2 Critical Photodiode Specifications

Photodiodes are one of the most common detectors for measuring a light source's optical power. A photodiode can be operated in one of two modes: photoconductive (reverse bias) or photovoltaic (zero bias). The photodiode bias condition depends upon the application's speed requirements and the amount of tolerable dark current (I_dark) on the sensor. Dark current is the current in the photodiode when there is no incident light and can be a large source of error during low photodiode current measurements.

In photoconductive mode, an external reverse bias is applied, and the measured output current is linearly proportional to the input optical power. Applying a reverse bias increases the width of the depletion junction, producing an increased responsivity and a decrease in junction capacitance, which creates a very linear response. However, operating in a reverse bias condition tends to increase the dark current. Figure 2-1 shows the dark current when the photodiode is reverse biased.

Figure 2-1 Photodiode Dark Current in Photoconductive Mode

This example shows a near-infrared (NIR) wavelength Indium Gallium Arsenide (InGaAs) photodiode for the design. In this application example, the photodiode operates in the photoconductive mode, where exposure to light causes a reverse current through the detector. A reverse bias is applied to the photosensor to reduce the junction capacitance. The reverse bias voltage (V_R) increases the depletion region width and consequently decreases the junction capacitance; therefore, increasing V_R improves the speed of response and linearity of the photodiode, with a trade-off of larger dark current.

Table 2-1 shows the NIR G8195-12 photodiode parameters.