1. DMT 121
ELECTRONIC DEVICES

2. Chapter 1 Introduction to Semiconductor
DMT 121
ELECTRONIC DEVICES
Chapter 1
Introduction to Semiconductor

3. Semiconductor Materials

4. Semiconductor Materials
Definition: Semiconductors are a special class of elements having a conductivity between that of a good conductor and that of an insulator

5. Semiconductor Materials
Single crystal – Germanium (Ge) and Silicon (Si)
Compound Semiconductor – Gallium Arsenide (GaAs), Cadmium Sulfide (CdS), Gallium Nitride (GaN) and Gallium Arsenide phosphide (GaAsP).
Mostly used : Ge, Si and GaAs

6. Semiconductor Materials
Ge – First discovered. Used as Diode in 1939, transistor in Sensitive to changes in temperature – suffer reliability problem.
Si – Introduced in 1954 (as transistor), less sensitive to temperature. Abundant materials on earth. Over the time – its sensitive to issue of speed.
GaAs – in 1970 (transistor), 5x speed faster than Si. Problem – difficult to manufacture, expensive, had little design support at the early stage.

7. Periodic Table • Columns: Similar Valence Structure 1e inert gases 2e
Electropositive elements:
Readily give up electrons
to become + ions.
Electronegative elements:
Readily acquire electrons
to become - ions. He N e Ar Kr Xe Rn
inert gases
accept 1e
accept 2e
give up 1e 2e 3e F Li Be
Metal
Nonmetal
Intermediate H Na Cl Br I At O S Mg Ca Sr Ba Ra K Rb Cs Fr Sc Y Se Te Po

8. Electropositive elements:
Readily give up electrons
to become + ions.
Electronegative elements:
Readily acquire electrons
to become - ions.

9. Semiconductors, Conductors & Insulators
Material that easily conducts electrical current.
The best conductors are single-element material (e.g copper,silver,gold,aluminum,ect.)
One valence electron very loosely bound to the atom- free electron
Insulators
Material that does not conduct electric current under normal conditions.
Valence electron are tightly bound to the atom – less free electron
Semiconductors
Material between conductors and insulators in its ability to conduct electric current
in its pure (intrinsic) state is neither a good conductor nor a good insulator
most commonly use semiconductor- silicon(Si), germanium(Ge), and carbon(C).
contains four valence electrons

10. Covalent Bonding & Intrinsic Materials
Atom = electron + proton + neutron
Nucleus = neutrons + protons
Protons
(positive charge)
Neutrons
(uncharged)
Nucleus
(core of atom)
Electrons
(negative charge)
ATOM

11. Atomic Structure No. of electron in each shell Ne = 2(n)2
n = no of shell.

12. Covalent Bonding Covalent bonding of the Silicon atom
Covalent bonding of the GaAs crystal

13. Intrinsic Carrier
Table 1.1
Intrinsic Carriers
Semiconductor
(per cubic centimeter)
GaAs
1.7 x 106 Si
1.5 x 1010 Ge
2.5 x 1013
Intrinsic (pure) carriers – The free electrons in a material due to only external causes
Ge has the highest number of carriers and GaAs has the lowest intrinsic carriers.
The term intrinsic (pure) is applied to any semiconductor material that has carefully refined to reduce the number of impurities to a very low level – essentially as pure as can be made available through modern technology

14. Relative Mobility Factor µn
Table 1.2
Relative Mobility Factor
Semiconductor
µn (cm2/V-s) Si
1500 Ge
3900
GaAs
8500
Relative mobility – the ability of the free carriers to move throughout the material.
GaAs has 5X the mobility of free carriers compared to Si, a factor that results in response times using GaAs electronic devices is 5X those of the same device made from Si.
Ge has more than twice the mobility of electrons in Si, a factor that results in the continued of Ge in high-speed radio frequency applications.

15. Difference between Conductors & Semiconductors
Conductors – Resistance increases with the increase in heat, because their vibration pattern about relatively fixed location makes it increasingly difficult for a sustained flow of carriers through the material – positive temperature coefficient.
Semiconductors – Exhibit an increased level of conductivity with the application of heat. As the temperature rises, an increasing number of valence electron absorb sufficient thermal energy to break the covalent bond and contribute to the number of free carriers – negative temperature effects

16. Energy Level
Figure: Energy levels: conduction and valence bands of an insulator, a semiconductor, and a conductor.

17. Extrinsic Materials : n-Type and P-Type Materials
The characteristics of a semiconductor material can be altered significantly by the addition of a specific purity atoms to relatively pure semiconductor materials – this process is known as doping process
A semiconductor that has been subjected to the doping process is called an extrinsic materials.
Extrinsic Materials are n-type material [five valence electrons (pentavalent)] and p-type material [three valence electrons atom (trivalent)]

18. N-Type Materials
n-Type material is created by introducing the impurity (bendasing) elements that have five valence electrons (pentavalent).
There are antimony (Sb), Arsenic (As) and phosphorous (P).
Diffused impurities with five valence electrons are called donor atoms
Figure: Antimony impurity in n-type material

19. N-Type Materials
The effect of this doping cause the energy level (called the donor level) appears in the forbidden band with Eg significantly less than intrinsic material.
This cause less thermal energy to move free electron (due to added impurity) into conduction band at room temperature.
Figure: Effect of donor impurities on the energy band structure

20. N-Type Material
Pentavalent atoms is an n-type semiconductor (n stands for the negative charge on electrons).
The electrons are called the majority carrier in n-type materials.
In n-type material there are also a few holes that are created when electrons-holes pairs are thermally generated
Holes in n-type materials are called minority carrier.

