# The Quanta Image Sensor: Every Photon Counts
## 5 Jot Device
### Photoelectron Counting Capability  
Both the PG and TPG jots are demonstrated to have deep sub-electron read noise, and the PCH-VPM method was used to characterize their photoelectron counting capability [17; 18]. The jots in each 32 \(\times\) 32 array were readout by single CDS under room temperature (RT). TPG jots have an average read noise of 0.29 e\(-\) r.m.s., or 129 \(\upmu\)V r.m.s. voltage noise, and a \"golden\" TPG jot achieved 0.22 e\(-\) r.m.s. read noise. The PCH of the \"golden\" TPG jot is depicted in Figure 12a. It was the first time that a CIS pixel without avalanche gain achieved deep sub-electron read noise and photoelectron counting capability. PG jots have an average read noise of 0.38 e\(-\) r.m.s., or 95.3 \(\upmu\)V r.m.s. voltage noise. Shared readout PG jots have an average read noise of 0.48 e\(-\) r.m.s., or 110 \(\upmu\)V r.m.s. voltage noise [20]. The PCHs of these jots are also shown in Figure 12b,c.  
A more straightforward method was also used to illustrate the photoelectron counting capability of TPG jot. The TPG jot was kept in an integration state under a low illumination and the FD voltage was read out continuously. The quantized voltage steps generated by photoelectrons (and possibly by some thermally generated electrons) can be clearly seen in Figure 13, in which the FD voltage (\(y\)-axis)  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Quantity** & **TPG Jot** & **Non-Shared PG Jot** & **Shared PG Jot** \\ \hline CG & 410 \(\upmu\)V/e\(-\) & 250 \(\upmu\)V/e\(-\) & 230 \(\upmu\)V/e\(-\) \\ Read Noise & 0.29 e\(-\) r.m.s. (129 \(\upmu\)V r.m.s.) & 0.38 e\(-\) r.m.s. (95.3 \(\upmu\)V r.m.s.) & 0.48 e\(-\) r.m.s. (110 \(\upmu\)V r.m.s.) \\ SF Size & 0.2 \(\times\) 0.2 \(\upmu\)m\({}^{2}\) & 0.2 \(\times\) 0.4 \(\upmu\)m\({}^{2}\) & 0.2 \(\times\) 0.4 \(\upmu\)m\({}^{2}\) \\ Dark Current @ RT & 0.09 e\(-\)/s (0.73 pA/cm\({}^{2}\)) & 0.12 e\(-\)/s (0.98 pA/cm\({}^{2}\)) & Not measured \\ Dark Current @ 60 \({}^{\circ}\)C & 1.29 e\(-\)/s (10.5 pA/cm\({}^{2}\)) & 1.26 e\(-\)/s (10.2 pA/cm\({}^{2}\)) & 0.71 e\(-\)/s (11.4 pA/cm\({}^{2}\)) \\ Lag @ RT & \textless{}0.1 e\(-\) & \textless{}0.1 e\(-\) & \textless{}0.12 e\(-\) \\ \hline \hline \end{tabular}
\end{table}
Table 1: Summary of characterization results of PG jot devices.  
Figure 11: TCAD 3D model of a 4-way shared readout jot, from [20].  
is normalized by CG. This is a very basic electrical engineering demonstration of putting one electron on a capacitor and seeing a step in the voltage, but we have not found many prior examples of such an elementary measurement in the literature. It is possible the unfiltered noise in Figure 13 is related to RTS but detailed exploration of this noise has not yet been performed.  
Figure 12: **(a)** PCH of a “golden” TPG jet with 0.22 e\(-\) r.m.s. read noise for a quanta exposure of 9. Presented in [18]; **(b)** PCH of a PG jet with 0.32 e\(-\) r.m.s. for a quanta exposure of 6.5. Presented in [17]; **(c)** PCH of a shared readout PG jet with 0.42 e\(-\) r.m.s. read noise for a quanta exposure of 2.4. Presented in [20].  
Figure 13: Illustration of photoelectron counting. The signal is the continuously sampled FD voltage from a TPG jet (with 0.28 e\(-\) r.m.s. read noise when operated in a CDS mode.) The FD voltage was changed by photoelectrons from SW (and possibly dark generated electrons.) Each single electron generates a fixed voltage jump on FD, and with deep sub-electron read noise, the electron quantization effect is visible.