In general, the operation regions of photo detectors can be divided into three modes, including linear integration photo diodes (PD), linear-mode avalanche photo diodes (APD) and Geiger-mode avalanche photo diodes (APD) or namely single photon avalanche photo diodes (SPAD).
Please refer to FIG. 1, which illustrates a bias operating region and an optical gain of varied photoelectric detectors. As the PD is operated at a low reverse bias region, the optical gain of the PD is not high and each photon induces at most one electron-hole pair.
The PD operated just below its breakdown voltage is known as a linear-mode APD. A working voltage (also known as a bias voltage) of the APD is near and larger than a breakdown voltage level which is −Vbd. That is to say, the absolute value of the bias voltage is smaller than the absolute value of the breakdown voltage level −Vbd.
The SPAD is operated at a Geiger mode. A bias voltage of the SPAD is smaller than a breakdown voltage level −Vbd. That is to say, the absolute value of the bias voltage is larger than the absolute value of the breakdown voltage level −Vbd. For example, the bias voltage is −(Vbd+Ve), Ve is an excess bias level, and Ve is positive. Under a high electric field, an optical gain of the SPAD whose order of magnitude is as high as 106 is very good in its sensitivity.
Please refer to FIG. 2, which illustrates a relationship between a load current Iload and the excess bias level Ve of the SPAD with the charge pump as the power supply. The SPAD and the load are cascoded between a supply voltage Vop and a ground voltage, and assume a temperature is kept at a particular value. As known from FIG. 2, when the load current Iload is quiescent, the excess bias level Ve is at a maximum; when the load current Iload is increased, the excess bias level Ve is decreased with the supply voltage Vop. Furthermore, the excess bias level Ve will affect the sensitivity of the SPAD. As the load current Iload is increased, the excess bias level Ve will be decreased, then desensitizing the SPAD.
Please refer to FIG. 3, which illustrates a relationship between the temperature T and the excess bias level Ve of the SPAD with the charge pump as the power supply. The SPAD and the load are cascoded between the supply voltage Vop and the ground voltage, and assume the load current Iload is kept at a particular value. As known from FIG. 3, as the temperature T is increased, the breakdown voltage level Vbd will be raised, thus decreasing the excess bias level Ve. The reduced excess bias level Ve will deteriorate the sensitivity of the SPAD. When the temperature T is increased, the excess bias level Ve will be decreased, then desensitizing the SPAD.
Based on the above, it is known that the breakdown voltage level Vbd and the supply voltage Vop may drift upon the environment. Therefore, it's very important to stabilize the bias voltage of the SPAD and keep a constant excess bias level Ve against process, voltage, and temperature (PVT) variations.
Please refer to FIG. 4, which illustrates a bias adjusting circuit of an ordinary SPAD. The bias adjusting circuit adjusts the bias of the operating diode 54 according to the dark count rates (DCR) of a reference diode 26, such that the excess bias is kept at a fixed value.
As shown in FIG. 4, the reference diode 26 shielded by a light opaque housing 36 is electrically connected to an active quenching circuit (AQR) 44 and a reference voltage Vref. Further, a gate counter 46 can count the dark count rate (DCR) in a predefined time period. Then, the gate counter 46 outputs a first digital word to a controller 48 according to the DCR. The controller 48 outputs a second digital word to a digital-to-analog converter (DAC) 50 according to a lookup table. The DAC 50 controls a variable voltage source 52 to output a bias voltage Vbias to the operating diode 54.
Based on the above, the bias adjusting circuit of the ordinary SPAD estimates the level of the breakdown voltage according to the DCR of the reference diode 26, and then adjusts the bias of the operating diode 54. In the bias adjusting circuit of the ordinary SPAD, the reference diode 26 is independent from the operating diode and shielded by a light opaque housing 36.
Please refer to FIG. 5, which illustrates a temperature compensated and control circuit of an ordinary SPAD. The temperature compensated and control circuit measures a breakdown voltage of the reference diode 58 and adjusts the bias voltage of another operating diode.
