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18.6 Semiconductors

주요 반도체 재료에서 전자와 홀의 mobility 대표적인 값은? 어떤 carrier가 빠른지?

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When a voltage is applied to a semiconductor, the electrons move through the conduction band, while the electron holes move through the valance band in the opposite direction.

[Fig. 18-16] 18.6 Semiconductors

+ -

반도체 재료에서 전자와 홀이 이동해서 전기전도에 기여하는 각각의 band의 이름은?

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Intrinsic semiconductor:

)2

exp()(

)()2(2

)2

exp(

)(pn :torsemiconduc Intrinsic

0

43**23

20

0

ii

TkE

qn

mmh

Tkn

TkE

npnn

nq

qpqn

B

gpe

pnB

B

gii

pei

pn

−+⋅=

=

−===

+⋅=∴=

⋅⋅+⋅⋅=

µµσ

π

µµσ

µµσ

18.6 Semiconductors

주요 공식들: Conductivities in Semiconductors

: 온도 의존성

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The distribution of electrons and holes in the valance and conduction bands (a) at absolute zero and (b) at an elevated temperature.

[Fig. 18-17] 18.6 Semiconductors

반도체에서 전기가 흐른다는 의미? 전자는 CB에서 정공은 VB에서 유동을 해야함!!

VB

CB

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The electrical conductivity versus temperature for intrinsic semiconductors composed with metals. Note the break in y-axis scale.

[Fig. 18-18] 18.6 Semiconductors

온도증가에 따른 전기전도도 변화: 금속과 반도체의 경향 차이 발생이유? 전기전도도의 절대값 차이

Example 18-7 참조

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Extrinsic Semiconductor

The conductivity depends primarily on the number of impurity, or dopants, atom, and, in a contain temperature range, is independent of temperature. [Fig. 18-20: Extrinsic range]

• Electrical neutrality: • p0 + Nd = n0 + Na

• n-type (Nd>>ni) : n0 ~ Nd

• P-type (Na>>pi) : p0 ~ Na

• The amount of dopants controls the conductivity of a semiconductor.

18.6 Semiconductors

Extrinsic semiconductor를 device에 사용하는 이유 tunable conductivity !!

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When a dopant atom with a valence greater than four is added to silicon, an extra electron is introduced and a donor energy state is created.

Now electrons are more easily excited into the conduction band.

[Fig. 18-19] 18.6 Semiconductors

n-type semiconductor의 특징: donor level Ed위치 in energy band structure

Ed << Eg

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When a dopant atom with a valence of less than four is substituted into the silicon structure, a hole is created in the structure and acceptor energy level is created just above the valence band. Little energy is required to excite the holes into motion.

[Fig. 18-21] 18.6 Semiconductors

p-type semiconductor의 특징: acceptor level Ea 위치 in energy band structure

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18.6 Semiconductors

Ed와 Ea의 크기는 Eg에 비해서 아주 작지만 P와 B의 경우 대략 0.05 eV 정도임.

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The effect of temperature on the carrier concentration of n-type semiconductor.

[Fig. 18-20] 18.6 Semiconductors

100 K 357K[84 °C]

Extrinsic semiconductor에 doping량의 조절 conductivity 조절 가능 !!

Example 18-8 참조

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Temperature dependent of extrinsic semiconductor

[Fig. 18-20 해설 / 3개의 범위 설명]

Impurity range - electron are excited from donor level to the conduction band or from valance band to the acceptor level.

Exhausting range - donor level becomes exhausted or the acceptor level becomes saturated.

Intrinsic range - thermal energy excited a large number of electron from the valance to the conduction bands.

18.6 Semiconductors

Direct and Indirect Bandgap Semiconductor를 정의하시오 Radiative recombination과 nonradiative recombination를 비교해서 정의 하시오 [ Eq. (18-16a)와 (18-16b)의 차이 ]

Example 18-9 참조

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18.7 Applications of Semiconductors

Behavior of a p-n junction device

(a)When no bias is applied electron and hole currents

(b) Under a forward bias

(c) Under reverse bias

The internal electric field that develops is shown as ε.

