Active solid-state devices (e.g. – transistors – solid-state diode – Thin active physical layer which is – Low workfunction layer for electron emission
Patent
1998-05-05
2000-03-28
Tran, Minh Loan
Active solid-state devices (e.g., transistors, solid-state diode
Thin active physical layer which is
Low workfunction layer for electron emission
257433, 257434, 257462, H01L 2906
Patent
active
060435089
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND
Measurement of weak light levels is a common procedure in science and technology.
One of the most sensitive photodetectors is a photomultiplier tube (PMT), or just photomultiplier. The basic structure of this device is a vacuum tube containing a light sensitive photocathode and an electron multiplier structure. Electric field by a high voltage is applied over the system. Photons to be detected hit the photocathode from which they liberate photoelectrons by a photoemission process. The electron multiplier consists of a series (typically 6-16) of secondary emission electrodes, called dynodes, with rising electric potential arranged between them. The photoelectrons from the cathode are directed to the first dynode where they produce several secondary electrons which are, in turn, directed to the next dynode where secondary emission is repeated, and so on. This results in amplification so that the signal from the output electrode, anode, is high enough to be handled electronically. Disadvantages of photomultiplier tubes are relatively high cost and need for high voltage which limit and complicate their versatility.
A different class are various semiconductor photodetectors, e.g. photodiodes, phototransistors and charge coupled devices (CCDs). Common to them is that light is allowed to affect a semiconductor material, where it generates charge carriers (electrons and holes) that are collected to produce an electrical signal. A problem with semiconductor detectors is that the carriers have to migrate in the bulk of semiconductor material where thermal energy produces a high background noise.
SUMMARY OF THE INVENTION
The present invention discloses a new type of photodetector which is cheap, sensitive and easy to construct. It comprises an evacuated chamber containing a photoemissive surface capable of liberating electrons (photoelectrons) through photoelectric effect in response to light photons. Characteristic to the present invention is that the photoelectrons are detected by a metal oxide semiconductor type of field effect transistor (MOSFET) having a floating gate, the gate being suitably charged before measurement. Photoelectron emission causes a change in gate charge, the change being indicative of the amount of light received by the detector.
According to one embodiment, the photoemissive surface is unattached to the gate, the latter being charged to a positive potential before measurement. The positive charge attracts photoelectrons and directs them to the gate where they neutralize its positive charge leading to a decrease in gate potential, the decrease being indicative of the amount of light received by the detector.
According to another embodiment, the photoemissive surface is processed directly on the floating gate which, in this case, is charged negatively before measurement. The liberated photoelectrons are collected to a separate anode electrode or just to the metal wall of the device casing. This causes an increase in the gate potential, the increase being indicative of the amount of light received by the detector.
During the photoelectron collection phase the presented photodetector does not require any electric power (voltage). Obviously, however, an additional electric field can also be applied, in order to optimize photoelectron collection.
OPERATION PRINCIPLE OF THE INVENTION
It is characteristic of the invention that electrons (photoelectrons) liberated from a photoemissive surface by a photoelectric effect in response to light photons are allowed to affect the surface of the floating gate of a MOSFET (metal oxide semiconductor type of a field effect transistor). The invention is based on measuring the effect of photoelectrons on the charge stored in the capacitance of the floating gate of the MOSFET before measurement.
The photoelectrons are collected by means of the effect of the electric field created by the gate after it has first been charged to a suitable potential. This initial charging is accomplished, for example, by applying the FN tunneling technique.
By measur
REFERENCES:
patent: 5471051 (1995-11-01), Niigaki et al.
patent: 5804833 (1998-09-01), Stettner et al.
Haaslahti Jukka
Kahilainen Jukka
Oikari Timo
Rados Technology Oy
Tran Minh Loan
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