Lecture 2
: Semiconductors -Basics
• 도체, 부도체, 반도체(Conductors, Insulators, Semiconductors)
• 실리콘 결정 (Silicon crystals)
• 진성반도체 (Intrinsic semiconductors)
• 두 유형의 흐름 (Two types of flow)
• 반도체의 도핑 (Doping a semiconductor)
• 두형의 외인성 반도체 (Two types of extrinsic semiconductors)
• 바이어스되지 않은 다이오드 (Unbiased diode)
• 순 / 역 방향 바이어스 (Forward / Reverse bias)
• 항복현상 (Breakdown)
• 에너지 준위/ 언덕 (Energy levels / hills)
• 전위장벽과 온도 (Barrier potential & Temperature)
• 역방향 바이어스된 다이오드 (Reverse-biased diode)
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Bohr model of an atom
As seen in this
model, electrons
circle the nucleus.
Atomic structure
of a material
determines its
ability to conduct
or insulate.
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Bohr model of an atom (cont’d)
.The orbit paths of the electrons surrounding a nucleus are called “shells (각)”.
.Each shell has a defined number of electrons it will hold. This is a fact of nature and can
be determined by the formula :
Ne = 2n2 ; n = 1, 2, 3, 4,… for K, L, M, N,… shell
.The outer shell (최외각) is called the “valence shell (가전자각)”.
.The less complete a shell is filled to capacity, the more conductive the material is.
⇔
.최고 에너지 준위를 가지는 전자는 원자의 최외각에 존재함.
.가전자각에 있는 전자를 “가전자 (valence electron)”이라고 함.
.가전자는 상대적으로 원자에 대한 구속력이 약하므로 물질 내에서 화학적 반응/결합에
관여하여 물질의 특성을 결정함.
.가전자가 충분한 에너지를 얻게 되면 최외각으로 부터 이탈 : “이온화(ionization)”
.이탈된 가전자 = “자유전자 (free electron)”
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A Copper atom has only 1
electron in its valence(가전대)
ring. This makes it a good
conductor. It takes 2n2
electrons or in this case(n=4)
32 electrons to fill the valence
shell.
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반도체(Semiconductor)
-도체 (conductor)와 절연/부도체(insulator) 의 중간적 성질을
가진 원소
-가장 좋은 도체 : 가전자 1개만 가짐
-가장 좋은 절연체 : 가전자 8개를 가짐
- Semiconductors typically have 4 valence electrons
: Silicon (Si), Germanium(Ge)
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Core diagrams for copper & silicon
One valence electron
Copper
+1
Four valence electrons
Silicon
+4
The nucleus + the inner electron orbits
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Silicon atoms in a crystal share electrons.
Valence saturation: n = 8
Because the valence electrons are bound, a silicon
crystal at room temperature is almost a perfect insulator.
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Inside a silicon crystal
• Some free electrons & holes are created by
thermal energy.
• Other free electrons and holes are recombining.
• Recombination varies from a few nanoseconds
to several microseconds.
• The time between creation & recombination of
a free electron and a hole is called the life time.
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자유전자(e) & 전공(h)의 생성(creation) /재결합(recombination)
그림2-5 (a) 열에너지가 전자와 전공을 생성한다. (b) 자유전자와 전공의 재결합
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Silicon crystals are doped to provide permanent carriers
Free electron
(n type semiconductor)
Pentavalent dopant
(비소(As), 안티몬(Sb), 인(P) 등)
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Hole
(p type semiconductor)
Trivalent dopant
(알루미늄(Al),붕소(B), 갈륨(Ga))
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This crystal has been doped with a pentavalent impurity.
minority carriers
majority carriers
The free electrons in n type silicon support the flow of current.
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This crystal has been doped with a trivalent impurity.
minority carriers
majority carriers
The holes in p type silicon support the flow of current.
Note that hole current is opposite in direction to electron current.
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Semiconductors in Summary
• The most popular material is silicon.
• Pure crystals are intrinsic semiconductors.
• Doped crystals are extrinsic semiconductors.
• Crystals are doped to be n type or p type.
• An n type semiconductor will have a few minority carriers
(holes).
• A p type semiconductor will have a few minority carriers
(electrons).
• A semiconductor can be doped to have an excess of free
electrons or holes
• The two types of doped semiconductors are n type and p
type
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Doping a crystal with both types of impurities
→ forms a pn junction diode.
Some electrons will cross the junction and fill holes.
→ A pair of ions is created each time this happens.
→ As this ion charge builds up, it prevents further
charge migration across the junction.
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The pn barrier potential (PN 전위장벽)
• Electron diffusion creates ion pairs called
dipoles(쌍극자).
• Each dipole has an associated electric field.
• The junction goes into equilibrium when the
barrier potential prevents further diffusion.
• At 25 degrees C, the barrier potential for a silicon
pn junction is about 0.7 volts.
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The pn barrier potential (PN 전위장벽)
Each electron that migrates across the junction and fills a hole
effectively eliminates both as current carriers.
P
N
Depletion layer
This results in a region at the junction that is
depleted of carriers and acts as an insulator.
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The Depletion Region & the pn barrier potential
.전기적 중성 영역:
총전하 = 0
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.공간전하 영역:
carrier없고 ion만 있는 영역
=“공핍층(depletion region)”
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The Depletion Region & The pn barrier potential
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Forward bias
The carriers move toward the junction
and collapse the depletion layer.
If the applied voltage is greater than
the barrier potential (=0.7V), the diode conducts.
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Reverse bias
The carriers move away from the junction.
The depletion layer is reestablished
and the diode is off.
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Diode bias
• Silicon diodes turn on with a forward bias of
approximately 0.7 volts.
• With reverse bias, the depletion layer grows
wider and the diode is off.
• A small minority carrier current exists with
reverse bias.
• The reverse flow due to thermal carriers is
called the saturation current.
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Energy Levels
• Extra energy is needed to lift an electron into a
higher orbit.
• Electrons farther from the nucleus have higher
potential energy.
• When an electron falls to a lower orbit, it loses
energy in the form of heat, light, and other
radiation.
• An LED is an example where some of the
potential energy is converted to light.
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The P side of a pn junction has trivalent atoms with a core
charge of +3. This core attracts electrons less than a +5 core.
Energy
Abrupt junction
Conduction band
Valence band
P-side
N-side
In an abrupt junction, the p side bands
are at a slightly higher energy level.
Real diodes have a gradual change from one material
to the other. The abrupt junction is conceptual.
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Energy bands after the depletion layer has formed
Energy
Conduction band
Energy hill
Valence band
P-side
N-side
To an electron trying to diffuse across the junction,
the path it must travel looks like an energy hill. It
must receive the extra energy from an outside source.
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Junction temperature
• The junction temperature is the temperature
inside the diode, right at the pn junction.
• When a diode is conducting, its junction
temperature is higher than the ambient.
• There is less barrier potential at elevated
junction temperatures.
• The barrier potential decreases by 2 mV for
each degree Celsius rise.
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Reverse diode currents
• Transient current occurs when reverse voltage
changes.
• IS, the saturation or minority-carrier current,
doubles for each 10 degree Celsius rise in
temperature. It is not proportional to reverse
voltage.
• The surface of a crystal does not have complete
covalent bonds. The holes that result produce a
surface-leakage current that is directly
proportional to reverse voltage.
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Homework
[HW #2] 다음 문제를 아래의 풀이과정 시작부를 참조하여 끝까지 풀어라.
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