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ECE G201: Introductory Material
ECE G201: Introductory Material
Semiconductors and Physical Operation of Diodes
Semiconductors and Physical Operation of Diodes
Periodic Table of Elements
Periodic Table of Elements
The Silicon Atom
The Silicon Atom
Silicon Crystal Each Si atom has four nearest neighbors — one for each
Silicon Crystal Each Si atom has four nearest neighbors — one for each
Two-dimensional Picture of Si note: each line ( —) represents a
Two-dimensional Picture of Si note: each line ( —) represents a
Silicon at Room Temperature
Silicon at Room Temperature
Silicon at Room Temperature
Silicon at Room Temperature
Silicon at Room Temperature
Silicon at Room Temperature
Current Flow in Silicon
Current Flow in Silicon
Some important facts
Some important facts
Important Facts (cont
Important Facts (cont
Doping
Doping
Periodic Table of Elements
Periodic Table of Elements
n-type silicon add atoms from column V of the periodic table
n-type silicon add atoms from column V of the periodic table
VERY IMPORTANT POINT
VERY IMPORTANT POINT
Periodic Table of Elements
Periodic Table of Elements
p-type silicon add atoms from column III of the periodic table
p-type silicon add atoms from column III of the periodic table
p-type silicon add atoms from column III of the periodic table
p-type silicon add atoms from column III of the periodic table
VERY IMPORTANT POINT
VERY IMPORTANT POINT
The pn Junction
The pn Junction
Dopant distribution inside a pn junction
Dopant distribution inside a pn junction
Dopant distribution inside a pn junction
Dopant distribution inside a pn junction
Voltage in a pn junction
Voltage in a pn junction
Zero Bias
Zero Bias
Forward Bias
Forward Bias
Reverse Bias
Reverse Bias
Breakdown
Breakdown
Solar Cell (Photovoltaic)
Solar Cell (Photovoltaic)
Light Emitting Diode (LED)
Light Emitting Diode (LED)
Junction Capacitance
Junction Capacitance
Junction Capacitance (Cj)
Junction Capacitance (Cj)

Презентация: «ECE G201: Introductory Material». Автор: Hopwood. Файл: «ECE G201: Introductory Material.ppt». Размер zip-архива: 93 КБ.

ECE G201: Introductory Material

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1 ECE G201: Introductory Material

ECE G201: Introductory Material

Goal: to give you a quick, intuitive concept of how semiconductors, diodes, BJTs and MOSFETs work as a review of electronics and an overview of this course This discussion will be qualitative no equations for now, these will be added later Note that the concepts are often over-simplified!

From Prof. J. Hopwood

2 Semiconductors and Physical Operation of Diodes

Semiconductors and Physical Operation of Diodes

Semiconductors Doping n-type material p-type material pn-Junctions forward, reverse, breakdown solar cells, LEDs, capacitance

3 Periodic Table of Elements

Periodic Table of Elements

Relevant Columns: III IV V

4 The Silicon Atom

The Silicon Atom

10 core electrons: 1s22s22p6

-

-

Nucleus: 14 protons 14 neutrons

4 valence electrons

-

-

The 4 valence electrons are responsible for forming covalent bonds

5 Silicon Crystal Each Si atom has four nearest neighbors — one for each

Silicon Crystal Each Si atom has four nearest neighbors — one for each

valence electron

0.5 nm

6 Two-dimensional Picture of Si note: each line ( —) represents a

Two-dimensional Picture of Si note: each line ( —) represents a

valence electron

At T=0 Kelvin, all of the valence electrons are participating in covalent bonds There are no “free” electrons, therefore no current can flow in the silicon ? INSULATOR

covalent bond

Si

7 Silicon at Room Temperature

Silicon at Room Temperature

For T>0 K, the silicon atoms vibrate in the lattice. This is what we humans sense as “heat.” Occasionally, the vibrations cause a covalent bond to break and a valence electron is free to move about the silicon.

8 Silicon at Room Temperature

Silicon at Room Temperature

For T>0 K, the silicon atoms vibrate in the lattice. This is what we humans sense as “heat.” Occasionally, the vibrations cause a covalent bond to break and a valence electron is free to move about the silicon.

-

= free electron

-

9 Silicon at Room Temperature

Silicon at Room Temperature

The broken covalent bond site is now missing an electron. This is called a “hole” The hole is a missing negative charge and has a charge of +1. = a hole

-

+

hole

10 Current Flow in Silicon

Current Flow in Silicon

Bond breaking due to: heat (phonons) light (photons) Conductance is proportional to the number of electrons and holes: Si resistance depends on temp. and light

11 Some important facts

Some important facts

The number of electrons = the number of holes that is, n = p in pure silicon this is called intrinsic material High temp ?more electrons/holes?lower resistance Very few electrons/holes at room temperature n=1.5x1010 per cm3, but nSi = 5x1022 per cm3 n/nSi = 3x10-13 (less than 1 in a trillion Si bonds are broken This is a SEMICONDUCTOR

12 Important Facts (cont

Important Facts (cont

Band Gap: energy required to break a covalent bond and free an electron Eg = 0.66 eV (germanium) Eg = 1.12 eV (silicon) Eg = 3.36 eV (gallium nitride) Metals have Eg= 0 very large number of free electrons?high conductance Insulators have Eg > 5 eV almost NO free electrons ? zero conductance

13 Doping

Doping

Intentionally adding impurities to a semiconductor to create more free electrons OR more holes (extrinsic material) n-type material more electrons than holes (n>p) p-type material more holes than electrons (p>n) HOW???

