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There is a class of design problems that can easily be solved by using human vision. Consider sensing the proper location of the paper in a printer. It is easy for a human to see the alignment, but difficult for a microprocessor to verify it. The camera in a cell phone needs to measure the ambient light to determine whether the flash needs to be activated. How can the oxygen level in blood be assessed in a non-invasive manner?
The solution to these design issues is the use of photodiodes or phototransistors. These optoelectronic devices convert light (photons) to electrical signals, and so enable a microprocessor (or microcontroller) to “see”. This allows it to control the positioning and alignment of objects, determine light intensity, and measure the physical properties of materials based on their interaction with light.
This article explains the theory of operation of both photodiodes and phototransistors and provides designers with the basic knowledge of their application. Devices from Advanced Photonix, Inc., Vishay Semiconductor Opto Division, Excelitas Technologies, Genicom Co., Ltd., Marktech Optoelectronics, and NTE Electronics are presented by way of example.
Photodiodes and phototransistors are sensitive to a range of optical wavelengths. In some cases, this is a design consideration, for instance, in making the operation invisible to the human eye. The designer should be aware of the optical spectrum in order to match the devices to the application.
The optical spectrum extends from longer wavelength infrared (IR) to shorter wavelength ultraviolet (UV) (Figure 1). The visible wavelengths are in between.
Most optoelectronic devices are specified using their operating wavelengths in nanometers (nm); frequency values are rarely used.
Silicon (Si) photodiodes tend to be sensitive to visible light. IR sensitive devices use indium antimonide (InSb), indium gallium arsenide (InGaAs), germanium (Ge), or mercury cadmium telluride (HgCdTe). UV sensitive devices commonly use silicon carbide (SiC).
The photodiode is a two-element P-N or PIN junction that is exposed to light through a transparent body or cover. When light strikes the junction, a current or voltage is developed depending on the mode of operation. The photodiode operates in any of three modes depending on the biasing applied to it. These are the photovoltaic, photoconductive, or avalanche diode modes.
If the photodiode is unbiased, it operates in the photovoltaic mode and produces a small output voltage when illuminated with a light source. In this mode, the photodiode acts like a solar cell. The photovoltaic mode is useful in low-frequency applications, generally under 350 kilohertz (kHz), with low light intensities. The output voltage is low, and the photodiode output requires an amplifier in most cases.
The photoconductive mode requires that the photodiode be reverse biased. The applied reverse bias will generate a depletion region at the P-N junction. The greater the bias, the wider the depletion region. The wider depletion region results in reduced capacitance, compared to the unbiased diode, resulting in faster response times. This mode has higher noise levels and may require bandwidth limiting to control them.
If the reverse bias is increased further, the photodiode operates in avalanche diode mode. In this mode, the photodiodes operate in a high reverse bias condition, permitting the multiplication of each photo-produced electron-hole pair due to avalanche breakdown. This results in internal gain and higher sensitivity in the photodiode. This mode is functionality similar to a photomultiplier tube.
In most applications, the photodiode operates in photoconductive mode with a reverse bias (Figure 2).
The reverse biased, unilluminated photodiode junction has a depletion zone with few free carriers. It looks like a charged capacitor. There is a small current caused by thermally excited ionization, called the “dark” current. An ideal photodiode would have zero dark current. Dark current and thermal noise levels are proportional to the temperature of the diode. The dark current can conceal the photocurrent due to extremely low light levels, so devices having low dark currents should be selected.
When light impinges on the depletion layer with sufficient energy, it ionizes the atoms in the crystal structure and generates electron-hole pairs. The existing electric field, due to the biasing, will cause the electrons to move to the cathode and the holes to move to the anode, giving rise to a photocurrent. The greater the light intensity, the greater the photocurrent. The current-voltage characteristic of the reverse biased photodiode shows this in Figure 3.
The graph plots the diode reverse current as a function of the applied reverse bias voltage with light intensity as a parameter. Note that increasing light levels produce a proportional increase in the reverse current levels. This is the basis for using photodiodes for measuring light intensity. The bias voltage, when greater than 0.5 volts, has little effect on the photocurrent. The reverse current can be converted to a voltage by applying it to a transimpedance amplifier.
The variety of light detection and measurement applications has given rise to a variety of distinctive photodiode types. The basic photodiode is the planar P-N junction. These devices offer the best performance in unbiased, photovoltaic mode. They are also the most cost-effective devices.
The 002-151-001 from Advanced Photonix, Inc., is an example of a planar diffusion InGaAs photodiode/photodetector (Figure 4). It comes in a surface-mount device (SMD) package measuring 1.6 x 3.2 x 1.1 millimeters (mm), with an active optical aperture measuring 0.05 mm in diameter.
This InGaAs photodiode has a spectral range of 800 to 1700 nm, covering the IR spectrum. Its dark current is less than 1 nanoampere (nA). Its spectral responsivity, which specifies the current output for a specific optical power input, is typically 1 ampere per watt (A/W). It is intended for applications including industrial sensing, security, and communications.
The PIN diode is formed by sandwiching a high-resistivity intrinsic semiconductor layer between the P-type and N-type layers of a conventional diode; hence the name PIN reflects the structure of the diode.
The insertion of the intrinsic layer increases the effective width of the diodes depletion layer, resulting in lower capacitance and a higher breakdown voltage. The lower capacitance effectively increases the speed of the photodiode. The larger depletion region offers a larger volume of photon-induced electron-hole generation and greater quantum efficiency.
The Vishay Semiconductor Opto Division’s VBP104SR is a silicon PIN photodiode covering the spectral range from 430 to 1100 nm (violet to near IR). It has a typical dark current of 2 nA and a large optically sensitive area of 4.4 mm² (Figure 5).
The avalanche photodiode (APD) is functionally similar to a photomultiplier tube in that it uses the avalanche effect to create gain in the diode. In the presence of a high reverse bias, each hole-electron pair generates additional pairs by means of avalanche breakdown. This results in gain in the form of a larger photocurrent per photon of light. This makes the APD an ideal choice for low light sensitivity.
An example of an APD is the C30737LH-500-92C from Excelitas Technologies. It has a spectral range of 500 to 1000 nm (cyan to near IR) with a peak response at 905 nm (IR). It has a spectral responsivity of 60 A/W @ 900 nm with a dark current less than 1 nA. It is intended for high bandwidth applications such as automotive light detection and ranging (LiDAR) and optical communication (Figure 6).
The Schottky photodiode is based on a metal-to-semiconductor junction. The metal side of the junction forms the anode electrode, while the N-type semiconductor side is the cathode. Photons pass through a partially transparent metallic layer and are absorbed in the N-type semiconductor, freeing charged carrier pairs. These free-charged carriers are swept out of the depletion layer by the applied electric field and form the photocurrent.
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