# A Roadmap for Gigabit to Terabit Optical Wireless Communications Receivers
## 2 Materials and Methods  
### Characteristics of Commercially Available SiPMs  
SiPMs are available from AdvanSiD, Broadcomm, First Sensor, Hamamatsu, and onsemi. All of these manufacturers supply SiPMs that contain arrays of microcells, with each microcell containing an APD in a series with a resistor. These microcells are connected in parallel and biased above the breakdown voltage of the APD. This means that, if a photon initiates an avalanche event in a microcell, the resulting current causes a voltage drop across the resistor. The resulting reduction in the voltage across the APD then quenches the avalanche event. The capacitance within the microcell is then recharged until the bias voltage across the APD is restored. During this recovery time the microcells' ability to detect photons is reduced [19]. Consequently, the need to recharge the microcells creates a non-linear SiPM response [19]. An important difference when selecting SiPMs for receivers is that onsemi creates an output that is capacitively coupled to each microcell. It is this capacitive coupling that creates fast output pulses on this second output. These narrower, fast output pulses are typically an order of magnitude narrower than the output pulses of other SiPMs. Since this significantly reduces ISI at higher data rates, SiPMs from onsemi have often been used to create receivers [5; 6; 7; 10; 11; 12; 13; 14; 15; 16; 17; 18]. Onsemi produce three series of SiPMs: The C and J series are manufactured on p-on-n substrates [23] and have a peak PDE at approximately 425 nm. The difference between these two series is that the J series uses a through-silicon via (TSV) process to minimize the dead space between microcells. In addition, onsemi produce the RB series of SiPMs. Unlike the other two series, the RB SiPMs are manufactured using an n-on-p substrate [23]. This change in substrate means that the RB devices have a peak PDE at approximately 600 nm. The important parameters for the RB, C, and J series SiPMs sold by onsemi were obtained from the relevant data sheets and are listed in Table 1. In this table, the PDE at 405 nm has been listed because this is the wavelength that has been used to reduce the impact of ambient irradiance from white LEDs [12]. The data for the C and J series devices with the same area and microcell size, for example, the J30035 and C30035 devices, show that the TSV process increases SiPMs' PDE and number of microcells, as well as significantly reducing their recovery time. This means that when the transmitter operated at 405 nm the J series has been favored [5; 6; 7; 10; 11; 12; 14; 15; 17; 18]. As expected, within any series of SiPMs, reducing the microcell size increases the number of microcells per unit area. However, the data in Table 1 show that this also reduces the PDE. The higher PDE of the larger microcells is an obvious advantage; however, the need to recharge microcells after quenching creates a non-linear response. This means that, at higher photon count rates, a larger number of microcells will be more important than the PDE. The best choice of microcell size will therefore depend upon the expected count rate of photons. Most importantly, in free space, a transmitter generates particular irradiances at different relative locations [12]. The irradiance required to support a particular data rate and BER is therefore the most important performance metric for free-space receivers. For any irradiance a larger SiPM will detect more photons from the transmitter. Using a larger area should therefore improve a receiver's performance; however, the data in Table 1 show that increasing the area of a SiPM also increases its fast output pulse width. This means that smaller SiPMs are expected to be able to operate at higher data rates before their output pulses cause ISI. The trade-off in pairs of characteristics, such as the number of microcells per unit area and PDE or SiPM area and fast output pulse width, has been investigated by numerical simulations.