4.5: Photodiodes

So far we have discussed how to generate light and amplify, how to modulate the intensity of light to send information, and how to control the flow of light. The final step in the process is to detect the signal. This is done with photodiodes and can be a separate device or part of an integrated optical circuit. Since not all materials support the detection of light (passive materials), this may also lead to hybride devices, where integrated optical chips with photodiodes are glued to integrated optical devices that are entirely passive. One might choose to do this to manage losses: silicon nitride and glass are very low loss, so demultiplexing might best be performed on a chip made of silicon nitride, while detection requires an active material like silicon, or InP. Thus, the two chips must be bonded to each other.

A photodiode consists of a junction between two materials, one doped with an excess of holes (p-type materials) and one doped with an excess of electrons (n-type materials). Where the two materials join, the holes and electrons recombine to create a neutral region (called the depletion region). Like an ordinary diode, if a reverse bais voltage is applied, then electrons and holes are driven away from the junction, and no current flows. If a photon is absorbed in the depletion region, it will generate an electron and a hole (assuming the photon energy is greater than the bandgap of the material). Due to the reverse bias, the electron and hole are separated from each other, and a current flows (called a photocurrent). This is illustrated in Figure \(\PageIndex{1}\).

Figure \(\PageIndex{1}\): A photodiode. The p-doped and n-doped materials are brought together to form a junction. The excess positive and negative charges eliminate each other at the junction to create a depletion region (top diagram). When a photon is absorbed in the depletion region, the electron and hole are separated because the applied voltage drives them apart, creating a photocurrent (\(I_{ph}\)) that opposes the applied voltage.

Now, assuming that the absorption and conversion of photons to electrons has an efficiency of 100%, we can see that if the average number of photons per bit is 10, then a 10 GB/s data stream will result in an average photocurrent of 16 nA. This is certainly large enough to readily amplify with a transimpedance amplifier to give a digital level. The depletion region is, unfortunately, rather small, meaning that most of the light does not contribute to the photocurrent. Furthermore, the tiny depletion region results in a large junction capacitance (the capacitance is \(C = \frac{\epsilon_r\epsilon_0A}{d}\), where \(A\) is the junction area, so a small \(d\) means a high capacitance), which acts as a low pass filter. Thus, a photodiode as described is not an efficient transducer for a high-rate optical data signal. To improve performance, the depletion region is artificially extended by separating the p and n materials with an undoped material (called an intrinsic material), creating a pin photodiode. The intrinsic material has the effect of increasing the chance of a photon being absorbed in a neutral region, and reducing the capacitance of the photodiode.

Commercial high speed pin photodiodes have a rise times as short as 25 ps, and responsivities of ~1 A/W. Taking the numbers used above, this leads to a photocurrent of 13 nA.

To achieve a voltage of 1.3 V, a transimpedance gain of 100 MΩ is required, which is unrealistic in a single stage. Recall that the gain bandwidth product (GBP) is a constant, so to achieve a gain of 100 MΩ requires a GBP of at least 750\(\times\)10¹⁵ Hz, while typical values for high-speed opamps are between 50-8000 MHz. Thus, for very high speed applications, it may be necessary to use discrete components rather than opamps for the first stages.

Note

It is not necessary to bias photodiodes, since the current is generated by the absorption of photons. However, biasing reduces the junction capacitance and reduces the losses due to electron-hole recombination in the intrinsic region (increased responsivity). On the other hand, biasing increases the dark current (a current that flows even in the absence of light), and increases the noise. Thus, biasing must be carefully chosen to balance responsivity and noise. The connection of the photodiode to the measuring circuit must also be carefully designed. The rise time of the circuit is given by \(\sqrt{RC}\), where \(R\) is the internal resistance of the photodiode and any external resistances. To keep the rise time high, \(R\) must be kept small.

The avalanche photodiode (APD) is a high-gain alternative to the pin diode. When a large bias voltage is applied to a photodiode, the electrons and holes are accelerated to very high energies. The electrons (and holes) will inevitably collide with atoms in the material, but now have enough energy to knock additional electrons free, which are themselves accelerated. This process can cascade (avalanche) such that a single absorbed photon generates thousands of electrons. The APD response time is limited by two factors: the junction capacitance and a dead time, which is the time the APD takes for the avalanche medium to recover. In order to reduce the dead time, most APD circuits quench the avalanche once the photocurrent reaches a certain threshold--in a digital system, the absolute value of the current is unimportant, only that it is above or below a set of threshold values. To increase the efficiency of the avalanche process, the avalanche region is separated from the area where photons are initially absorbed and the collector, as shown in Figure \(\PageIndex{2}\).

Figure \(\PageIndex{2}\): Avalance photodiode. Light is absorbed in the intrinsic region (i), and the high electric field in the second p region causes impact ionization and gain. Holes are also generated with every impact, but these are not shown. For some semiconductors the gain for holes and electrons is not the same, while for others it is. In most designs, photons enter through the p-doped region on the lefthand side.

A typical avalanche photodiode (APD) can have a responsivity of more than 10 A/W. More importantly, APDs can be designed to detect single photons, which is important for advanced telecommunication applications, such as communication between satellites and quantum communication channels. It is important to note that using APDs does not allow one to escape the GBP dilemma: quenching is a way to limit the gain in order to increase the bandwidth. For extremely high bandwidth systems (100 Gb/s), APDs must be operated such that their responsivity is less than unity.