Teknologi Fabrikasi Mikroelectronik/ Microelectronic ...portal.unimap.edu.my/portal/page/portal30/Lecture Notes/Fakulti... · • Some compounds, such as GaAs, SiC and SiGe. • Most

Department of Electronic Engineering Technology

PGT 330/3

Teknologi Fabrikasi Mikroelectronik/

Microelectronic Fabrication Technology

Basic of Semiconductors and Silicon Wafer

ManufacturingDepartment of Electronic Engineering Technology,

Faculty of Engineering Technology (FTK),

Universiti Malaysia Perlis (UniMAP),

Kampus Kampus UniCITI Alam,

Sungai Chuchuh,

02100 Padang Besar, Perlis,

MALAYSIA.


Basic of Semiconductor Devices

OBJECTIVES


• Identify at least two semiconductor materials from the periodic

table of elements

• List n-type and p-type dopants

• Describe a diode and a MOS transistor

• List three kinds of chips made in the semiconductor industry

• List at least four basic processes required for a chip

manufacturing

TOPICS


• What is semiconductor

• Basic semiconductor devices

• Basics of IC processing

WHAT IS A SEMICONDUCTOR


• Semiconductors are materials with electrical conductivitybetween conductors and insulators.

• The most commonly used semiconductor materials are siliconand germanium.

• Some compounds, such as GaAs, SiC and SiGe.

• Most important property is its conductivity can be controlled byadding certain impurities in the process called doping.





• The study of semiconductor materials – 19th century

• Early 1950s – Ge was the major semiconductor material, andlater in early 1960, Si has become a practical substitute withseveral advantages:

- Better properties at room temperature

- Can be grown thermally – high quality silicon oxide

- Lower cost, and

- Easy to get, silica and silicates comprises25% of theEarth’s crust







Semiconductor Substrate and Dopants





• Atom is basic building block of all materials

• Classical mechanics – every atom has it

own orbit structure

• Electron orbits are called shells

• The outermost shell is called valence shell

• When e leaves the valence shell, it

becomes a free electron and can conduct

electric current

Band gap



• When 2 or more identical atoms bond together

to form solid materials, their orbit overlap and

form so called energy bands. Can be

represented by the energy band diagram

• The bottom of conduction band is called Ec,

and the top of the valence band is called Ev.

• Eg = Ec – Ev

• Eg is defined as the energy required to break a

bond in semiconductor to free an e to cond

band and leave the hole in the valence band.

• Electrons in conduction band are free to move

and can conduct electric current

• Electrons in the valence band are bonded with

nuclei and connot move freely, therefore

cannot conduct electric current

Band gap



• Resistivity is the capability of a material resisting electric current.

• A good conductor has a very low resistivity and a good insulator has a very high resistivity.

• Unit: Ohm.cm

Resistivity



• For most metals, conduction and valence bands almost overlap or very small band gap. Electron can easily jump from valence to conduction band. Therefore the conduction band has a lot of e.

• For insulators, the band gap is so large that electrons cannot jump across it.

Resistivity and band gap



• Si is used in standard CMOS fabrication.

• GaAs and SiGe are commonly used in high speed devices.

• GaAs and GaP commonly used in LED.

• ZnS(II-VI) television screen.

• InSb, CdSe, PbTe, HgCdTe as light detectors.

• InP for microwave devices.

• GaAs, AlGaAs as semiconductor laser.

Semiconductor materials and its applications

CRYSTAL PROPERTIES OF SEMICONDUCTORS


• Amorphous

• Single Crystal

• Poly Crystal

Classification of Solids (Based on Atomic

Arrangement)



Crystal structures

• Amorphous - no repeated structure at all

• Polycrystalline - Some repeated structures

• Single crystal - One repeated structure



Crystal structures

• Amorphous - no repeated structure at all

• Polycrystalline - Some repeated structures

• Single crystal - One repeated structureAmorphous structure

Polycrystalline structure Single crystal structure



Silicon crystal structures

• Silicon has four electrons in the outermost shell.

• In a single crystal structure, every atom is bonded with four atoms shares a pair of electrons with each of them.



Crystal lattice

• LATTICE is the periodic arrangement of atom in a crystal.

• UNIT CELL is the basic building block of crystal lattice repeatingitself for entire lattice.

• The simplest 3D lattice is in cubic form(basic lattice forsemiconductor material is diamond lattice)

• Properties of periodic crystal lattice determine the allowedenergies of electron that participate in the conduction process.



Unit cell



Unit cell of single crystal structure



Crystal Plane and Miller Indices



Why Silicon Dominating

• Abundant, inexpensive

• Thermal stability

• Silicon dioxide is a strong dielectric and relatively easy to form

• Silicon dioxide can be used as diffusion doping mask

DOPING SEMICONDUCTOR


Semiconductor material types

• Intrinsic Semiconductor

• Extrinsic Semiconductor



Intrinsic semiconductor

• Pure semiconductor materials with no impurity atoms and no

lattice defect.

• At T=0 K, all energy states in valence band are filled with

electrons, states in conduction band are empty.



Electrical Conduction in Intrinsic Semiconductor




Si Si Si Si

Si Si Si Si

Si Si Si Si

Si Si Si Sie-

e

•As the temperature increase above 0K, a few valence bond

electrons may

gain enough thermal energy to break the bond and jump into the

conduction band.•As temperature

increase further, more

bonds broken, more

electrons jump

to the conduction band

and more “empty

states or holes”

created in the valence

band.