[Fig. 18-22]

p-n Junction에서 forward 와 reverse bias의 의미와 band 구조의 변화?

Drift velocity의 의미: in Glossary Current flow의 방향 ??

Depletion layer(W)의 두께??

참고: Example 5-2 / diffusion과 drift?

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(a) The current-voltage characteristic for a p-n junction.

(b) If an alternating signal is applied, rectification occurs and only half of the input signal passes the rectifier. [diode의 응용: AC to DC converter]

[Fig. 18-23] 18.7 Applications of Semiconductors

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(a)A circuit for an n-p-n bipolar junction transistor(BJT). The input creates a forward and reverse bias that causes electrons to move from the emitter, through the base, and into the collector, creating an amplified output.

(b) Sketch of the cross-section of the transistor. [Emitter, Base, Collector]

[Fig. 18-24] 18.7 Applications of Semiconductors

BJT의 동작원리와 Application??

Base layer should be very thin!!!

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[Fig. 18-25] 18.7 Applications of Semiconductors

FET(field effect transistor, MOSFET)의 동작원리와 주요 application Switching! 3극: (Source, Gate, Drain) in FET / Unipolar junction transistor(UJT)

+ 가 많음

- 가 몰려감

Channel 통해 전자 흐름: on

Channel 사라짐: off

[N channel 형성 NMOS]

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[Fig. 18-26] 18.7 Applications of Semiconductors

Czochralski growth technique으로 단결정 Si 잉곳의 제조 원리 이해 잉곳 Wafer IC 제조공정 Chip(die) Packing 공정 이해

Si = 1414

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[Fig. 18-26] 18.7 Applications of Semiconductors

반도체 IC제조공정: 코팅은 전체면적에, 에칭은 선택적 방법[photolithography]으로 !!

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18.8 Insulators & Dielectric Properties

Electrical insulators obviously must have a very low conductivity, or high resistivity, to prevent the flow of current.

Insulators are produced from ceramic and polymeric materials in which there is a large energy gap between the valence and conduction bands.

The resistivity of most of these is >1014 Ω-㎝ and the breakdown electric fields are ~5 to 15 kV/mm.

In order to select an insulating materials properly, we must understand how the material stores, as well as conducts, electrical charge.

Insulator의 적당한 용도는 다음의 몇가지 측면에서 고려되어야 함 resistivity breakdown electric field how the material stores electrical charge dielectric properties

* Store the electrical current in the capacitor

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18.9 Polarization in Dielectrics

Polarization mechanism in materials:

(a) electronic [Example 18-10]

(b) atomic or ionic

(c) high-frequency dipolar or orientation

(d) low-frequency dipolar

(e) interfacial-space charge at electrodes

(f) interfacial-space charge at heterogeneities

[Fig. 18-27]

Polarization(분극)은 전계에 반응하는 원자/분자/이온 등 ** What is dipole?

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Schematic representation of different mechanisms of polarization.

18.9 Polarization in Dielectrics

Electric field에 반응하는 Polarization 기구의 종류 charged 입자의 거동!!

A dipole : a pair of opposite charges separated by a certain distance!!

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(a) The Oxygen ions are at face centers, Ba2+ ions are at cube corners and Ti4+is at cube center in cubic BaTiO3.

(b) In tetragonal BaTiO3, the Ti4+ is off-center and the unit cell has a net polarization.

[Fig. 18-28] 18.9 Polarization in Dielectrics

[Dipolar or Orientation Polarization의 예]

An electric dipole is a separation of positive and negative charges.

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A charge can be stored a the conductor plates in a vacuum (a). However, when a dielectric is placed between the plane (b),

dielectric polarizes and additional charge is stored.

[Fig. 18-29] 18.9 Polarization in Dielectrics

Dielectric 특성의 응용: charge의 store Capacitor에 응용 됨

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Shift in the distribution of charge in a ceramic insulator when it is placed in an

electric field between two electrically conductive electrodes.

