Nano-optical/ Magnetic/ Electronic Materials and Uses

By Mona Kumari|Updated : June 17th, 2021

Photoconductive Material         

Let us consider a photo conducting slab. It is simply a light-sensitive semiconductor material with ohmic contacts on both ends.

 

Photoconductive Material         

Let us consider a photo conducting slab. It is simply a light-sensitive semiconductor material with ohmic contacts on both ends.

 

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       Photo-conducting material

When the material is illuminated with photons of energy E ≥ Eg electron hole pairs are generated, and the electrical conductivity of the material increases. Where Eg is the bandgap energy of the semiconductor material given by-

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Where λ is the wavelength of the incident photon.

Let I0 be the intensity of monochromatic light falling normally onto the slab. Then the intensity of transmitted light I is given by-

 I = I0 exp (- α D)

Where α is the absorption coefficient of the material and D is the thickness of the slab. Let L and B be the length and breadth of the photoconductive slab respectively. Also let us assume that the slab absorbs the entire light falling on it.

Now the light energy falls on the sample per sec is given by I0BL where I0 is the light energy falling per second on unit area of the slab. Therefore, the number of photons falling on the photoconductor per second

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Let η- be the quantum efficiency of the absorption process. It is nothing but the fraction of incident energy absorbed. Therefore the number of photons absorbed per second

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Now the average generation rate of charge carriers is given by-

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Under illumination the conductivity will increase by an amount is

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When a voltage is applied to the contacts, electrons and holes move in opposite directions resulting in a photocurrent given by

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The quantum efficiency of a photoconductor device is defined by the term photoconductor gain G.

Photoconductive gain

It is defined as the ratio of rate of flow of electrons per second to the rate of generation of electron hole pairs within the device.

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Rate of generation of electron hole pairs = rgBLD

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The photoconductive gain G can be increased by increasing the voltage V and decreasing the length, L of the device.

The photoconductive gain can also be defined as the ratio of the minority carriers life time τc and the transit time t.

Characteristics of photoconductive materials

They are:

i) High spectral sensitivity in the wavelength region of interest

ii) Higher quantum efficiency

iii)  Higher photoconductive gain

iv) Higher speed of response and

v) Lesser noise

PHOTOCONDUCTIVE MATERIALS

1.   Cadmium sulfide (CdS) and Cadmium selenide (CdSe)

These are highly sensitive in the visible region of radiation. They have high photoconductive gains (103 to 104) but the poor response time (about 50 ms). The response gets reduced at higher illumination levels indicating the presence of traps.

2.   Lead sulfide (PbS)

It has spectral responsitivity from 1 to 3.4 μm and hence very much suitable for fabricating near-infrared detectors. It has maximum sensitivity in the region of 2 μm with a typical response time of about 200 μs.

3.   Indium antimonide (InSb)

These detectors have wavelength response extending out to 7 μm and exhibit response times of around 50 ns.

4.   Mercury cadmium telluride (HgxCd1-x Te)

This is an alloy composed of the semi-metal HgTe and the semi-conductor CdTe. Semi-metals have overlapping valence and conduction bands. Depending on the composition of the alloy, a semiconductor can be formed with a bandgap varying between zero and 1.6eV. Correspondingly the detector sensitivities lie in the range 5 to 14 μm. Photoconductive gains of up to 500 are possible.

APPLICATIONS OF PHOTOCONDUCTIVITY DEVICES

  1. Light meters
  2. Infrared detectors
  3. TV cameras
  4. Voltage regulator
  5. Relays
  6. Detecting ships
  7. Air crafts

PHOTODIODES

Semiconductor light sensors that generate current or voltage when PN junction is illuminated by light. The cut off wavelength is given by:

λc = 1240/Eg

Eg - the bandgap energy(eV)

 Photodiodes are mostly constructed using Si, Ge, Lead Sulphide.

Photodiode Types

  1. PN Photodiode
  2. PIN Photodiode
  3. Schottky Photodiode
  4. Avalanche Photodiode

1. PN Photodiodes

Light incident generates electrons in the conduction band P-type material.

Holes in the valence band of N-type material.

Photodiode Reverse biased

  • Photoinduced electrons move down the potential hill from P to N side
  • Photoinduced holes add to the current flow from P to N side.

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           PN Photodiode

2. PIN Photodiode

  • High Resistance intrinsic layer added between P and N layer.
  • Reduces diffusion time of electron-hole pairs
  • Improves response time.
  • Suitable for high-speed photometry.

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                  PIN Photodiode

3. Schottky Photodiode

  • The thin gold coating is sputtered onto the N material.
  • Schottky photodiode has enhanced ultraviolet response.

4. Avalanche Photodiode

  • High speed, high sensitivity.
  • Constructed to provide a uniform junction that exhibits avalanche effect at reverse bias.
  • Electron-hole pairs generated by incident photons
  • Excellent SNR (Signal to Noise) ratio.

SOLAR CELLS

  • The operating principle of solar cells is based on the photovoltaic effect
  • Solar cell when exposed to sunlight open circuit voltage is generated.
  • Open circuit voltage leads to the flow of current through a load resistor connected across.
  • Incident energy leads to the generation of electron-hole pair.
  • Electron-hole pairs recombine or drift.
  • Accumulation of positive and negative charge carriers constitute open-circuit voltage.

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LIGHT-EMITTING DIODES (LED)

  • LED is a semiconductor PN junction diode designed to emit light when forward-biased.
  • It is one of the most popular optoelectronic sources.
  • LEDs consume very little power and are inexpensive.

 

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