7.4 UV, Visible and Near IR Detectors

• in the photoelectric effect light ejects an electron from a metal surface • a vacuum phototube converts a light flux into an electrical current, and is useful for detecting high levels of light • a photomultiplier converts a single photon into a current pulse, and is useful for detecting low levels of light • photodiodes are based on the promotion of electrons from the valence band to the conduction band of semiconductors, and are useful for detecting both high and low levels of light

7.4 : 1/8 Photoelectric Effect

• because metals contain free electrons they can absorb UV, visible, and near IR radiation • if the energy of the absorbed photon is greater than the ______of the metal, an electron is ejected into the vacuum • the energy of the photon equals the work function of the metal plus the kinetic energy of the electron hc1 hc =+mv2 ωλ = λ 2 0 ω

where ω is the work function, and λ0 is the wavelength that just barely ejects an electron • alkali metals are commonly used in detectors metal Li Na K Rb Cs ω 2.9 eV 2.75 2.3 2.16 2.14

λ0 428 nm 451 539 574 579

• mixtures of alkali metals can give λ0 as high as ______nm • because ______energy can combine with optical energy, the onset of photo-ejection is gradual

7.4 : 2/8 Vacuum Phototube

A metallic surface with a low work function is placed inside an evacuated tube. When light interacts with the metal, electrons are photo-ejected. By placing a 90 V electric potential between the photocathode and anode, the electrons are drawn to the anode. The resultant current is measured by a micro-ammeter.

glass tube hν e!

photocathode anode ! +

Absorption spectrophotometers have light levels in the tens of ______domain. This level of light produces currents near 1 μA. Vacuum phototubes have a ______originating from thermal ejection of electrons from the metallic photocathode.

7.4 : 3/8 Photomultiplier

A photomultiplier is nothing more than a vacuum phototube followed by an ______.

hν

1 e! 4 e! 16 e! 106 A

PC D1 D2 D3 D9

100 kΩ I

-1,000 V +0 V

The secondary electron emitters are called ______and are made from a beryllium alloy. The number of secondary electrons varies from _____. For an average of 4, the gain of the multiplier shown above is 410 = 106. This is a current of 1.6×10-13 A per photon.

Like a phototube, photomultipliers have a dark current.

7.4 : 4/8 Photocathode Response

90 9558 80 9558Q The spectral 70 response of a 60 C31034 photocathode 50 depends upon the 40 ______used and 1P28 931B whether the tube is 30 ______. 20 10

photocathode response (mA/W) 0 200 400 600 800 1000 wavelength (nm)

• 931B is a side window tube with "squirrel cage" dynodes; it is inexpensive and rugged; the 1P28 is a 931 with a quartz tube • 9558 is an end-on tube with high response and very low dark current; it is used primarily when measuring current • C31034 is an end-on tube with a wide spectral response; it is used primarily when photon counting

7.4 : 5/8 Doped Semiconductors

• partially filled bands are capable of carrying significant current • pure silicon has one filled and one empty band making it a "semi- conductor" • silicon doped with group V atoms picks up extra electrons in the ______band, thus conducts • silicon doped with group III atoms is missing electrons in the ______band, thus conducts

p-doped Si Silicon n-doped Si

conduction band conduction band conduction band

band gap excess excess electrons holes

Fermi level

valence band valence band valence band

7.4 : 6/8 p,n Junctions

• when no electrical potential is applied the Fermi levels are equal and no current flows • when ______biased, electrons flow to +V and holes flow to -V; current is sustained • when ______biased, conduction electrons will flow to +V while holes flow to -V; when all the electrons and holes are transferred the current stops • when ______biased, light promotes an electron into the conduction band and creates a hole in the valence band; current then flows (1 electron per photon)

reverse biased forward biased !V no applied hole potential flow excess electron holes electron hν flow flow

depletion region !V +V hole +V excess flow electrons

7.4 : 7/8 Photodiodes

A photodiode circuit consists of a power supply to provide the reverse bias and a current

measuring device. Current is proportional to ! hν intensity. A normal photodiode has a gain of unity, thus it cannot measure _____ levels of +5 V light. In an avalanche photodiode the initial I electron causes a pulse of charge. Avalanche diodes are good for ______.

Spectral response is ______of wavelength up to the bandgap. The graph would be flat if plotted as quantum efficiency versus wavelength. The drop-off in the UV is caused by a glass envelope. The magenta line is for a quartz envelope.

7.4 : 8/8