Pin

Photodetector Diodes

Marjan Alavi School of Electrical Engineering Sharif University of Technology

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Outline Intoduction APD PIN Diodes IIMPATT Devices Applications References

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Photodiode Basic Principle

O-E Converter The Electron-Hole Pair Give Rise to an Electrical Current Conduction Band

Electron

ΔE
Absorption

E=hc/λ Photon

Hole

Valence Band

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Photodetectors P-I-N photodiodes: Intrinsic

layer of semiconductor between the p-doped and n-doped side to extend the usable area to receive photons

Avalanche Photo Diode (APD): strongly

biased (reverse biasing) pn diode that creates many electron-hole pairs per each photon received; Aplifies the signal :improved sensitivity

http://hepunx.rl.ac.uk/BFROOT/

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General Principles The photogenerated current, Iph, increases linearly with:  The

optical power Magnitude  Ratio of Iph to the incident optical power, Φ: responsivity, ℜ of the photodiode.

 The

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The Ideal Photodiode All the incident light would be absorbed The quantum efficiency would then be unity Minimizing reflection at the incident surface; Maximizing the absorption within the depletion layer Avoiding recombination before the carriers are collected. 6

Basic Photodiode Configuration  Good

linearity  Low noise  High frequency response  Easy to manufacture

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PIN Photodiode Absorptive p InP

i

n

InGaAs

InP

Optical Input

Transparent

VR bock-bock.cisco.com/~abarbier

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Response Time CR time constant of the diode capacitance and its load Carrier transit time within the diode local recombination time Bandwidths exceeding 1GHz are practical. 9

Quantum Efficiency Quantum efficiency, η, for a given device: 



the ratio between Iph, measured in electrons per second and the incident optical flux, measured in photons per second. η = Iph/e ÷ Φ/εph = (Iph/Φ).(hc/eλ) = ℜ. (hc/eλ)

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Bandwidth Bandwidth = 1/ ttr

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Noise

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Singal to Noise Ratio

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PIN Sample P+ layer: 103nm (1/4 l ) I-layer: 2µm 11(1/2 l), h=0.99 N- layer: 2µm 22(1/4 l) Responsivity: 1.04A/W Response time: 28.6 psec Minimum detectable signal: 250W/cm2 14

APD principle of operation Incident photons generate charge carriers in the depletion region Accelerated to high speeds by an applied electric field Ionize atoms within the avalanche region

http://optics.org/articles

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APD Application High sensitivity at high-speed operation:  optical

communications

http://optics.org/articles 16

APD optimum operating

point Detector's signalto-noise ratio is APDs are the preferred choice if: 



light levels are limited (microwatts or nanowatts) A fast response (up to gigahertz) is required.

http://optics.org/articles

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IMPATT Diode Impact Avalanche And Transit Time diode Operates in reverse-breakdown (avalanche) region Applied voltage causes momentary breakdown once per cycle This starts a pulse of current moving through the device Utilizes impact ionization and transfer time delay to generate negative resistance

Frequency depends on device

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IMPATT Structure High density next to the junction reduce E from 300500 for avalanche to 15 KV/cm for Vsat in drift region P+ n n+ or P+ nin+

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Static Characteristics High Electric field (E=3 – 6 e5 V/cm) Avalanche multiplication Generating E/H pairs Drift velocity Negative resistance

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Satic Characteristics 

with proper choice of doping and region width, avalanche region can be entirely contained within region with doping N1 21

Dynamic Characteristics Avalanche region introduces phase shift of π /2 in the injected carrier density For π<ωt<2π, carriers drift across to n+-contact Carriers will drift at saturation velocity Drift region transit time introduces an additional delay Negative resistance characteristic is observed

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IMPATT Ossilator

http://www.ecn.purdue.edu/WBG/Device_Research/IMPATT_Diodes/ 23

Critical Field

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Drift Velocity

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Ionization rates The number of electron-hole pairs generated by an electron/hole per unit distance traveled.

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Limitations High noise oscillation frequency 

Limitations of semiconductor materials

Critical field, at which avalanche breakdown occurs. 

Vmax = EcW

Saturation velocity which is the maximum attainable velocity in semiconductors 27

Noise - Microwave Devices

IMPATT

BARITT

TED GaAs

TED InP

TUNNEL

MESFET

Bipolar

Noise Figure in dB

35.0 30.0 25.0 20.0 15.0 10.0 5.0 0.0

Devices

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Applications Single Diode Waveguide Switch Traditional electronic sources of THz light Receivers Source of CW Electromagnetic vawe

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Applications • Laser diode transmitters on Port Cards inside detector Total 570 transmitters 128 Port Cards, • PIN diode receivers on FTM board out in VME crates

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References Physics of semiconductor Devices, S. M. Sze, Bell Lab www.ctr.kcl.ac.uk/lectures/Johnk www.bock-bock.cisco.com www.ecn.purdue.edu Microwave devices, quantum effect, and hot-electron devices, Catlech Watson Lab presentation

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