Ch (2)
Semiconductors Diodes
SPECIAL – PURPOSE DIODES
Pn junction diode F widely used F in rectifying Û
waveshaping functions
Other diodes F similar in F structure F quite
different F in external characteristics
Tunnel Diodes
Increasing doping concentrations F increasing the
concentration of uncoverd charges F the width ot
the depletion region Û the potential barrier F are
decreased
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Ch (2)
Semiconductors Diodes
If the doping F greatly increases F the depletion
region F less than 10 nm wide F produced new
conduction mechanism Û V-I characteristic changed
Leo Esaki (1958) F quantum mechanics theory
indicates F for very thin potential there is F finite
probability F electron tunnels F through the barrier
without possessing enough energy to climb over it
Esaki diode V-I characteristic F shows
™ Tunneling effect F solid colored line
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Ch (2)
Semiconductors Diodes
At V F well below threshold for normal forward
current F electrons tunnel F from n to p region F if
there are available holes F electrons become free
IT F increases with V until the available hole
reduced
IT
becomes Ip F I decreases with increased v F
injection current F dominates
Peak current Ip Û valley current Iv F stable operating
points
Tunneling F wave phenomenon
Tunnel Diodes applications
Electron transfers with Speed of light
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Ch (2)
Semiconductors Diodes
Switching between Ip Û Iv F very fast for computer
applications
The region between Ip
Û Iv F negative resistance F
it can be used as a very high-frequency oscillator
Metal-Semiconductor Diodes
Ohmic contact F provides easy flow of current ¾
without creating additional VB
Aluminum in contact with Si F acts as p-type
impurity ¾
Hole current F flows easily by recombination with
electrons supplied by the external circuit
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Ch (2)
Semiconductors Diodes
Aluminum in contact with n-type material create F
rectifying contact F provide low-potential transition
F between semiconductor Û metal
Rectifying contact differs from pn junction in two
respects
1. forward V of schottky diode F ≅ 0.5 V of pn junction
F at the same current
2. Al-n diode F has only majority carriers ¾
Switching is very fast F no wait for the recombination
Schottky diode F useful for IC
Photodiodes
Radiant energy used to createF electron-hole pairs
F near the junction
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Ch (2)
Semiconductors Diodes
Under reverse-biased conditions F light-injected
minority carriers F cross the junction F Is increases
F fraction of mA ∝ to the total illumination
The ratio ofF illuminated current to the dark current
F very high
Photodiodes F used to the
Conversion F solar energy to electricity
Light-Emitting Diodes (LED)
Reversible process F energy released F at holes Û
electron recombines
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Ch (2)
Semiconductors Diodes
Defect in crystal F traps F moving electron or hole
F absorb its energy F hold it until another
recombination comes along
recombination F occasionally in F Si Û Ge F
frequently in F III-V compound
In gallium-arsenide F electron drops directly into a
hole F photon of energy F generated
Gallium-arsenide junction F generates radiation in
the visible range F (LED)
Gallium-arsenide junction F used as F junction
laser
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