Lecture 32

Lecture 32 - EEE 352: Lecture 32 Semiconductor Optical...

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EEE 352: Lecture 32 Semiconductor Optical Detectors Cooled, quantum well infrared photodetector in camera mount. Photoconductivity Photodiodes Other approaches Light Amplification by Stimulated Emission of Radiation: The Laser * The laser * Transitions between atomic levels Momentum conservation Metastable states * Stimulated emission of radiation * Laser operation An application of lasers
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p-n diodes also have important OPTOELECTRONIC applications SOLAR CELLS use photons to create ADDITIONAL electron-hole pairs by exciting electrons into the conduction band * A current is produced by this PHOTOVOLTAIC EFFECT The created charges move TOWARDS the junction interface And so essentially FORWARD BIAS the diode Optoelectronic Applications 0.5 μ m LIGHT BACK CONTACT p-TYPE n-TYPE TOP CONTACTS PHOTONS WITH ENERGY GREATER THAN THE BAND GAP ARE ABSORBED IN THE n-TYPE REGION CREATING ELECTRON-HOLE PAIRS THESE CARRIERS THEN DRIFT TO THE TOP AND BOTTOM CONTACTS AND SO CREATE AN ELECTRICAL CURRENT FLOW
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Photo Induced Carriers J x , E x z y When we shine light upon the semiconductor, we create EXCESS CARRIERS , and In optical GENERATION of excess carriers, the number of extra electrons is equal to the number of extra holes: new electron new hole photon induced transition The photo-generated density decays away from the surface—the number of photons decreases into the material as they are absorbed to create electron-hole pairs .
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Excess Carriers and Diffusion y Δ n The spatial variation of the excess density leads to a current flow— DIFFUSION CURRENT . We can follow two approaches here: In one, we make the thickness d << L (the diffusion length) and measure the photo- induced current through this thin slab. In the second, we make d > L , and measure the induced voltage between the front and back surface when no current flows.
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d << L Photoconductivity J = ( n 0 + Δ n ) e μ e + ( p 0 + Δ p ) e h [ ] E z ~ J 0 + J ph J 0 = ( n 0 e + p 0 h ) eE z J ph = e Δ n ( e + μ h ) E z x z E z J z Measurement consists of measuring the photo-induced current in the external circuit.
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Surface Photovoltage y V ph We constrain the device to have no current flow, particularly in the y -direction. Photo-generated charge flows from the illuminated surface into the semiconductor until a voltage builds up to oppose this diffusion. We measure the voltage. y Δ n
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Surface Photovoltage J y = e μ h + μ e ( ) Δ p + n 0 e e [ ] F y + e D e D h ( ) ∂Δ p y The current will flow in response to built-in fields and the density gradient: We now require that this current be zero:
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Lecture 32 - EEE 352: Lecture 32 Semiconductor Optical...

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