1.0 INTRODUCTION TO SEMICONDUCTORS

1.1 Characteristics and electrical properties of semiconductors.

1.1.1 Semiconductor and

include silicon and germanium.

1.1.2 Characteristics of N-type and P-type semiconductors.

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1.2 Characteristics of P-N junction and its reaction towards voltage biasing.

1.2.1 Formation of a junction a. Free electrons mobility b. Formation of depletion region and its

properties. c. Existence of threshold voltage and its values

for silicon and germanium. 1.2.2 Forward biased voltage and reverse biased

voltage supplied across P-N junction.

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1.2.3 Effects when a P-N junction is supplied with forward biased voltage and reverse biased voltage on the following items :

a. Area of depletion region. b. Junction resistance c. Current flow (including leakage current)

1.2.4 Breakdown when P-N junction is reverse biased.

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INTRODUCTION OF ATOMIC STRUCTURE• Atomic structure is the smallest element in a material.• Atomic structure model was introduced by Niels Bohr in

1913.• Atom consists of a nucleus at the center which is

surrounded by electrons. The nucleus contains proton (positive charge) and neutron (neutral).

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• The maximum orbit in an atom is 7 layers.• The orbits known as K, L, M, N, O, P and Q layer.

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• Maximum number of electrons in each orbit determine by formula:

• Maximum number of electrons in each layer :

2 x n2 n is the number of layer

Layer

No. of layer

Calculation

Maximum Electron

K 1 2 x 12 2

L 2 2 x 22 8

M 3 2 x 32 18

N 4 2 x 42 32

O 5 2 x 52 50

P 6 2x 62 72

Q 7 2 x 72 98

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• The number of layers are depend on the number of electrons in an atom.

• The outer layer of an atom named valence layer.• The electron in valence layer called electron valence

(it is current carrier).

• The maximum electrons in valence layer is 8 electrons. • The number of electron in valence layer determine the

electrical characteristics of the materials (conductor, semiconductor or insulator). 7

Valence electron

Valence layer

Figure 3 : Valence layer and Electron valence position

ELECTRON VALENS, TYPES OF MATERIAL & CHARACTERISTICS.Number

of ValenceElectro

n

Types of Material

Characteristic

1 to 3 Conductor • Can conduct an electrical current.• Low resistant that ease the current flow.• The atom always release its valence electrons. So, the

electrons are free to move from one atom to another.• Example : Gold, copper.

5 to 8 Insulator • Cannot conduct an electrical current.• High resistant.• The atom always receive valence electrons from another

atom to fill its valence layer. So, its become stable and capable to avoid any electrical activities.

• Example : Rubber.4 Semiconductor • Its electrical conductivity is between conductor and

insulator.• Cannot release/receive valence electrons.• Example : Silicon, Germanium.

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Example 1 :An Aluminium has 13 electrons. i). Determine the number of electrons in each layer.ii). Sketch its atomic structure.iii). State the material type.Solution : i). Number of electrons in each layer : ii). Atomic Structure :

K layer (1) : 2 x 12 = 2 electrons L layer (2) : 2 x 22 = 8 electrons M layer (3) : 13-2-8 = 3 electrons

iii). Material Type : Conductor (3 electron valens) 

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Example 2 :Germanium has 32 electrons.i). Determine the number of electrons in each layer.ii). Sketch its atomic structure.iii). State the material type. Solution :i). Number of electrons in each layer : ii). Atomic Structure : K Layer (1) : 2 x 12 = 2 electrons L Layer (2) : 2 x 22 = 8 electrons M Layer (3) : 2 x 32 = 18 electrons N Layer (4) : 32-2-8-18 = 4 electrons

iii). Material Type : Semiconductor (4 electron valens)

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1.1.1 Semiconductor

• Semiconductor is a material that has electrical conductivity between conductor and insulator.

• Always use in electronic components manufacturing (example: diode, transistor and integrated circuit).

• It has 4 valence electrons in its valence layer. Valence electrons are current carrier.

• An atom that has 4 valence electrons is unstable. It has to complete its valence electrons from 4 to 8 by sharing electrons with another atom. The electron sharing is called ‘ Covalent Bonds’.

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• Silicon and Germanium are a sample of semiconductor because it has 4 valence electrons in its valence layer.

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Valence layer has4 valence electrons

Covalent Bonds• Definition : Covalent Bond is when an atom sharing valence

electrons with their neighboring atom. So that it looks like having 8 valence electrons.

• It makes the atom becomes stable and has strong bonding.

• By this state, the atoms act as an insulator at the room temperature.

13Figure 6 : Covalent Bonds

• Electron and hole are created when the stability of the covalent bond is disturbed by several factors such as increase of temperature, voltage potential or doping process.

• Electron has a negative charge. So, free electrons are called ‘Negative current carrier’.

• Hole has a positive charge. So, holes are called as ‘Positive current carrier’.

