Lecture 15 Semiconductor Devices:
Depletion Layer:
n-type: Assume the surface of an n-type semiconductor
has been negatively charged. The free electrons near the
surface will be repelled. Thus, the region near the
surface has less free electrons than the interior
→ Depletion layer (space-change region)
Band diagram
for an n-type semiconductor with negatively charged
surface.
The depletion layer is a potential barrier for electrons.
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Band diagram for a p-type semiconductor with
positively charged surface.
Metal semiconductor contacts:
n-type metal
1. Schottky Rectifier
2. Ohmic contact
𝜑𝑀 > 𝜑𝑠
𝜑𝑀 < 𝜑𝑠
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Rectifying contacts (Schottky barrier contacts):
n-type
𝜑𝑀 > 𝜑𝑠
Band diagram for a metal and n-type semiconductor
𝜑: 𝑤𝑜𝑟𝑘 𝑓𝑢𝑛𝑐𝑡𝑖𝑜𝑛
𝜑𝑀 > 𝜑𝑠
If the metal and semiconductor are brought into
contact, electrons flow from the semiconductors down
into the metal until the Fermi energies of both sides are
equal.
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Look to the following drawing
Fig 9-16 on book
Thus, the metal will be charged negatively and the
energy band in the semiconductor will be lowered. In
equilibrium, electrons from both sides cross the
potential barrier
→ Diffusion Current
Contact potential:
The potential barrier for the electrons diffusing from
the semiconductor into the metal
𝜑𝑀 − 𝜑𝑠
Electron affinity (X):
From the bottom of the conduction band to the
ionization energy.
Drift current:
When an electron hole pair is thermally created in or
near the depletion layer. The excited electrons in the
conduction band is swept down the barrier, and so the
holes “the drift current”
Note: the drift current is very small specifically for large
band gap.
Total current=diffusion current + drift current
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How Schottky diodes work??
“metal and n-type” connected to D.C source
Reverse Bias: The metal is connected to the negative
terminal, the electrons in the semiconductor will be
repelled, the depletion layer, one potential barrier
increases, no diffusion current (negligible)
However, the drift current doesn’t depend on voltage.
Forward Bias: Metal is connected to the positive
terminal of the battery, the potential barrier of
semiconductor decreases “becomes narrower”
Characteristics Curve of Schottky Diodes
Ohmic Contact (Metallization)
Similar to metal-metal contact, the (I-V) curve is linear
(obeys Ohm’s Law)
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Ohmic contact can occur in metal-semiconductor
contact for the following cases:
- metal-n type (if 𝜑𝑀 < 𝜑𝑠 )
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look to the figure (9-18) from your textbook
- metal-p type (if 𝜑𝑀 > 𝜑𝑠 )
Metal p-type:
Similar
- Example: Al-Si p type
-
Metal n-type:
Electron flows from the metal into the semiconductor
and the bands of semiconductor bend downward (no
barrier) electron flows in the two directions. This
configuration allows the current to flow into and out of
the semiconductor without power loss.
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p-n junction (diode):
Similar potential barrier to reaction is formed
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Electrons flow from the higher level (n-type) to the ptype
The n-type loses electrons to the p-type.
- The p-type loses holes to the n-type
- The n-type becomes negatively charged at the
junction
- The p-type becomes positively charged at the
junction
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- An electric field is formed at the junction
This proceeds until equilibrium and both Fermi
energies are at the same level
Note: The bands move up to the p-side and down to the
n-side
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Lecture 15a: Continue semiconductor devices
Let’s continue on p-n junction
p-n junction (Diode):
Semi potential barrier to reaction is formed
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Electrons flow from the higher level (n-type) to the ptype
The n-type loses electrons to the p-type.
 The p-type loses holes to the n-type
 The n-type becomes positively charged at the
junction
 The p-type becomes negatively charged at the
junction.
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 An electric field is formed at the junction
This proceeds until equilibrium is reached and both
Fermi energies are at the same level.