21. Figure: Boron impurity in p-type material.
Si or Ge doped with impurities atoms having three valence electrons.
Mostly used are boron (B), gallium (Ga) and indium (In).
The void (vacancy) is called ‘hole’ represented by small circle or a ‘+’ sign.
Figure: Boron impurity in p-type material.
Diffused impurities with three valence electrons are called acceptor atoms

22. P-Type Material
In p-type materials the hole is the majority carrier and the electron is the minority carrier.
Holes can be thought as +ve charges because the absence of electron leaves a net +ve charge on the atom.

23. Electron vs Hole Flow
With sufficient kinetic energy to break its covalent bond, the electron will fills the void created by a hole, then a vacancy or hole, will be created in the covalent bond that released the electron.

24. Semiconductor Diode Diode Simple construction of electronic device
It is a joining between n-type and p-type material (joining one with majority carrier of electron to one with a majority carrier of holes)

25. No Bias (VD=0V)

26. Forward Bias (VD > 0 V)
Figure: Forward-biased p–n junction. (a) Internal distribution of charge under forward-bias conditions; (b) forward-bias polarity and direction of resulting current.

27. Reverse Bias (VD < 0 V)
Figure: Reverse-biased p–n junction. (a) Internal distribution of charge under reverse-bias conditions; (b) reverse-bias polarity and direction of reverse saturation current.

28. Diode Characteristics Curve
Figure: Silicon semiconductor diode characteristics.

29. Figure: Comparison of Ge, Si, and GaAs diodes.

30. Figure: Variation in Si diode characteristics with temperature change.
Temperature Effects
Figure: Variation in Si diode characteristics with temperature change.

31. Ideal Vs Practical
Semiconductor diode behaves in a manner similar to mechanical switch that can control the current flow between it’s two terminal
However, semiconductor diode different from a mechanical switch in the sense that it permit the current flow in one direction

32. Figure: Ideal versus actual semiconductor characteristics.
Ideal Vs Practical
Figure: Ideal semiconductor diode: (a) forward-biased (b) reverse-biased.
Figure: Ideal versus actual semiconductor characteristics.
(Short circuit equivalent –fwd bias, actual case R ≠ 0)
(Open circuit equivalent – Reverse bias, actual case saturation current Is ≠ 0)

33. Approximate Diode

34. Resistance Levels DC or Static Response
Application of dc voltage will result in an operating point on the characteristic curve will not change with time.
In general, the higher the current
through a diode, the lower is the
dc resistance level.
Figure: Determining the dc resistance of a diode at a particular operating point.

35. Figure: Defining the dynamic or ac resistance.
Resistance Levels
AC or Dynamic Response
Figure: Defining the dynamic or ac resistance.

36. Resistance Levels Average AC Response
Figure: Determining the average ac resistance between indicated limits.

37. Diode Equivalent Model

38. Example 1
Determine the forward voltage (VF) and forward current [IF]. Also
find the voltage across the limiting
resistor. Assumed rd’ = 10 at the determined value of forward.

39. Example 2 Determine the Reverse voltage (VR). Also
find the voltage across the limiting resistor. Assumed IR = 1 µA.
Answer:
VRLIMIT =1mV
VR=4.999V

40. Figure: Checking a diode with an ohmmeter.
Diode Testing
Analog MM (or Ohm meter testing)
Figure: Checking a diode with an ohmmeter.

41. Figure: DMM diode test on a properly functioning diode.
Diode Testing
Digital MM
Figure: DMM diode test on a properly functioning diode.

42. Diode Testing – Defective diode
Digital MM (Testing Defective Diode)
Diode failed open: get open circuit reading (2.6 V) or ‘OL’
Diode is shorted: get 0 V reading in both forward and reverse bias test.

43. Diode Notation

44. Figure: Characteristics of Zener region.
Zener Diode
Figure: Conduction direction: (a) Zener diode (b) semiconductor diode (c) resistive element.
Figure: Characteristics of Zener region.

45. Zener Region The Zener region is in the diode’s reverse-bias region.
At some point the reverse bias voltage is so large the diode breaks down and the reverse current increases dramatically.
This maximum voltage is called avalanche (runtuhan) breakdown voltage
The current is called avalanche current.