# Modulation Characteristics of High-Speed Transistor Lasers
## 3 Modulation Characteristics of TL
### Intracavity Photon-Assisted Tunneling  
By applying heavy doping to the collector and bottom base materials, [PERSON] et al. reported an intracavity photon-assisted tunneling transistor laser (ICPAT-TL; hereinafter, tunnel junction transistor laser (TJ-TL)) [61]. This process is called the Franz-Keldysh intracavity photon-assisted tunneling because of the use of the Franz-Keldysh effect. The basic process of Franz-Keldysh photon-assisted tunneling involves applying a voltage at the collector-base interface to change the energy band of the transition region, which causes the top carriers of the valence band to transition to the bottom of the conduction band by absorbing the photons emitted by the QW; the excess holes diffuse to the QW valence band and promote the occurrence of radiative transitions within the QW (see Figure 10). This is a self-regulating process of reorganization and absorption, which guarantees that \(\Delta\text{I}_{\text{E}}=\Delta\text{I}_{\text{C}}\) and \(\text{I}_{\text{B}}\) is constant [62]. The concept of a TL voltage-driven switch that converts stimulated emission (highly coherent light field) to spontaneous emission (low incoherent light field) was demonstrated using the Franz-Keldysh photon-assisted process [63]. Figure 9: TI–TL band model [57]. Reproduced from Ref. [57], with the permission of AIP Publishing. Although the modulation bandwidth of TL cannot be directly improved by FK photon-assisted tunneling, it is possible to directly modulate the voltage using transition region tunneling (I\({}_{\text{TT}}\)) and strong FK transition (I\({}_{\text{RKT}}\)) coupled to the radiative transition inside the QW. FK photon-assisted tunneling enables the modulation of light output by changing the collector voltage rather than only by current modulation. Compared with increasing the bandwidth, FK photon-assisted tunneling has a greater impact on the output power. The output power of TL devices reported to date cannot meet the demands of optical fiber communication; thus, FK photon-assisted tunneling is an important research direction for improving TL power output. The reason why FK photon-assisted tunneling can regulate the optical power by changing the voltage can be determined by analyzing Figure 11a. In region (1), during the TJ-TL operation under a weak collector junction field, collector tunneling enables holes to be effectively supplied to the active QW region (i.e., I\({}_{\text{RKT}}\) > 0, photon-assisted tunneling occurs), thereby increasing the laser output light quantity. In region (2), the voltage exceeds 0.8 V, and then the output light intensity of the device weakens owing to the FK light absorption effect. Because the output light intensity of TJ-TL can be easily regulated through voltage control, TJ-TL can be directly modulated through current and voltage controls. The sensitivity of optical power to V\({}_{\text{CE}}\) changes is shown in Figure 11b [64]. It shows that as I\({}_{\text{B}}\) increases, the curve of the light intensity change \(\Delta\)L versus \(V_{CB}\) becomes steeper. This shows that TJ-TL can be used for high-speed optoelectronic devices (e.g., optical switches) and voltage modulation. Compared to current modulation, voltage modulation is easier to implement in integrated circuits. Therefore, TJ-TL should be considered in future modulation schemes for optical fiber transmission. As reported by [PERSON] et al. [65], the bandwidth of the current modulation is higher than that of the voltage modulation. However, the peak frequency amplitude of the voltage modulation is flatter due to the sloping distribution of the base carrier density and the collector voltage-controlled photon absorption (i.e., FK tunneling), which eliminates charge accumulation. It follows that the voltage modulation has a wider adjustable frequency range and lower error rate [65]. Figure 10: TJ–TL energy band and intracavity photon-assisted tunneling diagrams [61]. Reproduced from Ref. [61], with the permission of AIP Publishing.