14 Periodic Table of Elements

Periodic Table of Elements

Relevant Columns: III IV V

15 n-type silicon add atoms from column V of the periodic table

n-type silicon add atoms from column V of the periodic table

Column V elements have 5 valence electrons Four of the electrons form covalent bonds with Si, but the 5th electron is unpaired. Because the 5th electron is weakly bound, it almost always breaks away from the P atom This is now a free electron.

16 VERY IMPORTANT POINT

VERY IMPORTANT POINT

Si

-

P+

The number of electrons is equal to the number of phos. atoms: n = Nd

The phosphorus atom has donated an electron to the semiconductor (Column V atoms are called donors) The phosphorus is missing one of its electrons, so it has a positive charge (+1) The phosphorus ion is bound to the silicon, so this +1 charge can’t move!

17 Periodic Table of Elements

Periodic Table of Elements

Relevant Columns: III IV V

18 p-type silicon add atoms from column III of the periodic table

p-type silicon add atoms from column III of the periodic table

Column III elements have 3 valence electrons that form covalent bonds with Si, but the 4th electron is needed. This 4th electron is taken from the nearby Si=Si bond

Si

B

19 p-type silicon add atoms from column III of the periodic table

p-type silicon add atoms from column III of the periodic table

Column III elements have 3 valence electrons that form covalent bonds with Si, but the 4th electron is needed. This 4th electron is taken from the nearby Si=Si bond This “stolen” electron creates a free hole.

Si

B

hole

20 VERY IMPORTANT POINT

VERY IMPORTANT POINT

Si

+

B-

The number of holes is equal to the number of boron atoms: p = Na

The boron atom has accepted an electron from the semiconductor (Column III atoms are called acceptors) The boron has one extra electron, so it has a negative charge (-1) The boron ion is bound to the silicon, so this -1 charge can’t move!

21 The pn Junction

The pn Junction

anode

cathode

integrated circuit diode

p-type

n-type

metal silicon oxide doped silicon wafer (chip)

22 Dopant distribution inside a pn junction

Dopant distribution inside a pn junction

excess holes diffuse to the n-type region

p>>n

n>>p

excess electrons diffuse to the p-type region

23 Dopant distribution inside a pn junction

Dopant distribution inside a pn junction

excess holes diffuse to the n-type region

excess electrons diffuse to the p-type region

DEPLETION REGION:

+

-

p~0, and acceptor ions are exposed

n~0, and donor ions are exposed

24 Voltage in a pn junction

Voltage in a pn junction

charge, r(x)

electric field, E(x)

voltage, V(x)

+

-

~0.7 volts (for Si)

25 Zero Bias

Zero Bias

voltage, V(x)

At zero bias (vD=0), very few electrons or holes can overcome this built-in voltage barrier of ~ 0.7 volts (and exactly balanced by diffusion) ? iD = 0

~0.7 volts (for Si)

26 Forward Bias

Forward Bias

voltage, V(x)

vD

As the bias (vD), increases toward 0.7V, more electrons and holes can overcome the built-in voltage barrier .? iD > 0

0.65 volts

0.50 volts

0.0 volts

27 Reverse Bias

Reverse Bias

voltage, V(x)

Is

vD

As the bias (vD) becomes negative, the barrier becomes larger. Only electrons and holes due to broken bonds contribute to the diode current. ? iD = -Is

1/2Is

1/2Is

0.0 volts

-5 volts

28 Breakdown

Breakdown

voltage, V(x)

|I| >> Is

vD

As the bias (vD) becomes very negative, the barrier becomes larger. Free electrons and holes due to broken bonds are accelerated to high energy (>Eg) and break other covalent bonds – generating more electrons and holes (avalanche).

large reverse current

0.0 volts

-50 volts

29 Solar Cell (Photovoltaic)

Solar Cell (Photovoltaic)

light

voltage, V(x)

Iph

Light hitting the depletion region causes a covalent bond to break. The free electron and hole are pushed out of the depletion region by the built-in potential (0.7v).

~0.7 volts (for Si)

Rload

30 Light Emitting Diode (LED)

Light Emitting Diode (LED)

voltage, V(x)

vD

In forward bias, an electron and hole collide and self-annihilate in the depletion region. A photon with the gap energy is emitted. Only occurs in some materials (not silicon).

photon

2.0 volts

1.5 volts

0.0 volts

31 Junction Capacitance

Junction Capacitance

A

semiconductor-”insulator”-semiconductor

The parasitic (unwanted) junction capacitance is Cj = eA/W, where W depends on the bias voltage

W n=p~0

e=11.9

32 Junction Capacitance (Cj)

Junction Capacitance (Cj)

The junction capacitance must be charged and discharged every time the diode is turned on and off Transistors are made of pn junctions. The capacitance due to these junctions limits the high frequency performance of transistors remember, Zc = 1/jwC becomes a short circuit at high frequencies (Zc ? 0) this means that a pn junction looks like a short at high f This is a fundamental principle that limits the performance of all electronic devices

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