# SPADs vs PMTs Single-photon avalance detectors (SPAD) are the evolution of the photo-multiplier vacuum tube (PMT) technology. The combination of multiple SPAD elements can ensure equal or even better timing and sensitivity performance compared vacuum-tube photomultipliers but they still perform low in acceptance area, linearity and dark counts. ## Working principles of PMTs The photomultiplier tubes are vacuum tube detectors capable of detecting light in the ultraviolet to near-infrared range of the electromagnetic spectrum. The photomultipliers consist of a photocathode, a glass housing which acts as a vacuum tube, several dynodes and an anode. When a photon strikes the photocathode material, electrons are ejected due to the photoelectric effect. These electrons then collide to a series of dynodes which multiply them to a level (~10^9 electrons) where a detectable signal can be retrieved from the anode. The dynodes need to be biased to a voltage of ~1-2kV. ## Working principles of SPADs The SPAD is a semiconductor device capable of detecting from ionizing radiation to infrared rays in the electromagnetic spectrum. It operates in inverse bias mode, above the breakdown voltage and exploits the photon-triggered avalance current to detect an incident radiation. The SPADs can be divided in two main categories. The thin-junction SPAD which has a breakdown voltage from 10 to 50V, an active area from 50 to 150μm and a photon efficiency of up to 45% on the visible range. The thick-junction SPAD which has a breakdown voltage from 200 to 500V, a wider active area of 100 to 500μm and efficiency of over 50% on the visible spectrum. ## Comparison of the two technologies SPADs have higher quantum efficiency and a wider operating range in the electromagnetic spectrum. The operational voltage as well as the power consumption is lower than the one of PMTs. The nature of the PMTs also brings some disadvantages, such as the fragility of the vacuum-tube, the reliability of the dynode chain, the vacuum itself and also the limited mass manufacturing capabilty compared to the SPADs sensors. The SPAD stays in an off state where I=0 until a photoelectron detected by the absorption of photon creates a first charge carrier in the active depletion layer. Then an avalance current is created and reaches a value which is limited by an external bias RC circuit. The time needed for the voltage to return to the detectable state is defined by the RC constant. During this time the SPAD is unable to detect any other photoelectron in contrast to the PMT which has no dead time. This also results to a photon-at-a-time response of the SPAD sensor compared to the PMT where the detected pulse is proportional to the single electron response, allowing the detection of N photoelectrons in a single burst of events. Even though the SPADs have a very small active area compared to PMTs and are unable to detect multiple photons in a single event they can be used in an x,y array formation (for example multiple 50 χ 50 μm^2 invidual SPADs or pixels). Due to the stochastic nature of light, each photon hits a different pixel of the array making it possible to balance out the dead time of each individual SPAD and ensure multiple photon detection capability. **Sources:** [1] Photon Counting https://www.rp-photonics.com/photon_counting.html [2] Single-Photon Detectors: From Traditional PMT to Solid-State SPAD-Based Technology https://ieeexplore.ieee.org/document/6882126 [3] Single Photon Counting – A Comparison between Two Types of Technology https://www.lasercomponents.com/us/news/single-photon-counting-a-comparison-between-two-types-of-technology-1/