Semiconductor Light Detectors ISAT 300 Foundations of Instrumentation and Measurement D. J. Lawrence Spring 1999

Semiconductor Light Detectors (1)  Energy Levels and Energy Gap in a Pure Semiconductor Electron Energy “Conduction Band” (Nearly) Empty “Valence Band” (Nearly) Filled with Electrons “Forbidden” Energy Gap Semiconductors have resistivities in between those of metals and insulators. Examples are Si, GaAs, CdS, and CdSe. When the valence band is full, the electrons there are unable to move. They are “locked” in chemical bonds.

Semiconductor Light Detectors (2)  Light absorption by a semiconductor  If a photon has an energy larger than the energy gap, the photon will be absorbed by the semiconductor, exciting an electron from the valence band into the conduction band, where it is free to move.  This absorption process underlies the operation of photoconductive light detectors, photodiodes, and photovoltaic (solar) cells. Electron Energy “Conduction Band” (Nearly) Empty “Valence Band” (Nearly) Filled with Electrons “Forbidden” Energy Gap

Semiconductor Light Detectors (3) -- Photoconductive Detectors Photons having energy greater than the energy gap of the semiconductor are absorbed, exciting electrons from the valence band into the conduction band, where they are free to move, and thus the resistivity, , of the semiconductor decreases. Since R semiconductor =  / A, the resistance of the semiconductor sample also decreases. semi- conductor I V out hf

Semiconductor Light Detectors (4) -- Photoconductive Detectors  A typical photoconductive cell: (a) cutaway view, and (b) symbol.

Semiconductor Light Detectors (5) -- Photoconductive Detectors  Characteristics of a typical CdSe photoconductive cell: (a) resistance versus illuminance, and (b) spectral response. (a) (b)

Semiconductor Light Detectors (6) -- Photodiodes  Photodiodes convert light energy (infrared, visible, or ultraviolet) into electrical energy.  Large area photodiodes that are designed for energy conversion are called Photovoltaic Cells or Solar Cells.  Photodiodes are made from semiconductors, e.g., Si, Ge, GaAs, and InGaAs.  In order for a photodiode to detect a photon, the photon energy (hf) must (usually) be greater than or equal to the semiconductor energy gap (E g ), i.e., hf  E g

Semiconductor Light Detectors (7) -- Photodiodes  Part of a photodiode consists of n-type material, meaning material to which specific impurities have been added so that there are free (negative) electrons to carry an electric current.  Part of a photodiode consists of p-type material, meaning material to which specific impurities have been added so that there are free “holes” to carry an electric current. Holes are electron vacancies and behave like positive charges.  The junction between these two types of material is called a p-n junction.

Semiconductor Light Detectors (8) -- Photodiodes  The structure of a silicon photodiode (cross section, not to scale): n-type silicon p-type silicon p-n junction cathode (metal) anode (metal) antireflection coating

p-n junction behavior p type n type Donor atom e.g. arsenic junction Acceptor atom e.g. indium + hole excess e -. p n silicon atom Charge migration sets up junction voltage

p-n junction behavior, cont’d hole excess e -. p n photon photoelectron - hole When an incident photon liberates an electron anywhere, the p-n junction field will cause the photo-electron to move across the junction from the p side to the n side. Likewise, the hole vacancies will migrate in the n-p direction.

Semiconductor Light Detectors (9) -- Photodiodes  Photons absorbed near the p-n junction generate electron-hole pairs, which are separated by the electric field at the junction.  These moving charges constitute a current in the resistor.  Optical energy is converted to electrical energy.  Energy Band Diagram: hf p n photocurrent Electron Energy - V photoelectrons vacant holes

Silicon Photodiodes ä Planar diffusion type silicon photodiodes are perhaps the most versatile and reliable sensors available. ä The P-layer material at the light sensitive surface and the N material at the substrate form a P-N junction - operates as a photoelectric converter, generating a current that is proportional to the incident light. - Silicon cells operate linearly over a ten decade dynamic range, and remain true to their original calibration longer than any other type of sensor. ä Silicon photodiodes are best used in the short-circuit mode, with zero input impedance into an op-amp. The sensitivity of a light-sensitive circuit is limited by dark current and Johnson (thermal) noise. VoVo R2R2 R1R1   

Semiconductor Light Detectors (10) -- Photodiodes  (a) Typical silicon photodiodes, and (b) symbol. (a) (b) I + - V p n Can manufacture in arrays for imaging applications

Can tailor wavelength sensitivity by choice of materials

Selecting a photodetector ä Sensitivity to the band of interest is a primary consideration when choosing a detector. ä You can control the peak responsivity and bandwidth through the use of filters, but you must have adequate signal to start with. Filters can suppress out of band light but cannot boost signal. ä The sensor should be blind to out of band radiation. If you are measuring solar ultraviolet in the presence of massive amounts of visible and infrared light, for example, you would select a detector that is insensitive to the long wavelength light that you intend to filter out.  Lastly, linearity, stability and durability are considerations. Some detector types must be cooled or modulated to remain stable. High voltages are required for other types. In addition, some can be burned out by excessive light, or have their windows permanently ruined by a fingerprint.