Measurement of Reverse Current of Avalanche Photodiode

Avalanche photodiode (APD) is a high-sensitivity, high-speed photodiode. When reverse voltage is applied, its internal gain mechanism can be activated. The gain of APD can be controlled by the amplitude of reverse bias voltage. The greater the reverse bias voltage, the higher the gain. APD works under the action of electric field strength, and the avalanche multiplication of photocurrent is similar to a chain reaction. APD is used in various applications that require high sensitivity to optical signals, such as fiber optic communication, scintillation detection, etc.

Measurements of APD generally include breakdown voltage, responsivity, and reverse bias current. The maximum rated current of a typical APD is 10-4 to 10-2A, while its dark current can be as low as 10-12 to 10-13A. The maximum reverse bias voltage varies with the material of the APD, and can reach 100V for devices made of indium gallium arsenide (InGaAs) and 500V for devices made of silicon.

Test introduction

Measuring the reverse bias current of an APD requires an instrument that can measure current over a wide range and output a scan voltage. Because of this requirement, instruments such as the Keithley Model 6487 Picoammeter Voltage Source or the Model 6430 Sub-Femtoamp Source-Meter are ideal for this type of measurement.

Figure 4-19 shows the Model 6430 Sub-Femtoamp Source-Meter connected to a photodiode. The photodiode is placed in an electrically shielded dark box. This dark box is connected to the low side of the Model 6430 Sub-Femtoamp Source-Meter in order to shield the sensitive current measurement from electrostatic interference.

Figure 4-20 shows the relationship between the current and reverse scan voltage of the InGaAs material APD measured by the 6430 digital source meter. Note that the current measurement range is very wide. As the light increases, the avalanche region becomes more obvious. The breakdown voltage will cause the current to flow freely because electron-hole pairs are formed at this time, and light is no longer required to illuminate the diode to generate current.