Silicon photodiodes are constructed from single crystal silicon wafers similar to those used in the manufacture of integrated circuits. The major difference is that photodiodes require higher purity silicon. The purity of silicon is directly related to its resistivity, with higher resistivity indicating higher purity silicon. Centro Vision products utilize silicon whose resistivities range from 10 Ohm-cm to 10,000 Ohm-cm.
A cross section of a typical silicon photodiode is shown in the figure. N type silicon is the starting material. A thin “p” layer is formed on the front surface of the device by thermal diffusion or ion implantation of the appropriate doping material (usually boron). The interface between the “p” layer and the “n” silicon is known as a pn junction. Small metal contacts are applied to the front surface of the device and the entire back is coated with a contact metal. The back contact is the cathode, the front contact is the anode. The active area is coated with either silicon nitride, silicon monoxide or silicon dioxide for protection and to serve as an anti-reflection coating. The thickness of this coating is optimized for particular irradiation wavelengths. As an example, a Centro Vision Series 5-T photodiode has a coating which enhances its response to the blue part of the spectrum.
The characteristics of pn junctions are well known. However, photodiode junctions are unusual because the top “p” layer is very thin. The thickness of this layer is determined by the wavelength of radiation to be detected. Near the pn junction the silicon becomes depleted of electrical charges. This is known as the “depletion region”. The depth of the depletion region can be varied by applying a reverse bias voltage across the junction. When the depletion region reaches the back of the diode the photodiode is said to be “fully depleted”. The depletion region is important to photodiode performance since most of the sensitivity to radiation originates there.
The capacitance of the pn junction depends on the thickness of this variable depletion region. Increasing the bias voltage increases the depth of this region and lowers capacitance until the fully depleted condition is achieved. Junction capacitance is also a function of the resistivity of silicon used and active area size. The relationship between junction capacitance, bias voltage and area is shown in the graph below.
When light is absorbed in the active area an electron-hole pair is formed. The electrons and holes are separated electrons passing to the “n” region and holes to the “p” region. This results in a current generated by light (usually abbreviated Isc). The migration of electrons and holes to their respective region is called “The Photovoltaic Effect”.
Silicon photodiodes are most useful as current generators although a voltage is also generated by illumination. Most of the data supplied in this manual refers to the short circuit current characteristics of the photodiodes. The short circuit current is a linear function of the irradiance over a very wide range of at least seven orders of magnitude. The Isc is only slightly affected by temperature, varying less than 0.2% per degree C for visible wavelengths. A recently published independent laboratory study has shown Centro Vision photodiodes to have Isc stability better than +/-0.25% per year.
Approximate Photdiode Short Circuit Currents for Various Light Sources
Part Number Sunlight at Noon, mA Room Light On Table, microA Super Red LED at 10 mA, 1 CM Away , microA Laser Pointer @ 1 meter, mA
OSD1-5T 0.47 0.45 0.32 0.71
OSD5-5T 1.80 2.10 1.70 1.00
OSD15-5T 4.50 5.60 2.60 1.00
OSD35-5T 11.00 14.00 3.80 1.10
OSD60-5T 28.00 39.00 7.20 1.10
It must be noted that when a reverse bias is applied some current will flow without illumination. The “dark current” is specified for every device. In cases where a very low bias voltage is used, shunt resistance is specified. This is determined by measuring dark current with +/-0.010 volts applied bias.
A photodiode has two terminals, a cathode and an anode. It has a low forward resistance (anode positive) and high reverse resistance (anode negative). Normal biased operation of most photodiodes described in this catalog calls for negative biasing the active area of the device which is the anode or positive biasing the backside of the device, which is the cathode.
In the photovoltaic and zero bias modes, the generated current or voltage is in the diode forward directi n. Hence the generated polarity is opposite to that required for the biased mode.
The measure of sensitivity is the ratio of radiant energy (in watts) incident on the photodiode to the photocurrent output in amperes. It is expressed as the absolute responsivity in amps per watt. Please note that radiant energy is usually expressed as watts/cm^2 and that photodiode current as amps/cm^2. The cm^2 term cancels and we are left with amps/watt (A/W). A typical responsivity curve that shows A/W as a function of wavelength is given below.
The wavelength of the radiation to be detected is an important parameter. As can be seen from the graph, silicon becomes transparent to radiation of longer than 1100 nm wavelength. It is not therefore suitable for use at wavelengths appreciably longer than this. Ultraviolet light is, conversely, absorbed in the first 100 nm thickness of the silicon. Even the most careful surface preparation leaves some surface damage which reduces the collection efficiency for this wavelength. Surface coatings further affect the spectral response of the device. It is normal to apply anti-reflection coatings which enhance the response (by up to 25%) at the required wavelength. These coatings may reduce the efficiency at other wavelengths which they reflect. The package window further modifies the spectral response. The standard glass window absorbs wavelengths shorter than 300 nm. For UV detection, a fused silica or UV transmitting glass window is necessary. Various filter windows are also available to tailor the spectral response to suit the application. One specific filter which is of great interest, modifies the normal silicon response to approximate the spectral response of the human eye.
In many applications the most important parameter is dynamic performance. Photodiode response time is the root mean square sum of the charge collection time and the RC time constant arising from series plus load resistances and the junction and stray capacitances. Charge collection time is voltage dependent and is made up of a fast and a slow component. The fast component is the transit time of the charge carriers (electrons and holes) through the depletion region, producing carriers that are collected by diffusion. The transit time of these carriers will be relatively slow. The figure below illustrates the transient response of a photodiode to a square pulse of radiation.
When a photodiode is operated in the unbiased mode, the slow diffusion component dominates, giving risetimes on the order of 0.5 microseconds.
For a fast response time, silicon resistivity and operating voltage must be chosen to produce a depletion layer within which the majority of the carriers are generated. In this case the transit time will be dependent on both the electron and hole drift velocities. The depletion depth necessary for full absorbtion increases rapidly with operationg wavelength. Response times increase correspondingly. This makes it difficult to achieve risetimes faster than 15-20 ns at 1064 nm, whereas risetimes of less than 2 ns are obtainable below 900 nm.
The Centro Vision -3T and -4X series take advantage of the increase in drift velocity resulting from a very high electric field. In this structure silicon thickness is reduced to just contain the required depletion depth, and a heavily doped back layer is used to supply the necessary charge to support the depletion region at higher voltage. In this way the operating field, and hence the carrier drift drift velocities, may be increased without a significant increase in depletion depth. Further increase in speed may be obtained at the expense of overall sensitivity by using silicon which is not thick enough to allow full absorbtion of incident radiation.