1. INTRODUCTION TO SEMICONDUCTORS
CHAPTER 16
INTRODUCTION TO
SEMICONDUCTORS

2. ATOMIC STRUCTURE AND SEMICONDUCTORS
The basic structure of semiconductors
Silicon and Germanium Atoms

3. ATOMIC BONDING
The atoms within the crystal structure are held together by covalent bonds
This sharing of valence electrons produces the covalent bonds that hold the atoms together

4. CONDUCTION IN SEMICONDUCTORS
An energy band diagram for silicon crystal occurs only at a temperature of absolute 0 K

5. Conduction Electrons and Holes
An intrinsic (pure) silicon crystal at room temperature has sufficient heat energy for some valence electrons to jump the gap from the valence band into the conduction band, which become free electrons
When an electron jumps to the conduction band, a vacancy is left in the valence band within the crystal, called a hole.

6. Creation of electron-hole

7. Electron-hole pairs
Recombination occurs when a conduction-band electron loses energy and fall back into a hole in the valence band

8. Electron and Hole Current
When a voltage is applied across a piece of silicon, the movement of free electrons is called electron current. The current which flow opposite with electron current is called hole current.

9. Hole current in intrinsic silicon

10. Comparison of Semiconductors to Conductors and Insulators
Pure semiconductive materials are neither insulators nor good conductors because current in a material depends directly on the number of free electrons

11. N-TYPE AND P-TYPE SEMICONDUCTORS
The conductivities of silicon and germanium can be increased and controlled by the addition of impurities to the intrinsic (pure) semiconductive material called doping
The two categories of impurities are n-type and p-type

12. N-TYPE SEMICONDUCTOR
To increase the number of conduction-band electron in intrinsic silicon, pentavalent impurity atom with five valence electrons (such as arsenic (As), phosphorus (P), and antimony (Sb) are added.
n-type

13. Majority and Minority Carriers of N-type Semiconductor
The electrons are called the majority carries in n-type material ( the n stand for the negative charge on an electron)
Holes which are not produced by the addition of the pentavalent impurity atoms are called minority carries

14. P-TYPE SEMICONDUCTOR p-type
Trivalent impurity atom (three valence electrons, such as aluminum (Al), Boron (B), and gallium (Ga)) are added to increase the number of holes in intrinsic silicon
Atoms with three valence electrons are known acceptor atoms because they leave a hole in the semiconductor’s crystal structure
p-type

15. Majority and Minority Carriers of P-type Semiconductor
The holes are the majority carries in p-type material and Electron in p-type material are the minority carries

16. THE PN JUNCTION
The junction of silicon which it has doped on one half with a trivalent impurity and the other half with a pentavalent impurity is called the pn junction
The pn junction is the feature that allows diodes , transistor, and other devices to work

17. Formation of the Depletion Region
The area on both sides of the junction is called depletion region
The existence of the positive and negative ions on the opposite sides of the junction creates a barrier potential (VB) that is the amount of voltage required to move electrons through the electric field

18. Energy Diagram of the PN Junction

19. BIASING THE PN JUNCTION
Forward Bias
Forward bias is the condition that permits current through a pn junction
The negative terminal of the VBIAS source is connected to the n region, and the positive terminal is connected to the p region

20. The Effect of the Barrier Potential on Forward Bias

21. BIASING THE PN JUNCTION
Reverse Bias
Reverse bias is the condition that prevents current through the pn junction
Reverse current is a very small current produced by minority carries during reverse bias

22. Energy Diagram for Reverse Bias
When a pn junction is reverse-biased, the n-region conduction band remain at an energy level that prevents the free electrons from crossing into the p-region
There are a few free minority electrons in the p-region conduction band that flow down the ‘energy hill’ into the n-region, and they combine with minority hole in the valence band

23. Reverse Breakdown
If the external reverse-bias voltage is increased to a large enough value, reverse breakdown occurs
When one minority conduction-band electron goes toward the positive end of the pn junction, during its travel, it collides with an atom and imparts enough to knock a valence electron into the conduction band
The rapid multiplication of conduction-band electrons, known as an avalanche effect

24. DIODE CHARACTERISTICS
Diode Characteristic Curve
Forward bias
As the forward voltage approaches the value of the barrier potential (0.7 V for silicon and 0.3 V for germanium), the current begins to increase

25. DIODE CHARACTERISTICS
Diode Characteristic Curve
Reverse bias
As the voltage (VR) increases to the left, the current remains near zero until the breakdown voltage (VBR) is reached
When breakdown occurs, there is a large reverse current that can destroy the diode

26. Diode symbol

27. Diode Approximations
The ideal diode model

28. Diode Approximations
The practical diode model

29. Diode Approximations
The complex diode model

30. Typical Diode package