1. Material Science and Technology 2
HACETTEPE UNIVERSITY
DEPATMENT OF CHEMICAL
ENGINEERING
Material Science and Technology 2
NECDET DALGIÇ YILMAZ ŞAHAN
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2. SEMICONDUCTORS

3. GOALS information about electrical characteristics of materials
Discussing how p-n junctions work
Describing the processing and properties of semiconductors

4. OUTLINE Semiconductor basics Definition of the semiconductor
Common semiconductor material
Semiconductor crystal structure
How silicon crystal conducts electiricity?
P-type and N-type silicon
The PN Junction
Basic semiconductor devices
Semiconductor Manufacturing Process
Summary
References

5. Semiconductor basics resistance resistivity ,f(T) conductor insulator
FIGURE 1 : Resistivity chart

6. WHAT IS SEMICONDUCTORS
have electrical properties between a conductor and insulator
do not conduct as well as conductors
do not insulate as well as insulators
their atoms are closely , have a crystal structure
electronic devices, computers, telephones made from semiconductors
conductivity can be controlled by doping

7. Most common semiconductor material four valence electron
SILICON
Most common semiconductor material
four valence electron
covalent bond ,very stable
FIGURE 2: structure and lattice of pure crystal of Silicon

8. How silicon crystal conducts electricity?
pure silicon electrical conductor
by adding impurity
introduce an impurity such as phosphorus, arsenic, aluminum,gallium
can be exist unbounded electron
can exist a hole

9. P-TYPE SILICON With elements that have three valence electrons
Such as B, Al and Ga
Fourth bond cannot be formed
Positive charge
Not a complete connection
There is a empty place ‘hole’
Hole movement will exist
By diffusion of B2H6

10. Illustration of the hole movement
Electron leave the covalent bond and move into hole
Therefore, another hole will be existed
Holes will move to

11. N-TYPE SILICON
These elements have five valence electrons to share with other atoms
Such as Sb, P and As
Fifth electron cannot form a bond
Negative charge
Greatly increasing the conductivity
By diffusion of PH3

12. The PN Junction
A single type (p or n ) semiconductor material is not very useful
For useful applications, semiconductor crystal must contain both P-type and N-type
PN junction can only be created by inserting different impurities into different parts of a single crystal

13. N region will has positive charge after missing some electrons
Those electrons will fill the holes in the P region, which will therefore has a negative charge.
So there will be potential difference and electrical current

14. Forming a PN Junction
p-type
n-type

15. n-type p-type Semiconductors p-type and n-type are brought together

16. n-type p-type Electrons and holes migrate across the junction

17. n-type
p-type
The depletion layer is formed

18. Electric field
n-type
p-type
A potential difference is set up across the depletion layer

19. Impurity(dopant) concentration
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

20. Basic semiconductor devices
Resistor
Transistors
Capacitors
Diode

21. resistor
Resistors are made by doped silicon or polysilicon on an IC chip.
Resistance is determined by length, line width,height, ana dopant concentration
V=I*R ( Ohm law)

22. capasitors Charge storage device
The capacitor's function is to store electricity, or electrical energy.
The capacitor also functions as a filter, passing alternating current (AC), and blocking direct current (DC).
blogs.courant.com/.../capacitor.jpg

23. diode P-N Junction(transistor),electron tube with anode ana cathode
Allows electric current go through only in one direction
Used commonly in bridge form to convert AC to DC voltage.
“Rectification”
Various types of Bridge Rectifiers

24. transistors PNP or NPN Switch Amplifier Anolog,digital circuit
Fast, high power device
injection
diffusion
collection

25. MOS transistors 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

26. Bipolar transistors NPN-PNP configuration Advantages
Market share reducing rapidly
Still used for anolog systems
power devices
TV,Cellar phone, etc.
Advantages
Switch signal high speed
Handle large circuit(high power amplifier)
Wireless transmitter
Not effective weak signaal amplification
A standard bipolar transistor
static.howstuffworks.com/gif/amplifier-transi...

