Semiconductor Basics

Chapter 1

Atomic Structure

Elements are made of atoms– 110 Elements; each has an atomic

structure– Today, quarks and leptons, and their

antiparticles, are candidates for being the fundamental building blocks from which all else is made!

Bohr Model– Atoms have planetary structure – Atoms are made of nucleus (Protons

(+) & Neutrons) and electrons (-)

110 th element is called Darmstadtium (Ds)

Atomic Structure

Atoms go around the nucleolus in their orbits – discrete distances

Each orbit has some energy level The closer the orbit to the nucleus the less

energy it has Group of orbits called shell Electrons on the same shell have similar energy

level Valence shell is the outmost shell Valence shell has valence electrons ready to

be freed Number of electrons (Ne) on each shell (n)

– First shell has 2 electrons– Second shell has 8 electrons (not shown here)

Ne = 2n2

Valence Shell

Atoms are made of valence shell and core

Core includes nucleus and other inner shells

– For a Carbon atom the atomic number is 6

– Core charge = 6 P + 2 e = (+6)+(-2)=(+4)

– Remember the first shell has 2 electrons

Elements

Basic categories– Conductors

Examples: Copper, silver One valence electron , the e can

easily be freed– Insulators

Valence electrons are tightly bounded to the atom

– Semiconductors Silicon, germanium (single

element) Gallium arsenide, indium

phosphide (compounds) They can act as conductors or

insulators

Conduction band is where the electron leaves the valence shell and becomes freeValence band is where the outmost shell is

Always free electrons

Free electrons

Semiconductors

Remember the further away from the nucleus the less energy is required to free the electrons

Germanium is less stable– Less energy is required to make the

electron to jump to the conduction band

When atoms combine to form a solid, they arrange themselves in a symmetrical patterns

Semiconductor atoms (silicon) form crystals

Intrinsic crystals have no impurities

Conduction Electrons and Holes

Electrons exist only within prescribed energy bands

These bands are separated by energy gaps

When an electron jumps to the conduction band it causes a hole

When electron falls back to its initial valence recombination occurs

Consequently there are two different types of currents

– Hole current (electrons are the minority carriers)

– Electron current (holes are the minority carriers)

Remember: We are interested in electrical current!

Doping

By adding impurities to the intrinsic semiconductor we can change the conductivity of the material – this is called doping

– N-type doping – P-type doping

N-type: pentavalent (atom with 5 valence electrons) impurity atoms are added

– [Sb(Antimony) + Si] – Negative charges (electrons) are generated N-type has lots of free electrons

P-type: trivalent (atom with 3 valence electrons) impurity atoms are added

– [B(Boron) + Si] – Positive charges (holes) are generated– P-type has lots of holes

Diodes

N region has lots of free electrons P region has lots of holes At equilibrium: total number positive and negative

charges is the same (@ room temp) At the pn junction the electrons and holes with

different charges form an electric field In order to move electrons through the electric field

(generate current) we need some force (voltage)– This potential difference is called barrier voltage– When enough voltage is applied such that electrons

are moved then we are biasing the diode– Two layers of positive and negative charges for

depletion region – the region near the pn-junction is depleted of charge carriers)

Biasing Types of a Diode

Forward bias– Bias voltage VBias > barrier voltage VBar

– Reduction in + and – ions smaller depletion region

– VBar Depends on material, doping, temp, etc. (e.g., for silicon it is 0.7 V)

Reverse bias– Essentially a condition that prevents

electrons to pass through the diode – Very small reverse break down current– Larger depletion region is generated

Cathode n region

Anode p region

Connected to the negative side of the battery

Connected to the positive side of the battery

A K

Biasing Types of a Diode (Forward)

Cathode n region

Anode p region

A K

Moving electrons

Small dynamic resistance

VBias

np

Conventional Current Flow

Conventional Current FlowI (Forward)

Very SmallMoving Electrons:Reverse Current)

Biasing Types of a Diode (Reverse)

Cathode n region

Anode p region

A K

Large resistance

VBias

np

Conventional Current Flow

Holes are left behind; large depletion region

Instant pull of electrons

I-V Characteristic of a Diode

Forward bias: current passes through – The knee is where VBias=VBar– At point B VBias < VBar Very little current– Note that at the knee the current increases rapidly but

V(forward) stays almost the same

Reveres bias: No current passes through – When VBias < VBar Very little current (mu or nano Amp)– At the knee, the reverse current increases rapidly but the

reverse voltage remains almost the same – Large reverse current can result in overheating and possibly

damaging the diode (V=50V or higher typically)

Overheating results from high-speed electrons in the p-region knocking out electrons of atoms in n-region from their orbit to the conduction band

– Hence, we use limiting resistors

Electrons moving from n to p region

Modeling a Diode (Forward Biasing)

Use r’d (internal resistance)- Not linear!

Complete Modeling of a Diode

Note that IF is the actual direction of electron current

Forward bias: VBias = VF + IF(RLIMIT+rd); rd is typically given, VF typically is 0.7 VReverse bias: VBias = VR + IR * RLIMIT; IR is typically given

VRVF

Showing the Actual electron direction

Example

Find the current through the diode and the voltage across the resistor.Assume rd = 10 ohm

Biasing?

Forward bias: VBias = VF + IF(RLIMIT+rd)10 = 0.7 + IF(RLIMIT+10) IF=9.21 mAVF=0.7+IF*rd = 792 mVVRLIMIT = IF * RLIMIT = 9.21V

VF

Forward biased

Example

Find the current through the diode and the voltage across the resistor.Assume IR = I uA

Note: Reverse biased

VR

Reverse bias: VBias = VR + IR * RLIMIT

VRLIMIT = IR*VRLI MIT = 1mAVR=VBIAS-VRLIMIT=4.999 V

Forward Bias

Calculate the voltage across the resistor.

Reverse Bias

Calculate the voltage across the resistor.

Do this example on your own:

R1

1k R21.5k

R34.7k

R44.7k

D2DIODE_VIRTUAL

U2DC 1M-19.459 V

+

-

U1

DC 1e-009

-0.021m A+ -

U3

DC 1e-009

-4.182m A+ -

V230 V

J1Key = Space

V130 V

Forward Bias

ReverseBias

+

-

R1

1k R21.5k

R34.7k

R44.7k

D2DIODE_VIRTUAL

U2DC 1M0.683 V

+

-

U1

DC 1e-009

2.984m A+ -

U3

DC 1e-009

6.114m A+ -

V230 V

J1Key = Space

V130 V

Forward Bias

ReverseBias

+

-

Make sure you can calculate andfind all currents- Hint: find Vn, first

Vn

Vn

i1

i2i3

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