Photoconductors

1
Photoconductors

• Rajeev V Shenoi
• Centre for High Technology Materials
• University of New Mexico

2
Photodetectors

• Convert Optical energy to electrical energy.
• Parameter changes by light detection.

3
Classification

• Extrinsic
• Lower energy than bandgap
• Interband transitions
• Trap states

• Intrinsic
• energy equal to bandgap
• Band to band transitions

• Photvoltaic
• Potential barrier to separate generated EHP
• Asymmetric characteristics

• Photoconductive
• Photogeneration of carriers.
• Change in conductivity.

4
Quantum Efficiency

• Number of Independent electons per photon
• Excludes gain mechanism
• Depends on absorption
• Upper and lower cutoff exhibited

5
Responsivity

• Current produced per unit radiation incident

• Linear function of wavelength
• Upper cutoff Bandgap of Semiconductor
• Lower cutoff High absorption

6
Noise Sources
Johnson Noise Shot Noise Generation-Recombinatio
n Noise Temperature Fluctuation Barkheusen
Noise 1/f Noise
7
Johnson Noise

• Thermal Motion of Carriers
• Contributes to dark current
• Gaussian Statistics
• White Spectral Density
• Dominant source at High Frequency
• Varies as square root of Detector Area

8
Shot Noise

• Stream of electrons generated at random times
• White spectral density
• Gaussian Statistics
• Correlated with Signal Current
• Varies as square root of Detector Area

9
Shot Noise

• Fluctuations in Generation and Recombination
  Process.
• Dominant in Low Bandgap structures
• Low temperature operation

Temperature Noise

• Changes in the temperature of the material
• Important in Thermal Detectors.

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Flicker Noise (1/f noise)

• Due to ohmic contacts at electrodes
• Surface traps
• Dominant at low frequencies

Barkheusen Noise (Popcorn Noise

• Impact ionization of neutral impurities.
• Due to defect in lattices and contacts.
• Observed as a spike in the output

Total Noise
11
Signal To Noise Ratio
Noise Equivalent Power

• Signal power when SNR1
• Measure of Sensitivity of Detector
• Lower NEP is better.
• Depends on Detector Area and Bandwidth

12
Detectivity

• Intrinsic Figure of Merit
• Normalized for Detector Area and Bandwidth
• Higher Detectivity is better
• Linear relationship with wavelength

Noise Equivalent Irradiance

• Input flux density when SNR1

13
Noise Equivalent Temperature Difference

• Measure of Sensitivity for FPA
• Temperature Difference equivalent to SNR1
• Smaller NETD is better
• Depends on and
• No information on spatial resolution

rms Noise
Signal measured
14
Minimum Resolvable Temperature Difference

• Combination of Thermal and Spatial Sensitivity

• Low Frequencies Thermal Sensitivity
• High Frequencies- Spatial Sensitivity
• Thermal Sensitivity as function of Spatial
  Frequency
• Higher MRTD is better.

15
What is the maximum Responsivity possible at 8?m?
Assume
16
Thank You