Note: The bands move up to the p-side and down to the
n-side
After the potential barrier is created, (at equilibrium):
At the conduction band, electrons from n-region find a
potential barrier that reduces its diffusion to p- region.
Electrons in p-region can easily diffuse down the
potential barrier to n-region, however their number is
small (they are only due to excitation). Thus, the
number of electrons crossing the junction in both
directions is equal.
Applying external potential to (p-n) junction
We will obtain similar effects to Schottky diode
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Look to the next page for more details
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Breakdown (Avalanche and Zener Diode)
Look to this phenomena, it occurs for the p-n diode
when the reverse voltage increases more than a certain
value (critical value). Its break down, it occurs for
avalanching or tunneling (Zener diode)
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-Uses for Zener Diode
Photo diode (solar cell)
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Consists of p-n junction. If light falls on or near the
depleted layer, electrons are lifted from the valence
band to the conduction band (electron-hole pairs are
created).
Let’s look to the band diagram
The electrons will move to the n-region and the holes
will move to the p-region.
- we can detect these charge carriers in two ways:
1. Open Circuit (photovoltaic mode of operation)
An external potential will appear
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2. Short Circuit the device (photo-conductive
mode of operation)
An external current will flow
The p-region is made very thin (≈ 1nm), so the light can
reach the depleted layer.
Note: Defects play a serious role
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Avalanche Photodiode:
p-n photo diode operated in a high reverse bias-mode
(near break down voltage)
Light→ create electron-hole pairs → are accelerated
through the depleted region to high velocity → ionize
lattice atoms and generate more hole-electron pairs =
photo current gain
Tunnel Diode:
Let’s discuss first “degenerate semiconductors”
At very high level doping, the Fermi level moves up into
the conduction band in the n-type material and moves
down to the valence in p-type material.
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The photo diode is degenerate p-type and degenerate ntype semiconductor. Look to the following figure:
Another phenomena: Electrons can tunnel through the
potential barrier in both directions. In equilibrium the
net tunnel current is zero.
Look to the Fermi energy in every mode of operation in
the next figure and look to the resultant I-V
characteristic curve of tunnel diode
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Reverse Bias:
Potential barrier is increased the Fermi energy in the parea is raised. Electrons flow from p-type to n-type.
Forward Bias (small):
Potential barrier is decreased. Electrons flow from ntype to p- type
Forward Bias (Medium):
The area opposite (facing) the filled conduction band is
forbidden the current decreases
Forward Bias (normal):
Electrons in the conduction band get enough energy to
climb the potential barrier of p-side
Transistors
The most important electronic device
Bipolar transistor
Unipolar transistor (field effect transistor)
Bipolar transistor: two junctions
n-p-n transistor
p-n-p transistor
Bipolar transistor: carriers are majority and minors
current control
Field effect resistor: carriers are only majority voltage
control
Bipolar Junction Transistor (n-p-n)
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Band diagram of an unbiased n-p-n biopolar junction
transistor
E: emitter
B: base
C: collector
Note: The base is thin compared to the emitter and
collector
Also,
Emitter→ ℎ𝑒𝑎𝑣𝑖𝑙𝑦 𝑑𝑜𝑝𝑒𝑑
Base → 𝑙𝑖𝑔ℎ𝑡𝑙𝑦 𝑑𝑜𝑝𝑒𝑑
Collector→ 𝑙𝑖𝑔ℎ𝑡𝑙𝑦 𝑑𝑜𝑝𝑒𝑑
It can work as:
 Signal amplifier
 Switch
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Signal Amplification:
Emitter and base diode: forward biased
Base-collector diode: reverse biased
Look to the figure of:
“Biasing an n-p-n transistor”
- Because of forwarding biasing the E-B diode, the
barrier will be reduced, a large electron flow to the
base.
Why the base is thing and lightly doped?
Reverse biasing the base-collector
→ Causes the electrons to accelerate down to the
collector
Switching
Base voltage can stop the electron flow from the emitter
to collector
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n-p-n
transistor
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