As shown in FIG. 5, the reference diode 58 is electrically connected to a recharging circuit 60. An analog-to-digital converter (A/D) 62 measures the breakdown voltage of the reference diode 58 and outputs a first digital word to a bias control circuit 64. The bias control circuit 64 outputs a second digital word to a DAC 66. The DAC 66 can control a variable voltage source 68 to output the bias voltage Vbias to the operating diode 70.
Based on the above, the temperature compensated and control circuit of the ordinary SPAD adjusts the bias of the operating diode 70 according to the level of the breakdown voltage of the reference diode 58. In the temperature compensated and control circuit of the ordinary SPAD, the reference diode 58 is independent from the operating diode and shielded.
Please refer to FIG. 6, which illustrates a temperature and load compensated method performed in a SPAD. The temperature and load compensated method adjusts an output voltage of a digital charge pump (DCP) 80 to control the excess bias of the SPAD according to the DCR and the pulse width of the reference diodes.
As shown in FIG. 6, the temperature and load compensated circuit includes the digital charge pump (DCP) 80, an array emulator 82, an environment monitor 84, a FPGA and host 86, and digital control oscillator (DCO) 88.
Since the DCR and the pulse width are related to the variations of the temperature and the excess bias, the FPGA and host 86 receives the signal generated by the environment monitor 84, calculates the DCR and the pulse width, and controls the DCO 88 and the DCP 80 to output the supply voltage Vop for adjusting the excess bias of the reference diodes 89a to 89c. 
Based on the above, the temperature and load compensated method of the SPAD adjusts the supply voltage Vop and accordingly the excess bias according to the variations of the DCR and the pulse width. To compensate the temperature and load variations of the ordinary SPAD, the reference SPADs 89a to 89c are independent from the operating diodes and shielded.
Please refer to FIGS. 7A and 7B, which illustrate an ordinary high dynamic photo detector and an operating method thereof. As shown in FIG. 7A, the high dynamic photo detector includes a PIN diode 112, a SPAD 116, a sensing transistor 114, a reading transistor 118 and a reset transistor 120. The PIN diode 112 is operated at the linear integration mode, and the SPAD 116 is operated at the Geiger mode. The high dynamic photo detector switches the operation between the linear mode and the Geiger mode according to a light flux.
As shown in FIG. 7B, in step 180, the high dynamic photo detector detects the light condition 180. In step 188, when the brightness is larger than or equal to the threshold value, then it is operated at the linear mode. In step 184, when the brightness is not larger than or equal to the threshold value, then it is operated at the Geiger mode. In the step 186, if the output is not saturated, it is kept at the Geiger mode. In the step 186, if the output is saturated, it is switched to the linear mode (step 188). In step 190, at the linear mode 188, if the noise is less than or equal to a threshold value, then it is switched to the Geiger mode (step 184). If the noise is not less than or equal to the threshold value, then it is kept at the linear mode (step 188).
Because the high dynamic photo detector achieves the performance by switching operation between the linear mode and the Geiger mode, the signal processing circuit is much complex.
Please refer to FIG. 8, which illustrates an environment light detecting system and method. As shown in FIG. 8, a sensing element 200 includes a substrate 202, a light emitting diode 204, a SPAD array 206, a filter 208, an etalon filter 210, a lens 209, a lens 211 and a brown window 212. An infrared light can pass the brown window 212. The SPAD array 206 includes a Raw SPAD (i.e. a first unfading pixel 214), IR light passed SPAD (i.e. a second unfading pixel 216, and an opaque metal SPAD (i.e. fading pixel 218).
When the brightness of the environment is larger than a threshold value, the sensing element 200 can be performed by the fading pixel 218. When the brightness of the environment is lower than the threshold value, the sensing element 200 can be performed by the first unfading pixel 214 and the second unfading pixel 216.
The sensing element 200 needs a calibration circuit calibrating the mismatch between the fading pixel 218 and the first unfading pixel 214 (or the second unfading pixel 216) to avoid an inaccuracy of the sensing element 200.
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