# Advances in High-Speed, High-Power Photodiodes: From Fundamentals to Applications
## 2 Overall Design Considerations for Photodiodes  
### Principles of PIN-PD and UTC-PD  
The band diagrams for PIN-PD [42; 43] and UTC-PD [5; 31; 32; 44; 45] are shown in Figure 2. PIN-PD has a simple three-layer structure, i.e., wide-bandgap P- and N-layers, and a depleted absorber. Both holes and electrons in the depleted absorber of PIN-PD contribute to the response, but the output response mainly depends on the low-velocity carrier holes, whose transport velocity is an order of magnitude lower than that of electrons, which limits the photoresponse to some extent. The UTC-PD band structure, by dividing the PIN-PD absorber into two layers, is composed of a p-type neutral absorber and a wide-bandgap (depleted) carrier collector [31; 32]. By employing a bandgap grading or/and doping grading in the absorber, the quasi-field is formed to reduce the electron traveling time effectively. In the meantime, the photogenerated majority holes in the InGaAs absorber respond quickly within several picoseconds due to the high concentration of electrons, leading to a negligible effect on the photoresponse; therefore, the high-velocity electrons dominate the photoresponse of UTC-PDs. Furthermore, the velocity of electrons (\(3\sim 5\times 10^{7}\) cm/s) is usually 6-10 bigger than that of holes (\(5\times 10^{6}\) cm/s) [5], together with the quasi-ballistic transport (overshoot velocity) [46] in the InP collector, which leads to a quite short total delay time in the whole UTC-PD. For PIN-PD and UTC-PD with the same absorber (W\({}_{\rm a}\)), the UTC-PD (W\({}_{\rm a}\cong\) W\({}_{\rm c}\)) represents a superior frequency response with a shorter traveling time in the collector due to the large difference in carrier velocities, though the carriers in UTC-PD travels \(\sim\)three times the average distance than that in PIN-PD [44]. Figure 2: Band diagrams of (**a**) PIN–PD and (**b**) UTC–PD. During high optical input, the charge intensity increases together with the increasing photocurrent in the depletion, producing a space charge effect due to the charge accumulation, while the field and potential profiles are modulated resulting in output saturation and a nonlinear response. For PIN-PD, as shown in Figure 2a, the charge storage happens at high injection conditions, resulting in the band bending and field screening, which finally causes output current saturation. However, in UTC-PD, shown in Figure 2b, only electrons participate in the space charge effect, whose velocity overshoot, even facing the decreased electrical field. Finally, the saturation occurs because of the current blocking produced from the heterojunction interface between the absorber and collector; while this state will be slower than that in PIN-PD, it is because UTC-PD can withstand a charge density an order of magnitude higher than PIN-PD [5, 44]. In addition, the space charge in UTC-PD can not only be compensated by doping the spacer layer at the absorber/collector (InGaAs/InP) heterojunction interface but also can be reduced by employing a heavily n-doped InP charge (cliff) layer to increase the electrical field of the spacer layer [47]. Therefore, UTC-PD is a typical representative of high-speed response and high-output current. In a typical UTC-PD structure, the bandgap discontinuity (the abrupt conduction band barrier) at the InGaAs absorber and InP collector interface causes the current blocking effect, thus leading to the device output current saturation. To smooth this discontinuity and suppress the blocking effect, the compositional graded quaternary system material In\({}_{1-x}\)Ga\({}_{x}\)As\({}_{y}\)P\({}_{1-y}\) (for spacer layers) is usually employed to improve device performance [48]. Therefore, the design of In\({}_{1-x}\)Ga\({}_{x}\)As\({}_{y}\)P\({}_{1-y}\) lattice-matched to InP [49] must be considered, while the cutoff wavelength-related refractive index and permittivity is shown in Figure 3a.