27. Semiconductor Manufacturing Process
Fundamental Processing Steps
1.Silicon Manufacturing
a) Wafer Manufacturing
b) Crystal structure
2.Photolithography
a) Photoresists
b) Photomask and Reticles
c) Patterning

28. 4. Diffusion & Ion Implantation
3.Oxide Growth & Removal
a) Oxide Growth & Deposition
b) Oxide Removal
c) Other effects
d) Local Oxidation
4. Diffusion & Ion Implantation
a) Diffusion
b) Other effects
c) Ion Implantation

29. CRYSTAL GROWTH
Czochralski Process is a Technique in Making Single-Crystal Silicon
A Solid Seed Crystal is Rotated and Slowly Extracted from a Pool of Molten Si
Requires Careful Control to Give Crystals Desired Purity and Dimensions

30. WAFER MANUFACTURING
The Silicon Crystal is Sliced by Using a Diamond-Tipped Saw into Thin Wafers
Sorted by Thickness
Damaged Wafers Removed During Lapping
Etch Wafers in Chemical to Remove any Remaining Crystal Damage
Polishing Smoothes Uneven Surface Left by Sawing Process

31. Photolithography
Photolithography is a technique that is used to define the shape of micro-machined structures on a wafer.

32. Photolithography and Photoresist
The first step in the photolithography process is to develop a mask, which will be typically be a chromium pattern on a glass plate.
Next, the wafer is then coated with a polymer which is sensitive to ultraviolet light called a photoresist.
Afterward, the photoresist is then developed which transfers the pattern on the mask to the photoresist layer.

33. There are two basic types of Photoresists Positive and Negative.
Positive resists
Positive resists decomposes ultraviolet light. The resist is exposed with UV light wherever the underlying material is to be removed.
Negative resists
Exposure to the UV light causes the negative resist to become polymerized, and more difficult to dissolve. Therefore, the negative resist remains on the surface wherever it is exposed, and the developer solution removes only the unexposed portions.

34. Photolithography Photomasks and Reticles
This is a square glass plate with a patterned emulsion of metal film on one side. The mask is aligned with the wafer, so that the pattern can be transferred onto the wafer surface. Each mask after the first one must be aligned to the previous pattern.

35. A common reticle is the 5X
When a image on the photomask is projected several time side by side onto the wafer, this is known as stepping and the photomask is called a reticle.
A common reticle is the 5X
The patterns on the 5X reticle are reduced 5 times when projected onto the wafer. This means the dies on the photomask are 5 times larger than they are on the final product. There are other kinds of reduction reticles (2X, 4X, and 10X), but the 5X is the most commonly used. Reduction reticles are used on a variety of steppers, the most common being ASM, Canon, Nikon.

36. Examples of 5X Reticles

37. Photolithography and Patterning
The last stage of Photolithography is a process called ashing. This process has the exposed wafers sprayed with a mixture of organic solvents that dissolves portions of the photoresist .
Conventional methods of ashing require an oxygen-plasma ash, often in combination with halogen gases, to penetrate the crust and remove the photoresist.  Usually, the plasma ashing process also requires a follow-up cleaning with wet-chemicals and acids to remove the residues and non-volatile contaminants that remain after ashing.  Despite this treatment, it is not unusual to repeat the "ash plus wet-clean" cycle in order to completely remove all photoresist and residues.

38. Oxidation of Silicon Dry oxide - Pure dry oxygen is employed
Disadvantage
- Dry oxide grows very slowly.
Advantage
- Oxide layers are very uniform.
- It has especially low surface state charges and thus make ideal dielectrics for MOS transistors.
Wet oxide - In the same way as dry oxides, but steam is injected
Disadvantage
- Hydrogen atoms liberated by the decomposition of the water molecules produce imperfections that may degrade the oxide quality.
Advantage
- Wet oxide grows fast.
- Useful to grow a thick layer of field oxide

39. Methods of planar process
Diffusion
--A uniformly doped ingot is sliced into wafers.
--An oxide film is then grown on the wafers.
--The film is patterned and etched using photolithography exposing specific sections of the silicon.
Diffusion is a cheaper and more simplistic method, but can only be performed from the surface of the wafers. Dopants also diffuse unevenly, and interact with each other altering the diffusion rate.
Ion Implantation
--A particle accelerator is used to accelerate a doping atom so that it can penetrate a silicon crystal to a depth of several micron
Ion implantation is more expensive and complex. It does not require high temperatures and also allows for greater control of dopant concentration and profile.

40. Summary Semiconductors; materials, conductivity between conductor
and insulator
Its conductivity can be controlled by dopant concentration and applied
voltage
Silicon, germanium, and gallium arsenate
Silicon most popular: abundant and stable oxide
Boron doped semiconductors is p-type, majority carriers are holes
P, As or Sb doped semiconductor is P-type, the majority carriers are
electrons
Higher dopant concentration, lower resistivity

41. Summary R=g *l / A C=KA/d Capacitors are mainly used in DRAM
Bipolar transistors can amplify electric signal, mainly used for analog systems
MOSFET electric controlled switch, mainly used for digital systems

42. References http://www.play-hookey.com/semiconductors/pn_junction.html