# Integrated Silicon Photonics for Enabling Next-Generation Space Systems
## 4 Space Radiation Effects
### Integrated Germanium Photodiodes
#### 4.1.3 Photodiodes: Single-Event Effects  
SEEs are caused by a high-energy particle imparting its energy to the crystal lattice, thereby generating a high density of electron/hole pairs (EHPs). This process creates current transients that can corrupt data encoded on the optical signal. There has only been one investigation, to date, that discusses the transient effects in lateral and vertical _p-i-n_ PD, as shown in Figure 7(a) (similar to the one shown in Figure 3) and Figure 7(b), respectively [66]. A high-energy laser pulse was used to induce transients in the PDs. Figure 7(a,b) show the resulting current transients for a lateral and vertical _p-i-n_ PD, respectively, induced by a 500-pJ laser pulse. The transient increases in magnitude with a larger reverse bias, for both cases. This is because the electric field within the junction increases and is able to separate and collect more EHPs, resulting in larger current spikes.  
The transient duration, however, appears to only depend on reverse-bias voltage for the lateral device. This phenomenon is attributed to the fact that the electric field in the lateral device (depicted by the dashed arrow in Figure 7) is strongest near the buried oxide (BOX), and decreases toward the top of the Ge layer. Therefore, the carrier transit-time is non-uniform and strongly dependent on the magnitude of the reverse-bias voltage. The vertical _p-i-n_ PD, in contrast, has a much more uniform electric field throughout the Ge layer. The study also claims that the electric-field magnitude in the vertical _p-i-n_ is much higher than its horizontal counterpart, even at low voltages. Carriers in the vertical _p-i-n_ accelerate to near saturation velocity which results in a weak dependence in transient duration to reverse-bias voltage.  
Figure 6: Photodiode (**a**) Dark current, (**b**) responsivity, and (**c**) frequency response show minimal to no degradation from DD caused by neutrons (after [65]).  
Typically, a transient with a small peak and short duration is desired in order to reduce the probability of data corruption. This investigation showed that there is a trade-off between these two transient characteristics: the horizontal _p-i-n_ has a smaller peak magnitude, while the vertical _p-i-n_ has a shorter transient duration. In this case, transients need to be examined from a system perspective to determine which structure is more desirable for reducing the probability of bit errors in a heavy-ion rich environment. Parameters such as bit-rate, modulation type, and modulation depth are just a few things to consider when evaluating this trade-space.  
As a final note on this study, it is relatively unclear how transients induced by a laser pulse in a laboratory compare to transients induced by high-energy ions, as you would see in space. An important distinction is that the laser pulse generates most of the EHPs within its \(\sim\)1 um spotsize. This is much larger than a high-energy ion, which could have an EHP generation radius on the order of several tens to hundreds of nanometers. Several studies have been conducted to compare and contrast laser- and ion-induced transients in electrical devices [67; 68; 69; 70; 71; 72; 73; 74]. While there have been some successes in developing techniques to correlate and predict the corresponding transient response from a particle, it is clear that this correlation varies between different technologies, materials, and operating device physics. Therefore, the correlation between laser- and ion-induced transients in silicon photonic devices remains an area of research yet to be addressed.  
Figure 8: Captured transients from (**a**) LPIN and (**b**) VPIN photodiodes (after [66]).  
Figure 7: Cross-section view of a (**a**) lateral and (**b**) vertical germanium-integrated, _p-i-n_ photodiode. The dashed white arrow shows the electric field shape and direction (after [66]).