Application of photodiodes - PowerPoint PPT Presentation

1 / 11
About This Presentation
Title:

Application of photodiodes

Description:

Application of photodiodes A brief overview Diode devices Check valve behavior Diffusion at the PN junction of P into N and N into P causes a depleted non-conductive ... – PowerPoint PPT presentation

Number of Views:184
Avg rating:3.0/5.0
Slides: 12
Provided by: bioenOkst
Category:

less

Transcript and Presenter's Notes

Title: Application of photodiodes


1
Application of photodiodes
  • A brief overview

2
Diode devices
  • Check valve behavior
  • Diffusion at the PN junction of P into N and N
    into P causes a depleted non-conductive region
  • Depletion is enhanced by reverse bias
  • Depletion is broken down by forward bias
  • When forward biased
  • High current flow junction voltage
  • When reverse biased
  • Very low current flow unless above peak inverse
    voltage (PIV) (damaging to rectifying diodes, OK
    for zeners)

3
Quantum devices
  • Absorption of a photon of sufficient energy
    elevates an electron into the conduction band and
    leaves a hole in the valence band.
  • Conductivity of semi-conductor is increased.
  • Current flow in the semi-conductor is induced.

4
Photodiode structure
Absorbtion in the depletion layer causses current
to flow across the photodiode and if the diode is
reverse biased considerable current flow will be
induced
5
Photodiode fundamentals
  • Based on PN or PIN junction diode
  • photon absorption in the depletion region induces
    current flow
  • Depletion layer must be exposed optically to
    source light and thick enough to interact with
    the light
  • Spectral sensitivity

6
Photodiode characteristics
  • Circuit model
  • I0 Dark current (thermal)
  • Ip Photon flux related current
  • Noise characterization
  • Shot noise (signal current related)
  • q 1.602 x 1019 coulombs
  • I bias (or signal) current (A)
  • is noise current (A rms)
  • Johnson noise (Temperature related)
  • k Boltzmans constant 1.38 x 1023 J/K
  • T temperature (K)
  • B noise bandwidth (Hz)
  • R feedback resistor (W)
  • eOUT noise voltage (Vrms)

7
Photodiode current/voltage characteristics
8
Trans-impedance amplifier function
  • Current to voltage converter (amplifier)
  • Does not bias the photodiode with a voltage as
    current flows from the photodiode (V1 0)
  • Circuit analysis
  • Note current to voltage conversion

9
Diode operating modes
  • Photovoltaic mode
  • Photodiode has no bias voltage
  • Lower noise
  • Lower bandwidth
  • Logarithmic output with light intensity
  • Photoconductive mode
  • Higher bandwidth
  • Higher noise
  • Linear output with light intensity

10
For the photovoltaic mode
  • I thermal component photon flux related
    current
  • where
  • I photodiode current
  • V photodiode voltage
  • I0 reverse saturation current of diode
  • e electron charge
  • k Boltzman's constant
  • T temperature (K)
  • n frequency of light
  • h Planks constant
  • P optical power
  • h probability that hv will elevate an electron
    across the band gap

11
Circuit Optimization
  • Burr-Brown recommendations (TI)
  • Photodiode capacitance should be as low as
    possible.
  • Photodiode active area should be as small as
    possible so that CJ is small and RJ is high.
  • Photodiode shunt resistance (RJ ) should be as
    high as possible.
  • For highest sensitivity use the photodiode in a
    photovoltaic mode.
  • Use as large a feedback resistor as possible
    (consistent with bandwidth requirements) to
    minimize noise.
  • Shield the photodetector circuit in a metal
    housing.
  • A small capacitor across Rf is frequently
    required to suppress oscillation or gain peaking.
  • A low bias current op amp is needed to achieve
    highest sensitivity
Write a Comment
User Comments (0)
About PowerShow.com