Basic Intent
This
lecture is intended to offer an overview of the basic types of sensors used to
detect optical and near infrared radiation. At the completion of this lecture,
the student should be familiar with some of the more common types of
photosensors, some of the principles which govern their performance, and some
applications.
Photosensors
Detection
of light is a basic need for everything from devices to plants and animals. In
the case of animals, light detection systems are very highly specialized, and
often operate very near to thermodynamic limits to detection. Device
researchers have worked on techniques for light detection for many years, and
have developed devices which offer excellent performance as well.
Clearly, a major sponsor of light detection device research has been the military. Devices for light detection are of fundamental importance throughout military technology, and the maturity and widespread availability of inexpensive photosensors is a direct result of this DOD research investment over many years.
Light is a quantum-mechanical phenomena. It comes in discrete particles
called photons. Photons have a wavelength 
, a
velocity 
, a
frequency 
,
energy 
, and even
a momentum 
. Among all of
this, it is important to remember the relationship between energy and
wavelength. In all cases, the energy of the photon determines how we detect it.
Light detectors essentially may be broken into two categories. The so-called Quantum detectors all convert incoming radiation directly into an electron in a semiconductor device, and process the resulting current with electronic circuitry. The Thermal detectors simply absorb the energy and operate by measuring the change in temperature with a thermometer.
We will start be examining the Quantum detectors, since they offer the best performance for detection of optical radiation.
In all of the quantum detectors, the photon is absorbed and an electron is liberated in the structure with the energy of the photon. This process is very complicated, and we will not examine it in detail. It is important to recognize that semiconductors feature the basic property that electrons are allowed to exist only at certain energy levels. If the device being used to detect the radiation does not allow electrons with the energy of the incident photon, the photon will not be absorbed, and there will be no signal.
On the other hand, if the photon carries an amount of energy which is `allowed' for an electron in the semiconductor, it can be absorbed. Once it is absorbed, the electron moves freely within the device, subject to electric fields (due to applied voltages) and other effects. Many such devices have a complicated `band structure' in which the allowed energies in the structure change with location in the device. On example of such a `band structure' is that offered by a p-n diode, such as shown in the textbook. In a diode, the p-n junction produces a step in the allowed energy levels, resulting in a direction in which currents flow easily and the opposite direction in which current flow is greatly reduced.
A photo-diode is simply a diode, biased against its easy flow direction (`reverse-biased') so that the current is very low. In a photon is absorbed and an electron is freed, it may pass over the energy barrier if it possesses enough energy. In this respect, the photodiode only produces a current if the absorbed photon has more energy than that needed to traverse the p-n junction. Because of this effect, the p-n photodiode is said to have a `cutoff wavelength' - photons with wavelength less than the cutoff produce current and are detected, while photons with wavelength greater than the cutoff do not produce current and are not detected.
Fig. 1: Connection of a photodiode in a photovoltaic mode
Photodiodes may be biased and operated in two basic modes: photovoltaic and photoconductive. In the photovoltaic mode, the diode is attached to a virtual ground preamplifier as shown in Fig. 1 (from the textbook), and the arrival of photons cause the generation of a voltage which is amplified by the op-amp. The primary feature of this approach is that there is no dc-bias across the diode, and so there is no basic leakage current across the diode aside from thermally-generated currents. This configuration does suffer from slower response because the charge generated must charge the capacitance of the diode, causing an R-C delay.
Fig. 2: Photoconductive operating mode
In the photoconductive mode, the diode is biased, and the current flowing across the diode is converted to a voltage (by a resistor), and amplified. A photoconductive circuit is shown in Fig. 2 (from the book). The primary advantage of this approach is that the applied bias decreases the effective capacitance of the diode (by widening the depletion region), and allows for faster response. Unfortunately, the dc bias also causes some leakage current, so detection of very small signals is compromised.
In addition to making optical detectors from diodes, it is also possible to construct them from transistors. In this case, the `photocurrent' is deposited in the base of a bipolar junction transistor. When subjected to a collector-emitter bias (for npn), the current generated y the photons flows from the base to the emitter, and a larger current is caused to flow from the collector to the emitter. For an average transistor, the collector-emitter current is between 10 and 100x larger than the photocurrent, so the phototransistor is fundamentally more sensitive than the diode.
Photodiodes and phototransistors are very widely available. Most semiconductor device manufacturers also offer photodiodes and transistors, so there are nearly 100 suppliers. More than 10 manufacturers specialize in photosensors. As a result, optimized photodiodes and transistors are available at very low cost.
These devices are also available in packages which are designed for particular applications. For example, it is common to use a light emitting diode and a detector mounted in a pair so that passing objects can interrupt the optical beam between them. "Opto-interruptors" which consist of such emitter-detector pairs are available in a wide variety of configurations. "Proximity detectors" which are situated side-by-side sense the presence of a reflecting surface by causing reflected light to strike the detector.
Fig. 3: Incremental Encoders
Other applications of optical detector-emitter pairs include measurement of the rotation rate of electric motors. In this case, a disk is mounted on the shaft of the motor with a large number of slits cut through it. The detector emitter pair is mounted so that the slits cause an oscillation in the signal - and the rotary position can be determined by counting the peaks in signal. This is called an optical encoder, and it is widely used in electric motors, as shown in the Fig. 3.
Fig. 4: Spectral Response of an Infrared Photodiode
Most phototransistors and photodiodes have their peak sensitivity in the near infrared (see Fig. 4; from the book). The peak sensitivity occurs near the cutoff wavelength (near 1 um) and extends to shorter wavelengths. The location of this peak sensitivity is due to the energy of the `band gap' in silicon, and is not easily adjusted.
