Silicon Photodiodes
[Product Guide(pdf)]

PRINCIPLE OF OPERATION

Figure 1 (a) shows a cross section of a photodiode. The P-layer material at the active surface and the N material at the substrate form a PN junction which operates as a photoelectric converter. The usual P-layer for a silicon photodiode is formed by selective diffu- sion of boron, to a thickness of approximately 1 µm or less and the neutral region at the junction between the P- and N-layers is known as the depletion layer. By varying and controlling the thickness of the outer P-layer, substrate N-layer and bottom N+layer as well as the doping concentration, the spectral response and frequency response can be controlled. When light strikes a photodiode, the electron within the crystal structure becomes stimulated. If the light energy is greater than the band gap energy Eg, the electrons are pulled up into the conduction band, leaving holes in their place in the valence band. (See Figure 1 (b)) These electron-hole pairs occur throughout the P-layer, depletion layer and N-layer materials. In the depletion layer the electric field accelerates these electrons toward the N-layer and the holes toward the P-layer. Of the electron-hole pairs generated in the N- layer, the electrons, along with electrons that have arrived from the P-layer, are left in the N-layer conduction band. The holes at this time are being diffused through the N-layer up to the depletion layer while being accelerated, and collected in the P-layer valence band. In this manner, electron-hole pairs which are generated in proportion to the amount of incident light are collected in the N- and P- layers. This results in a positive charge in the P-layer and a negative charge in the N-layer. If an external circuit is connected between the P- and N-layers, electrons will flow away from the N-layer, and holes will flow away from the P-layer toward the opposite respective electrodes.

Figure 1 (a): Photodiode Cross Section (b): Photodiode P-N Junction State

EQUIVALENT CIRCUIT

An equivalent circuit of a photodiode is shown in Figure 2.

Figure 2: Photodiode Equivalent Circuit

Using the above equivalent circuit, the output current lo is given as follows:

where

The open circuit voltage Voc is the output voltage when lo equals 0.

Thus Voc becomes

If I' is negligible, since Is increases exponentially with respect to ambient temperature, Voc is inversely proportional to the ambient temperature and proportional to the log of I_L. However, this relation- ship does not hold for very low light levels. The short circuit current Isc is the output current when the load resistance R_L equals 0 and Vo equals 0, yielding:

In the above relationship, the 2nd and 3rd terms limit the linearity of Isc. However, since Rs is several ohms and Rsh is 10⁷ to 10¹¹ Ohms, these terms become negligible over quite a wide range.

V-l CHARACTERISTICS

When a voltage is applied to a photodiode in the dark state, the V-l characteristic observed is similar to the curve of a conventional rectifier diode as shown in Figure 3 (1) . However, when light strikes the photodiode, the curve at (1) shifts to (2) and, increasing the amount of incident light shifts this characteristic curve still further to position (3) in parallel, according to the incident light intensity. As for the characteristics of (2) and (3), if the photodiode terminals are shorted, a photocurrent Isc or Isc proportional to the light intensity will flow in the direction from the anode to the cathode. If the circuit is open, an open circuit voltage Voc or Voc' will be generated with the positive polarity at the anode.

The short circuit current Isc is extremely linear with respect to the incident light level. When the incident light is within a range of 10^-12 to 10^-2 W, the achievable range of linearity is higher than 9 orders of magnitude, depending on the type of photodiode and its operating circuit. The lower limit of this linearity is determined by the NEP, while the upper limit depends on the load resistance and reverse bias voltage, and is given by the following equation:

where

When laser light is condensed on a small spot, however, the actual series resistance element increases, and linearity deteriorates. Voc varies logarithmically with respect to a change of the light level and is greatly affected by variations in temperature, making it unsuitable for light intensity measurements. Figure 4 shows the result of plotting Isc and Voc as a function of incident light illuminance.

Figure 3: I-V Characteristics

Figure 4: Output Signal vs. Incident Light Level (S2386-5K)

Glossary

Spectral Response

The photocurrent produced by a given level of incident light varies with the wavelength. This relation between the photoelectric sensitivity and wavelength is referred to as the spectral response characteristic and is expressed in terms of photo sensitivity, quantum efficiency, etc.

