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Lecture 7: Electrostatics and IV Characteristics of P-N DiodeECE5590: Nanoscale Devices and circuits
Mostafizur [email protected]
• R-G, Drift, Diffusion• Continuity Equation• P-N Junction
Recap
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Outline
• P-N Junctions• Drawing Band Diagram• Solution in Equilibrium• Non-Equilibrium Conditions (I-V)• Summary
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P-N Junction Devices..
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P and N doped Materials
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Time < 0
P-type piece N-type piece
Time < 0: Pieces separated
At time = 0, slam the two pieces together
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At time = 0, slam the two pieces together
Hole gradient
Jp, diffusion = -qDp dp/dx = current right, holes right
Electron gradient
Jn,diffusion = -qDn dn/dx = current right, electrons right
Gradients drive diffusion
left
E
E
Depletion Region
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Depletion Approximation
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Electrostatics
• Poisson’s Equation
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Drawing Band diagram
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Electric Field is Discontinuous for Heterojunction
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Homojunction
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Built-in Potential (Vbi)
How wide is the depletion region?
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Depletion Approximation-step junction
x
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Solution for E
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Solution for E
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Solution for V
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Depletion Width
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I-V Characteristics
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Applying a Bias
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Depletion Width
• Would current flow? Diffusion/Drift?
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Effect of Bias
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I-V Curve for Ideal Diode
qkT
V
eII VVA
0
/0 )1( 0
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Ideal P-N Junction Diode
Assumptions:
• Steady-State conditions• Non-degenerate doping• One-dimensional• Low Level Injection• Only drift, diffusion,thermal R-G (no photons)
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Applications
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Solar Cells
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Solar Cells
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Solar Cells
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Summary
• P-N diodes are simplest semiconductor devices
• Operation lies in fundamental of physics• Useful for various applications