5 - Measuring parameters

The conventional photoelectron multiplier (PEM) is a vacuum tube applying the photoelectron emission of metal surfaces for transduction. Electrons are emitted from the cathode when photons impinge on it. The electrons are ejected from the cathode surface when the energy of the radiation quanta is greater then the work function of the cathode material. In the PEM tube, additional electrodes (dynodes), at subsequently higher positive potential, are located between cathode and anode so as to amplify the electron current by means of secondary emission from the dynodes. The electrons are collected by the anode and cause a current flow, which can be used to produce an output voltage impulse on the load resistor connected in series with the anode. Despite of its large size and the necessary high voltage, the PEM has unique advantages such as low noise and great sensitivity.

Photoconductive sensors (photoresistors) are made of semiconducting materials that reduce their resistance in response to increasing illumination. This change is resulted by the electron-hole generation that is created by the absorption of the energy of incident photons. Polycrystalline films (e.g., lead salts and InSb), as well as single crystals (like doped Ge and Si) are used as photoconductor materials. A popular example is the CdS "cell" used in many cameras.

Photovoltaic sensors are generator type devices; they require no external excitation power. Their output voltage is a function of the illumination of a junction between two dissimilar materials. The junction acts as a potential barrier, across which electron flow is excited by incident photons. Several types of material pairs exhibit the photovoltaic effect, such as Fe-Se, Cu-CuO, and polycrystalline Se-CdO (used in the popular selenium cells). Single crystal semiconductor photovoltaic cells employing doped Si, Ge or CdSe use a pn-junction as their potential barrier. In a variant of this, the junction is formed by an n-type glass (phosphosilicate glass) deposited on p-type single crystal silicon. The wide variety of available materials enables to create photovoltaic cells that are sensitive for various wavelength ranges. InAs, InSb, PbSnTe, and HgCdTe based cells are sensitive within the IR range and, in order to improve the signal-to-noise ratio, are operated at cryogenic temperatures (typically at 77 K).

Photodiodes are based also on semiconductor pn-junctions. They apply the current-voltage characteristic shift of the device due to the electron-hole generation caused by the absorbed light. Practically, the reverse biased current of the diode is measured. The devices have a built-in field enabling them to operate in the photovoltaic cell mode but have much better performance in the photoconductive mode. Photodiodes apply Si or Ge pn-junctions and compound semiconductors often with heterojunctions. Devices operated within the visible range are based on the latter materials. An intrinsic region (which defines the depletion layer) characterizes the PIN diodes between the p- and n- type regions. Their operation is based on the excitation of electron-hole pairs in the intrinsic layer. Avalanche photodiodes (APDs) are depletion-layer photodiodes operated with a bias voltage at which electron-hole multiplication will be induced by the avalanche effect. The operation of these devices is similar to that of the PEM tubes and they have been often referred to as "silicon avalanche PEMs."

Phototransistors are light sensitive pnp- or npn-junctions based on the same effect as photodiodes but also providing inherent amplification of the photocurrent. An intrinsic region (which defines the depletion layer) characterizes the PIN diodes between the p- and n-type regions. Their operation is based on the excitation of electron-hole pairs in the intrinsic layer, which is caused by photons received at the surface of the p-type region. Because the latter ones are passing through the intrinsic layer generating secondary electron-hole pairs, the generation efficiency is improved in comparison with the simple photodiode. The main advantage of the PIN diodes is their extremely short switching time due to the small reverse biased capacitance.

Charge Coupled Devices (CCDs) are advanced sensor elements in image transfer processing. Their structure and charge coupling operation is illustrated on the animation. The charge induced by the incident light can be transferred by the appropriate series of impulses given on the gate electrode system developed on the top of the MIS (metal-insulator-semiconductor) structure. The readout can be performed at a location far from the generation site.