6.6: Photodetectors

Types of Photodetectors

Photodetectors are sensors used to convert light, at optical or other nearby frequencies, to electricity. One way to classify photodetectors is by their type of active material, which may be a solid or a gas. The first type of detectors are semiconductor photodetectors made from solid semiconductor pn junctions. The choice of semiconductor influences the wavelengths of light which can be absorbed because only photons with energy greater than or equal to the energy gap of the semiconductor can be absorbed. For example, silicon has an energy gap of 1.11 eV, so it is able to absorb the photons in both the visible range 1.9 eV \(< E <\) 3.1 eV as well as photons in the near infrared range 1.1 eV \(< E <\) 1.9 eV. In some semiconductor photodetectors, a thin intrinsic (undoped) layer is added between the p-type material and the n-type material at the junction. In these semiconductor p-i-n junction photodetectors, the added layer widens the depletion layer. It also decreases the internal capacitance of the junction thereby increasing the detector response time [10, p. 660]. The second type of detectors are made from gas filled vacuum tubes, and these detectors are called phototubes [10, p. 646]. A voltage is placed across electrodes in the tubes. When light shines on the phototube, energy from a photon of light can rip off an electron from a gas atom. The electron and ion flow towards the electrodes, thereby producing electricity. The most common type of phototube is the photomultiplier tube. This device has multiple electrodes, and when an electron hits one of these electrodes, additional electrons are emitted. These electrons can hit additional electrodes to produce even more electrons. Because each incoming photon produces a cascade of electrons, photomultiplier tubes have high internal amplification.

Another way to classify photodetectors depends on whether incoming photons have enough energy to rip off electrons or just excite them. The first type of detectors are called photoelectric detectors, and they operate based on a process called photoelectric emission [10, p. 645] [27, p. 171]. In these detectors, incoming light has energy greater than or equal to the energy from the valence band to the ground level at the top of an energy level diagram. These detectors convert light to electricity because incoming photons of light rip electrons off their atoms, and the flow of the resulting electrons is a current. The second type of detectors are called photoconductive detectors or sometimes photovoltaic detectors, and they operate based on a process called photoconductivity [10, p. 647]. In these detectors, incoming light has energy equal to the difference between the valence and conduction bands, not enough to rip off electrons. These detectors convert light to electricity because incoming photons excite electrons, and the conductivity of the detector is higher when light shines on it. Solid semiconductor photodetectors can operate based on either photoelectric emission or photoconductivity, but most operate based on photoconductivity. Phototubes typically operate based on photoelectric emission.

Some photodetectors have a single element while others are made from an array of elements. A digital camera may contain millions of individual photodetectors. These elements are integrated with a charge-coupled device (CCD), which is circuitry to sequentially transfer the electrical output of each photodetector of the array [9, p. 359]. The CCD was invented in 1969 by Willard S. Boyle and George E. Smith. For this invention, they shared the 2009 Physics Nobel Prize with Charles K. Kao, who was awarded the prize for his work on optical fibers [80].

Eyes in animals are photodetectors. The retina of the human eye is an array composed of around 120 million rod cells and 6 to 7 million cone cells [81]. These cells convert light to electrical impulses which are sent to the brain.

Measures of Photodetectors

The frequency response is one of the most important measures of a photodetector. Often it is plotted versus wavelength or photon energy instead of frequency. A photodetector is only sensitive within a particular wavelength range, and the frequency response is often not flat.

As with all types of sensors, signal to noise ratio is another important measure. While photodetectors have many sources of noise, one major source is thermal noise due to the random motion of charges as they flow through a solid [9, p. 220]. To mitigate thermal noise in photodetectors used to detect very weak signals, the detectors are cooled with thermoelectric devices or using liquid nitrogen. A measure related to signal to noise ratio is the noise equivalent power. It is defined as the optical power in watts that produces a signal to noise ratio of one [82].

Another measure of a photodetector is the detectivity, denoted \(D*\), in units \(\frac{cm \cdot (Hz^{1/2} )}{W}\). It is a measure of the strength of the output assuming a one watt optical input. By definition, it is equal to the square root of the area of the sensor times the bandwidth under consideration divided by the noise equivalent power [82] [83, p. 654].

\[D* = \frac{\sqrt{\text{Area} \cdot \text{Bandwidth}}}{\text{Noise Equivalent Power}} \nonumber \]

Figure \(\PageIndex{1}\) shows detectivity versus wavelength for optical detectors made of various semiconductors.

Photodetectors are also characterized by their response times. Response time is defined as the time needed for a photodetector to respond to a steplike optical input [82]. Typical response times can range from picoseconds to milliseconds [83, p. 656]. There may be a tradeoff between response time and sensitivity, so some detectors are designed for fast operation while others are design for higher sensitivity [9, p. 220].

Figure \(\PageIndex{1}\): Spectral response of a variety of photodetectors. This figure is used with permission from Hamamatsu [82].