We have developed a real time ultraviolet (UV) imaging system that can visualize both invisible UV light and a visible (VIS) background scene in an outdoor environment. As a UV/VIS image sensor, an organic photoconductive film (OPF) imager is employed. The OPF has an intrinsically higher sensitivity in the UV wavelength region than those of conventional consumer Complementary Metal Oxide Semiconductor (CMOS) image sensors (CIS) or Charge Coupled Devices (CCD). As particular examples, imaging of hydrogen flame and of corona discharge is demonstrated. UV images overlapped on background scenes are simply made by on-board background subtraction. The system is capable of imaging weaker UV signals by four orders of magnitude than that of VIS background. It is applicable not only to future hydrogen supply stations but also to other UV/VIS monitor systems requiring UV sensitivity under strong visible radiation environment such as power supply substations.

We present an analysis of carrier dynamics of the single-photon detection process, i.e., from Geiger mode pulse generation to its quenching, in a single-photon avalanche diode (SPAD). The device is modeled by a parallel circuit of a SPAD and a capacitance representing both space charge accumulation inside the SPAD and parasitic components. The carrier dynamics inside the SPAD is described by time-dependent bipolar-coupled continuity equations (BCE). Numerical solutions of BCE show that the entire process completes within a few hundreds of picoseconds. More importantly, we find that the total amount of charges stored on the series capacitance gives rise to a voltage swing of the internal bias of SPAD twice of the excess bias voltage with respect to the breakdown voltage. This, in turn, gives a design methodology to control precisely generated charges and enables one to use SPADs as conventional photodiodes (PDs) in a four transistor pixel of a complementary metal-oxide-semiconductor (CMOS) image sensor (CIS) with short exposure time and without carrier overflow. Such operation is demonstrated by experiments with a 6 µm size 400 × 400 pixels SPAD-based CIS designed with this methodology.

We have developed a direct time-of-flight (TOF) 250 m ranging Complementary Metal Oxide Semiconductor (CMOS) image sensor (CIS) based on a 688 × 384 pixels array of vertical avalanche photodiodes (VAPD). Each pixel of the CIS comprises VAPD with a standard four transistor pixel circuit equipped with an analogue capacitor to accumulate or count avalanche pulses. High power near infrared (NIR) short (<50 ns) and repetitive (6 kHz) laser pulses are illuminated through a diffusing optics. By globally gating the VAPD, each pulse is counted in the in-pixel counter enabling extraction of sub-photon level signal. Depth map imaging with a 10 cm lateral resolution is realized from 1 m to 250 m range by synthesizing subranges images of photon counts. Advantages and limitation of an in-pixel circuit are described. The developed CIS is expected to supersede insufficient resolution of the conventional light detection and ranging (LiDAR) systems and the short range of indirect CIS TOF.

A shot noise suppression process of the emission-limited or the “soft” reset in an image sensor is investigated by analytically solving a master (Kolmogorov–Bateman) equation for a continuous-time Markov chain with correlated carrier emission rates. An explicit form of the probability distribution function, the hypoexponential type, is derived. An analytical form of the soft-reset (SR) noise is derived as the second-order moment, that is, variance, of the distribution function. It describes all the main characteristics of the SR noise previously reported; time-dependent shot noise suppression approaching an asymptotic limit of the half amount of noise charges as those of the fast or “hard” reset. The noise suppression mechanism is attributed to cascaded transitions of the storage node potential on a potential ladder giving the variance composed of a uniformly decreasing convergent series. The noise reduction factor is shown to be determined by the ladder spacing of a single-carrier charging potential. From the first-order moment analysis, an average time or lifetime dependence identical to that of the deterministic diode-decay equation is derived. Thus, the SR noise can be characterized by the governing probability law and its moments.

A nonlinear LCR parallel circuit model of a single photon avalanche diode (SPAD) is derived from a Lienard-type nonlinear differential equation. The resistance and the inductance associated with avalanche multiplication (AM) are time-dependent and governed by the avalanche time constant due to the impact ionization ratio. Time dependences of current, voltage, resistance, and inductance in the model are analyzed by numerically solving the equation. During the initial generation of a Geiger-mode pulse, when the voltage reaches the breakdown voltage, the resistance diverges to limit the maximum current and the inductance reduces to give the maximum speed of the voltage variation with the avalanche time constant. In the frequency domain, avalanche impedance (AI) spectra are obtained as a ratio of voltage and current spectra calculated as Fourier transforms of the time domain signals. The AI spectra exhibit a negative-resistance and an inductance. An analytic expression for the impedance is derived and found to comprise only a quenching resistance and a stray capacitance as indicated by a constant radius of a Nyquist plot. Finally, the present model is shown to incorporate the models of impact-ionization-avalanche transit-time (IMPATT) diodes when a very small-signal limit is assumed.

Nonlinear carrier dynamics during quenching of a single photon avalanche diode (SPAD) are investigated. A Lienard type differential equation is derived from carrier continuity equations and it is solved analytically and numerically. Universal characteristics of the quenching carrier dynamics, i.e., stability, bifurcation, and quenching conditions, are analyzed on a trace-determinant plane of the Jacobian matrix and on a phase plane as functions of the quenching resistance (QR). With a finite QR or a resistive quenching (RQ), the breakdown voltage is found to be an attractor leading to a nonquenched final state. In order to produce a successful quenching, i.e., a status identified as an unstable fixed point (FP) with a zero-carrier state, two conditions are found to be necessary: 1) bias voltage dropping below breakdown voltage to ensure carrier decrease and 2) carrier extinction (CE) in this carrier decreasing period. The two conditions together lead to a threshold of QR. On the other hand, a capacitive quenching (CQ) appearing as a special case of RQ with an infinite resistance is found to show a completely different bifurcation character. CQ is derived to be equivalent to a logistic equation giving a transcritical bifurcation at the breakdown voltage and a final state identified as a stable FP with natural CE. Finally, two time constants both governed by an excess voltage are derived. In particular, one of them, a lifetime of impact ionizations, is found to be equivalent to the ``avalanche frequency'' of an impact ionization avalanche transit-time diode (IMPATT).