Shedding Light on Photodetector Selection

Choice is very application-specific, and requires knowledge of noise factors and amplifier circuits.

A great number of medical, industrial, and analytical applications require the detection of light; some of these are chemiluminescence, bioluminescence, fluorescence, and atomic absorption. All of the applications require the use of a detector to convert light into an electrical signal, and to accomplish this there are three basic technologies: photomultiplier tubes (PMTs), avalanche photodiodes (APDs), and photodiodes.

The question of when to use or not to use a PMT, APD, or silicon photodiode is not a simple one to answer. A photodiode is suitable in applications with excess light. In applications with very weak signals, a PMT is the best choice. But there are applications where the choice is not clear. Focusing on solid-state options, this article will review detector characteristics and the criteria for detector selection, and finally amplifier performance.

Silicon Photodiode

A silicon photodiode is essentially a P-N junction consisting of a positively doped P region and a negatively doped N region. Between these two regions exists an area of neutral charge known as the depletion region. When light enters the device, electrons in the crystalline structure become excited. If the energy of the light is greater than the bandgap energy of the material, electrons will move into the conduction band. This creates holes throughout the device in the valence band where the electrons were originally located. Electron-hole pairs generated in the depletion region drift to their respective electrodes--N for electrons and P for holes, resulting in a positive charge buildup in the P layer and a negative charge buildup in the N layer. The amount of charge is directly proportional to the amount of light falling on the detector. If an external circuit is connected to the electrodes, current will flow in the circuit.

That is the photovoltaic method of operation. It is also possible to apply a reverse bias to the photodetector, creating the photoconductive mode. This has the effect of increasing the electric field strength between the electrodes and the depth of the depletion region. The advantages of this kind of operation are higher speed, lower capacitance, and better linearity. However, dark current is directly dependent on reverse bias voltage, and thus becomes larger with increasing bias voltage. Generally, PIN photodiodes and APDs are operated in this fashion.

The noise in a photodiode can take two forms. The first is the shot noise of the dark current, which results from the statistical uncertainty in the arrival rate of photons. It is present in all signals and has the following form:

where i_dark = RMS noise current, q = electron charge, I_dark = photogenerated signal current, and B = frequency bandwidth of the detector-amplifier combination. The second noise source is the thermal noise of the shunt resistance, also known as Johnson noise, and it takes the form:

where I_Rsh = RMS noise current resulting from Johnson noise, k = Boltzman's constant, T = absolute temperature of the photodiode, and R_sh = shunt resistance of the photodiode. Shot noise will dominate in photoconductive operation, while Johnson noise will do so in the photovoltaic mode.

Avalanche Photodiode

The APD is a specialized silicon PIN photodiode designed to operate with high reverse voltages. These reverse voltages generate high electric fields at the P-N junction. Some of the electron-hole pairs passing through or generated in this field gain sufficient energy (greater than the bandgap energy) to create additional electron-hole pairs, a process known as impact ionization. If the newly created electron-hole pairs acquire enough energy, they also create new pairs. This effect, known as avalanche multiplication, is the mechanism by which APDs produce internal gain, an important attribute when the detector is combined with an amplifier.

Since the APD is always operated in the photoconductive mode, its noise takes the same form as the photodiode dark-current shot noise, with the addition of a few terms:

where M = detector internal gain and F = detector excess noise factor. Of the two additions, the gain simply amplifies the noise as it does the signal and has no net effect on the signal-to-noise ratio. The excess noise factor is noise added to the output signal by the multiplication process of the APD and is strongly dependent on wavelength as well as gain.

So far the only factor to play a role in detector selection has been noise equivalent power (NEP), the amount of light equivalent to the noise level of a device, or, put another way, the light level required to obtain a signal-to-noise ratio of unity. Since the noise level is proportional to the square root of the frequency bandwidth, the NEP is measured at a bandwidth of 1 Hz and expressed in units of W/Hz^1/2. But as the light level increases, the NEP no longer plays a role in the signal-to-noise ratio. The shot noise of the signal tends to dominate the signal-to-noise ratio.

where i_signal = photogenerated signal before gain. If the application has strong light signals, then the shot-noise performance of the detector is the only thing to consider, because the detector's dark noise and amplifier will be insignificant compared to the shot noise of the signal.

Understanding Amplifier Noise

Probably the most overlooked aspect of detector selection is that of an amplifier. The amplifier usually sets the lower noise floor for the detector-amplifier combination.

Figure 1 shows a transimpedance amplifier, the configuration used here in the calculation of noise.

Amplifier noise can be broken down into three major components. The first two take the familiar form of the photodiode shot noise and Johnson noise. The third term arises from the input voltage noise of the amplifier. It is the shot noise of the input bias current and the Johnson noise of the feedback resistor. It is very strongly related to the frequency bandwidth. The total detector-amplifier noise becomes:

It is now possible to calculate the NEP for a given detector and, based on these calculations, select the best detector for a particular application.

Referring to Table 1, the S1337 photodiode is designed for low capacitance, the S2387 for low dark current, and the APD to produce gain. The tables evaluate the noise performance of each device to facilitate the choice of detectors. Table 1 shows the specifications, selected with approximately the same active area size.

Table 2 sheds some light on detector selection. If it is chosen by the detector catalog's NEP alone, the total instrument performance has not been optimized. From the above information, the choice might seem to be the APD. But the APD requires a high-voltage power supply to bias it, is very temperature-sensitive, and generally costs more than a photodiode. So, in the above example, the S1337 would seem to be the best choice. Furthermore, when considering the detector's signal-to-noise performance at various light levels, it can be seen from Figure 2 that the photodiode's signal-to-noise ratio will be better than the APD when the amplifier noise is no longer a factor. This is because of the excess noise factor of the APD. Unless the application demands the lowest NEP possible, the photodiode would be the best choice under these conditions.

When is the APD a good detector selection? Based on the above equations, it is evident that the amplifier noise is strongly dependent on the frequency bandwidth. Figure 2 was calculated at 100 Hz. If the calculation were instead done at 1 MHz, the result would be much different, as shown by Figure 3. Of course, choosing a photodiode with lower capacitance would have closed the gap with the APD. However, the point is that the gain of the APD is necessary when the amplifier noise is large, as is the case with wide-frequency-bandwidth applications, or the light source is weak.

Choosing the correct detector is very application-specific, but here are some general guidelines.

• Try to use the smallest active area possible. If the light source for the application is diffuse, this might not be practical; however, from the standpoint of noise, small diodes have lower capacitance and dark current. They are also less expensive.
• In most applications, small capacitance will be more important than small dark current. Furthermore, the NEP in the catalogs does not take capacitance into account; therefore care should be exercised when comparing detectors by NEP.
• In small-frequency-bandwidth applications, photodiodes operated in a photovoltaic mode will generally outperform photoconductive devices. To reduce noise, the detector's shunt resistance should be much greater than the feedback resistance.
• To reduce the contribution of Johnson noise, use as large a feedback resistor as possible in the first amplifier stage
• In wide-frequency-bandwidth applications, PIN photodiodes operating in the photoconductive mode are preferred because of lower terminal capacitance. APDs with their internal gain perform very well in wideband applications as well. They should be considered when the light source is weak and the amplifier noise is large.

For more information on Hamamatsu's lines of photodetectors and amplifiers, please contact Earl Hergert at Hamamatsu Corporation, 360 Foothill Rd., Bridgewater, NJ 08807; (908) 231-0960; 800-524-0504.