18.9 Polarization in Dielectrics

Ceramic Insulator

Ceramic Insulator

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[Fig. 18-30] 18.9 Polarization in Dielectrics

Dielectric constant K의 의존성: 조성, 온도, 미세구조, electrical frequency, …

Dielectric constant K가 클수록 store 용량이 증가함.

Dielectric loss는 K가 변화하는 부분에서 발생 Dielectric loss가 작을 수록 좋음!!

Dielectric constant K가 클수록 store 용량이 증가함.

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18.9 Polarization in Dielectrics

Dielectric constant가 커지면??

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18.10 Electrostriction, Piezoelectricity, Pyroelectricity & Ferroelectricity

각각의 정의? electrostriction, piezoelectricity, pyroelectricity, and ferroelectricity

Electrostriction: polarization ions and electronic clouds are displaced Materials 에서 mechanical strain 발생 Piezoelectricity: 물질이 stress를 받으면, voltage를 develop [direct piezo- ]. Pyroelectricity: 물질에 온도 변화 dielectric polarization charge 발생 Ferroelectricity: spontaneous and reversible dielectrical polaization 발생 [ reversible using an electrical field ]

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The (a) direct and (b) converse piezoelectric effect. In the direct piezoelectric effect, applied stress cause a voltage to appear. In the converse (b), an applied voltage leads to development of strain.

[Fig. 18-31] 18.10 Electrostriction, Piezoelectricity, Pyroelectricity & Ferroelectricity

각각의 용도? electrostriction, piezoelectricity, pyroelectricity, and ferroelectricity

Strain(stress) voltage 발생!!

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Example of ceramic capacitor. (a)Single-layer ceramic capacitor (disk capacitor)

(b) Multilayer ceramic capacitor (stack ceramic layers)

[Fig. 18-32] 18.10 Electrostriction, Piezoelectricity, Pyroelectricity & Ferroelectricity

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18.10 Electrostriction, Piezoelectricity, Pyroelectricity & Ferroelectricity

(a) Different polymorphs of BaTiO3 and accompanying changes on lattice constant and dielectric constant.

[Fig. 18-33]

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(b) Single crystal. (c) Polycrystalline BaTiO3 showing the influence of the electric filed on polarization.

[Fig. 18-33] 18.10 Electrostriction, Piezoelectricity, Pyroelectricity & Ferroelectricity

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(a)The effect of temperature and grain size on the dielectric constant of barium titanate.

(b) Ferroelectric domain can be seen on the microstructure of polycrystalline BaTiO3.

[Fig. 18-34] 18.10 Electrostriction, Piezoelectricity, Pyroelectricity & Ferroelectricity

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Chapter 18. Electronic materials

이웃 원자간 bonding의 type 물질의 특성을 지배합니다 !! 마지막 부분의 Summary를 여러 번 읽어서 완전히 이해 해야 합니다. 내용 정리

마지막 부분의 Glossary를 여러 번 읽어서 이해 해야 합니다. 용어 정리

첫 페이지의 “Have You Ever Wondered?”부분이 이해 되어야 합니다. 의문점(목적)을 가지고 공부

[18장에서 공부의 Key Point]: 공부 잘하는 방법!!

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숙 제 1. Example 18-1 부터 18-1부터 10까지 풀이를 이해후, 작성해서 제출하세요.

2. 온도증가에 따른 전기전도도 변화는? 금속, 반도체 각각의 대해 논하시오

3. 반도체에서 intrinsic과 extrinsic의 차이는?

4. n-type과 p-type 반도체의 제조방법과 에너지 밴드 구조를 도시하시오.

5. BJT와 UJT의 동작특성과 용도를 비교 설명하세요.

6. Polarization 특성을 응용한 사례를 몇 가지 논하시오.

Chapter 18. Electronic materials

σ = n e µ (18-15a)

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18장에서 무엇을 새롭게 알았습니까?