• In intrinsic material, electrons and holes are created in pairs by

thermal energy. So the number of electrons in conduction band

is equal to the number of holes in the valence band

• Electron concentration = hole concentration

• ni = pi and

• nipi = ni2 (MASS ACTION LAW) – the product of n p is always a

constant for a given semiconductor material at given

temperature




• Extrinsic s/c is defined as a semiconductor in which controlled

amounts of specific dopant or impurity atoms have been added

so that the thermal equilibrium electron and hole concentration

are different from the intrinsic carrier concentration.

Extrinsic Semiconductor



• The purpose of doping is to alter the conductivity ofsemiconductor materials.

• Two types of dopant; p-type (B), n-type (P, As)

• N-type dopants provide an electron in s/c materials, hence calleddonors.

• P-type dopants provide a holein s/c materials, hence called acceptor



N-type Dopant

• P and As have 5 electron valens

• When doped into Si, 4 electrons used to form the covalence bond

with Si

• 1 extra electron is left in the outermost shell and will occupy a

new

• energy level called Donor Energy.



N-type Dopant

valence band

conduction bandEc

Ev

Ed

Ec

Ev

Ed+ ++

---

• Energy required to elevate donor electron is less than that for

electron involved in covalence bonding.

• With small thermal energy, donor electron is elevated to the

conduction band

• This process add electron to the conduction band without

creating holes in the valence band.

• The resulting material is referred as n-type semiconductor.



P-type Dopant

• B have 3 electron valens

• When doped into Si, one empty state is created in the

covalence bond

• This empty state will occupy a new energy level called

Acceptor Energy.



P-type Dopant

• Some valence electron gain a small amount of energy to move

around the crystal lattice.

• This electron would occupy the “empty” position associated with

B atom.

• The vacated electron position is considered as holes.



P-type Dopant

• This process generate holes in the valence band without

creating electrons in the conduction band.

• The resulting material is referred as p-type semiconductor



Illustration of hole movement



Illustration of hole movement



Dopant Concentration and Resistivity







• Higher dopant concentration, more carriers(electrons or

holes)

• Higher conductivity, lower resistivity

• Electrons move faster than holes

• N-type silicon has lower resistivity than p-type silicon at the

same dopant concentration

BASIC SEMICONDUCTOR DEVICES


• Resistor

• Capacitor

• Diode

• MOS transistor



• The simplest electronic device.

• In the IC fabrication pattern doped silicon normally used to make resistors determined by the length, line width, junction depth and dopant concentration.

• Polysilicon also used a resistor.

Resistor



ResistorExample 1

Question: Many people use polysilicon to form gates and local

interconnections. Resistivity of polysilicon is determined by

dopant concentration, which is very high, about 1022 cm-3, and

200 μΩ.cm. Assuming that the polysilicon gate and local

interconnection line width, height, and length are 1 μm, 1 μm,

and 100 μm, respectively.

What is the resistance?



Capacitor

• One of the most important IC component

• When two conducting materials are separated by a dielectric,

a capacitor is formed.

C = Κε0h l

d

ε0- Absolute permittivity of vacuum

(8.85 x 10-12 F/m)

K - Dielectric constant



Capacitor

• Charge storage device

• Memory devices, esp. DRAM

• Challenge : reduce capacitor size while keeping the

capacitance.

• High- K dielectric material.



Capacitor



Parasitic Capacitor

• Unwanted (parasitic) capacitor, as result of dielectric

sandwiched between 2 metal layers. This will result in the RC

delay of the IC circuit.

• Major limitation for current IC device speed

• This application required low k dielectric and better conduction

metal

= RC



Capacitor

Example 2

Question: What is the capacitance for a capacitor with the

same structure as Figure below with h = l = 10 μm? (The

dielectric between two conducting plates is silicon dioxide, with

= 3.9, and d = 1000 Å.)



Diode

• P-N Junction

• Allows electric current go through only in one way (positively

biased)



Diode• When p-type and n-type semiconductors join together, they form a p-n

junction diode.

• Holes in p-type region will diffuse to the n-type region, and electrons in

n-type region will diffuse to the p-type region (at thermal equilibrium,

without applied bias).

• The area dominated by minority carriers is called the transition region.

• The voltage across the transition region given by;



MOSFET

• Metal-oxide-semiconductor also called MOSFET (MOS Field

Effect Transistor)

• Simple, symmetric structure

• Switch, good for digital, logic circuit

• Most commonly used devices in the semiconductor industry



MOSFET

NMOS

• Conducting gate(metal or polysilicon)

• Heavily doped source and drain

• P-type substrate

• Ultra thin gate oxide



MOSFET

NMOS• When no bias voltage is applied to the gate, no current flow.

• When gate is positively biased, positive charge will appear at the gate.

• Positive charge at the silicon surface will be expelled from the region.

• At certain voltage (Threshold Voltage), electron will be accumulated at

silicon surface to form channel, and allow the electron flow from source

to drain.



MOSFET

PMOS• When no bias voltage is applied to the gate, no current flow.

• When gate is negatively biased, negative charge will appear at the

gate.

• Negative charge at the Si surface will be expelled from the region.

• At certain voltage(threshold), holes will be accumulated at the Si

surface to form channel, allow the holes flow from drain to source.



THANK YOU

Documents

Teknologi Fabrikasi Mikroelectronik/ Microelectronic ...portal.unimap.edu.my/portal/page/portal30/Lecture Notes/Fakulti... · • Some compounds, such as GaAs, SiC and SiGe. • Most