be explained by this point. This \"funnel-shaped\" energy band allows the carriers to be more strongly confined in the QW, where the carrier distribution is \"peaked\". Although the current gain of Structure (3) is higher than that of Structure (1), more carriers enter the QW under the action of the base electric field of the GRIN-SCH layer. Consequently, the current gain and optical gain of Structure (3) are both higher than those of Structure (1). Further, the threshold current of Structure (3) is lower. Because Structure (3) has more carriers drifting into the collector, its threshold current is higher than that of Structure (2). During their study of Structures (1) and (3), [PERSON] et al.