Photo Sensitivity

This measure of sensitivity is the ratio of radiant energy expressed in watts (W) incident on the device, to the resulting photocurrent expressed in amperes (A). It may be represented as either an abso- lute sensitivity (ANV) or as a relative sensitivity normalized for the sensitivity at the peak waveiength, usually expressed in percent (%) with respect to the peak value. For the purpose of this catalog, the photoelectric sensitivity is represented as the absolute sensitivity, and the spectral response range is defined as the region in which the relative sensitivity is higher than 5% of the peak value.

Quantum Efficiency (QE)

The quantum efficiency is the number of electrons or holes that can be detected as a photocurrent divided by the number of the incident photons. This is commonly expressed in percent (%). The quantum efficiency and photo sensitivity have the following relationship at a given wavelength:

where S is the photo sensitivity in ANV at a given wavelength and lambda is the wavelength in nm (nanometers).

Short Circuit Current (Isc), Open Circuit Voltage (Voc)

The short circuit current is the output current which flows when the load resistance is 0 and is nearly proportional to the device active area. This is often called "white light sensitivity" with regards to the spectral response. This value is measured with light from a tungsten lamp of 2856 K distribution temperature (color temperature), providing 100 Ix illuminance (1000 Ix for GaP photodiodes). The open circuit voltage is a photovoltaic voltage developed when the load resistance is infinite and exhibits a constant value independent of the device active area.

Infrared Sensitivity Ratio

This is the ratio of the output current (IR) measured with a light flux (2856 K, 100 Ix) passing through an R-70 (t=2.5 mm) infrared filter to the short circuit current (Isc) measured without the filter. It is commonly expressed in percent, as follows:

Dark Current (ID), Shunt Resistance (Rsh)

The dark current is a small current which flows when a reverse voltage is applied to a photodiode even in dark state. This is a source of noise for applications in which a reverse voltage is applied to photodiodes (for example, as with PIN photodiodes). In contrast, for applications where no reverse voltage is applied, noise characteristics are figured out from the shunt resistance. This shunt resistance is the voltage-to-current ratio in the vicinity of 0V. For the purpose of this catalog, the shunt resistance Rsh is defined as follows:

where I_D is the dark current at V_R=lOmV.

Terminal Capacitance (Ct)

An effective capacitor is formed at the PN junction of a photodiode. Its capacitance is termed the junction capacitance and is the major factor in determining the response speed of the photodiode. And it probably causes a phenomenon of gain peaking in I-V conversion circuit using operational amplifier. In this catalog, the terminal capacitance including this junction capacitance plus package stray capacitance is listed.

Rise Time (tr)

This is the measure of the time response of a photodiode to a stepped light input, and is defined as the time required for the output to change from 10% to 90% of the steady output level. The rise time depends on the incident light wavelength and load resistance. For the purpose of this catalog, it is measured with a light source of GaAsP LED (655 nm) or GaP LED (560 nm) and load resistance of IkOmega.

Cut-off Frequency (fc)

This is the measure used to evaluate the time response of high- speed avalanche photodiodes and PIN photodiodes to a sinewave- modulated light input. It is defined as the frequency at which the photodiode output decreases by 3dB from the output at 100 kHz. The light source used is a laser diode (830 nm) and the load resis- tance is 50 Omega.The rise time tr has a relation with the cut-off frequency fc as follows:

NEP (Noise Equivalent Power)

The NEP is the amount of light equivalent to the noise level of a device. Stated differently, it is the light level required to obtain a signal-to-noise ratio of unity. This catalog lists the NEP values at the peak wavelength. Since the noise level is proportional to the square root of the frequency bandwidth, the NEP is measured at a band- width of 1Hz and thus expressed in units of W/Hz^1/2.

Reverse Voltage (VR Max)

Applying a reverse voltage to a photodiode triggers a breakdown at a certain voltage and causes severe deterioration of the device performance. Therefore the absolute maximum rating is specified for reverse voltage at the voltage somewhat lower than this breakdown voltage. The reverse voltage shall not exceed the maximum rating, even instantaneously.