Radiant energy – Invisible radiant energy responsive electric signalling – Semiconductor system
Patent
1996-11-19
1999-06-22
Westin, Edward P.
Radiant energy
Invisible radiant energy responsive electric signalling
Semiconductor system
257 10, H01J 4304
Patent
active
059144912
DESCRIPTION:
BRIEF SUMMARY
The object of the invention is a photon or particle detector comprising one or more transmission elements surrounded by a sufficient vacuum.
1. PRIOR ART
To detect photons, photodetectors are used for converting the optical signals of light quanta, that is photons, to electrical signals. The photons are absorbed in the absorption region of the detector, thus forming charge carriers which are an electron and a hole, or an electron alone. Detectors functioning in the visible light region, or in the vicinity of this region, can be divided into two groups: solid state photodetectors and vacuum tube photodetectors. Both groups include several different detector types for different applications. It is also possible to detect particles by means of similar detectors, depending on the energy of the particles.
1.1 Conventional Solid State Detectors
As photons are absorbed or particles collide with a semiconductor, at least one free primary charge carrier is created in the solid state semiconductor detectors. If the number of photons reaching the detector is small, the primary signal formed by the electrons or holes created must be amplified in order for the photons to be detected. Amplification may be performed either by an external amplifier or by internal gain in the detector.
The most common photodetector incorporating a unity gain is a p-i-n photodiode by means of which an almost 100% quantum efficiency can be achieved. Internal amplification in a conventional avalanche photodiode (APD) is based on the fact that the primary charge carriers accelerated in an electric field have sufficient energy to ionize atoms by colliding with them. Sufficient energy is approximately 1.5 E.sub.g, when E.sub.g is the energy gap of a semiconductor, that is, the valence-conduction band gap. The electron and hole created by ionization are then capable of ionizing new atoms. This process is called avalanche multiplication.
However, a problem of both the external amplifier and avalanche multiplication is considerable noise. It usually impairs the sensitivity of the detector to such a degree that detecting low luminosity is impossible. In addition, the noise increases radically as multiplication increases. The noise caused by multiplication is a particularly serious problem in compound semiconductor avalanche photodiodes, by means of which good quantum efficiency can be achieved also at long (>1 .mu.m) wavelengths of light. In these diodes, the electron and hole--accelerated by a large electric field--ionize atoms with almost equal probability, which means that the noise caused by avalanche multiplication is at its highest level.
1.2 Conventional Unipolar Solid State Photomultiplier
In multiquantum well (MQW) structures consisting of semiconductors having small and large energy gaps (E.sub.g), it is possible to achieve so-called unipolar avalanche multiplication. The semiconductor layers with a smaller energy gap (E.sub.g), that is, quantum wells (QW), are doped with impurity atoms which, as a result of low thermal energy, ionize and release charge carriers-to the quantum well. In unipolar avalanche multiplication, the primary charge carrier--the electron or the hole--collides with the charge carriers stored in the quantum wells formed on the conduction band (electrons) or valence band (holes). Thus only one type of the charge carriers, the electron or the hole, is involved in multiplication. In this case multiplication causes little noise.
If the quantum wells are doped with donors, unipolar electron multiplication is detected. If, on the other hand, the quantum well layers are doped with acceptors, unipolar hole multiplication is detected. To achieve multiplication, the primary charge carrier must have sufficient kinetic energy to ionize charge carriers from the quantum well. The magnitude of the multiplication can be assessed by the expression
In order to maintain multiplication continuously on the same level, the wells must be kept full, for example, by means of thermal or optical excitement or by bringing the wells in
REFERENCES:
patent: 4586068 (1986-04-01), Petroff et al.
patent: 5374826 (1994-12-01), La Rue et al.
Jalonen Marko
Kojola Hannu
Pessa Markus
Salokatve Arto
Toivonen Mika
Hanig Richard
Westin Edward P.
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