Table 1: Band Gaps and Longest Wavelengths
Photosensors can be made from other electronic materials with different band-gaps, as shown in Table 1 (from the book). None of these materials are as widely available as silicon, and costs for detectors made from InSb can be substantially higher.
There is another important consideration to keep in mind when selecting photosensors. In addition to the photocarriers in the device, thermally-generated carriers can be produced. The distribution of energies generated by thermal processes is dependent on the thermodynamics of the device, and on the temperature. Because of this relationship, increasing the temperature causes an increase in the number of thermally generated carriers. Conversely, reducing the band gap of a room-temperature device will also cause an increase in the number of thermally-generated carriers. Silicon detectors work well at room temperature, but heating to more than 100C starts to cause substantial increases in `dark current'. Detectors made from materials other than silicon may offer increased cutoff wavelength, but may also require cooling below room temperature.
In general, there is a nearly linear relationship between the maximum operating temperature and the cutoff energy for the detector. By selecting a material with a cutoff energy 1/5 that of silicon (such as InSb), it is necessary to cool the device to about 1/5 of the maximum operating temperature of silicon (cooling to 77K is optimal for InSb). This tradeoff between cutoff and operating temperature imposes severe cost issues for operation of devices at fairly long wavelengths.
Fig. 5: Operating Ranges for Some Infrared Detectors
If cooling is affordable, a large selection of materials and devices with `engineered band-gaps' is available. The tremendous interest in devices with cutoff wavelengths near 10-20 um is a direct result of the DOD interest in infrared detectors for `night vision'. It turns out that the peak of the infrared spectrum for objects at room temperature is in this region, and so the maximum contrast in thermal detection is available by producing devices with sensitivity in this region.
There is a simple relationship between the temperature of an infrared source and the peak wavelength of the blackbody spectrum.
where the wavelength is in microns, and the temperature is in Kelvin. So, for room temperature, the max. wavelength is near 10 microns.
Of the materials most studied, the clear winner is Mercury Cadmium Telluride (`MCT'). It may be formulated to have cutoff between 10 and 20 microns, and offers excellent properties for infrared detection. In particular, it offers low dark current, high absorptivity, and low carrier scattering. Unfortunately, it is difficult and expensive to manufacture. As should be expected for anything containing mercury, its fabrication process is an environmental nightmare, and the basic material is not compatible with electronics. As a result, it is `bump-bonded' onto silicon substrates for readout and signal processing. In addition, it must be operated at or below 77K, which imposes operational complications. A commercial imaging system based on MCT detector arrays generally costs near 100K.
Most military applications (ballistic missiles, aircraft imaging systems, satellite systems) can afford this set of costs and complications, but commercial and civilian application are generally cost-constrained. Therefore, recent research activities have focused on other materials which might be less expensive to make and operate.
InSb does not offer sensitivity in the 10-20 um region, but is more easily made than MCT, is electronics compatible, and can be operated near 100K. Research to extend the operation to higher temperatures is underway throughout academia and industry.
Overall, the relationship between cutoff and operating temperature is pretty strict. MCT, which has been the focus of billions of dollars of materials research effort, has only been slightly extended to higher temperatures. There is not tremendous hope that InSb or other materials will benefit from a large change in operating requirements.
The other type of optical detector, the thermal detector, does offer some hope for this problem.
Thermal detectors operate by absorbing the infrared radiation and measuring the temperature rise of the detector with a thermometer. Generally, the performance of thermal detectors is limited by the availability of sensitive and small heat capacity thermometers.
An important advantage of thermal infrared detectors is due to the absence of any relationship between the wavelength of the absorbed radiation and the response of the detector. Any energy which is absorbed causes a response in the detector. Therefore, it is possible to use a thermal infrared detector at room temperature to detect radiation from room temperature blackbodies.
However, it is important to note that if the conditions allow use of a quantum detector, such a detector will outperform a thermal detector by several orders of magnitude. Thermal detectors come into their own in situations which simply don't allow quantum detectors.
Since the thermometer is mounted within the infrared detector structure, it is connected to a temperature reference by a finite thermal conductance. This finite conductance imposes dynamic constraints on the system behavior, and we may analyze the situation as follows:
Assume we have a thermometer which is a thermistor with a temperature coefficient given by:
Fig. 6: Voltage Divider
This thermistor is mounted in an electrical circuit with a load resistor which has a resistance of RL, and is biased by a dc voltage of Vin. The electrical circuit is shown in Fig. 6.
As for all voltage dividers, we have:
The sensitivity of this system is given by:
Fig. 7: Thermal Circuit
However, we must consider the thermal characteristics of this system as well. In this case, we model the thermometer as a finite heat capacity attached to an object by a finite thermal conductance. Infrared power is deposited into the thermometer, causing the temperature of the thermometer to oscillate. This thermal situation may be modeled as a thermal circuit as shown in the figure below.
By energy balance, the energy gained is equal to the change in energy of the thermometer:
Now, we assume that the power and the thermometer temperature oscillate:
Now, we insert these expressions into the energy balance equation, and we have:
We may take the constant and oscillating parts to be independent, and we have :
These reduce to:
So the sensitivity of this device to changes in infrared absorbed power is:
To improve the sensitivity, is it important to choose a thermometer with a large temperature coefficient and a small heat capacity. We can see from this expression that the response of the detector will have a simple 1-pole response, which is to say that it is frequency-independent below the cutoff frequency and decreases as 1/f above the cutoff. Its response is exactly the same as that of an electrical low-pass filter.
There are several different infrared detectors that are based on this detection concept. In fact, almost every well-established thermometer has also been optimized as an infrared detector.
In the next lecture, we will look at infrared detectors which are based on piezoelectric materials, resistance thermometers, the expansion of ideal gases, and on thermocouples.