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• There are several factors that effected the stability of an atom such as increase in temperature, potential difference and doping process.

i. Increase in temperature• In low temperature, covalent bonding is stable. • When temperature is increasing, the atom become

unstable.• Electrons free from their bonding, moving from one orbit

to another orbit randomly.

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ii. Potential difference• When a high voltage supplied to a semiconductor

materials, electrons will move to the positive potential. •In that time, it will act as a conductor.

• The electron movements are as figure below :

16Figure 7 : Effect of potential difference to a semiconductor.

iii. Doping Process• Process of adding impurity atoms into intrinsic

semiconductor to increase the number of any current carrier (either free electron or hole) of the semiconductor.

• Two elements used for doping are Trivalent and Pentavalent.

• If semiconductor is doped with Trivalent material (3 valence electrons), it will produce P-type semiconductor.

• If semiconductor is doped with Pentavalent material (5 electron valence), it will produce N-type semiconductor.

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Types Of Semiconductors i. Intrinsic semiconductor

• Instrinsic semiconductor is a pure semiconductor that has no foreign substance.

• Example : Germanium and Silicon. ii. Extrinsic semiconductor

• Not pure, mixed with foreign substances.• The mixing process known as ‘doping process’.• Produce N-type or P-type semiconductor. • Foreign subtances known as Trivalens (has 3 valence

electrons) or Pentavalens (has 5 valence electrons). • Example foreign substance of Trivalens : Aluminium, Boron,

Galium and Indium .• Example foreign substance of Pentavalens : Antimoni,

Arsenik dan Fosforus 18

1.1.2 Characteristic of N-type and P-type semiconductors.i. N-type semiconductors.It happens when pure semiconductors (eg Silicone) is doping with Pentavalent impurities (5 valence electrons) (refer figure 8) :

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●

Contributes free electron

Figure 8 : N-Type Semiconductor

• 4 of 5 valence electrons from the foreign substance atom will form Covalent bonds with Silicon atoms, but there will be one more electron that has no pair.

• The electron will free from its orbit and become free electron (negative current carrier).

• The material is called as N-type material.• Its majority current carriers are free electrons.• Its minority current carriers are holes.

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ii. P-type semiconductors• It happens when the intrinsic semiconductor is doping

with Trivalens impurities (3 valence electrons).

Figure 9 : P-Type Semiconductor.

• Example (Refer to Figure 9) : Silicon (Si) is an intrinsic semiconductor, while Indium (In) is the impurities.

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Producing Hole

• Silicon allowed its 3 valence electrons to form covalent bond with 3 Indium’s (In) valence electrons.

• But, one more valence electron of the silicon is not forming a covalent bond because lack of Indium valence electron. This empty space formed a hole, which is a positive current charge.

• The material is called as P-type material.• Its majority of current carriers are holes .• Its minority of current carriers are electrons.

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1.2 P-N Junction• A P-N junction is formed at the boundary between a

P-Type and N-Type semiconductor. • P-N junction is feature which enables diode, transistor

and the other devices work.

1.2.1 Formation of a junction • Figure below shows the combination of N-type

and P-type materials which form a P-N junction.

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P-typeN-type

P-N JUNCTION

Figure 10 : P-N junction

• After joining P-type and N-type semiconductors, electrons on the N-type region tend to diffuse into the P region.

• As the electrons diffuse, they leave positively charged ions in the N region. This creates a layer of positive charges near the junction.

• Likewise, holes on the P-type region begin to diffuse into the N-type region, leaving negative charge.

• When a free electron meets a free hole, it makes the electron hole neutral pair. This means the hole and electrons cancel each other and vanish. Thus, at the boundary, there is an area that does not have any mobile charge carrier. This is call Depletion Region.

• This neutral area provides the barrier for the further movement of the charge carriers, which is called potential barrier.

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• The minimum forward voltage applied to overcome the potential barrier is called threshold voltage.

• The threshold voltage is the minimum forward voltage value across the semiconductor at which the semiconductor start to conduct current.

• The threshold voltage is approximately 0.3V for germanium and 0.7V for silicon.

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Barrier Potential

Figure 11: Formation Of Depletion Region and Barrier Potential.

P-typeN-type

Depletion Region

-

-

+

+

1.2.2 Voltage Biases

Definition : Voltage bias is voltage that supplied across P-N junction.

1.2.3 Types of voltage bias : i) Forward Biasii) Reverse Bias

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i). Forward Bias

• Definition : P-type semiconductor material is connected to the positive terminal and the N-type semiconductor material is connected to the negative terminal of a battery (Figure 12)

Figure 12 : Forward Bias 27

• Circuit Operation : - Electron in N material will be push towards the

combination area cause depletion region become thin (small).

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Figure 13 : Depletion Region In Forward Bias

Depletion Region

P-type N-type

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- The resistance of the P-N junction will decrease.

- When the forward bias voltage increase until over the knee voltage (0.3V for Germanium and 0.7V for Silicon), the electron will be able to cross the combination region and also towards the positive supply.

- So, the current can flow.

- The current is known as Forward Current.

- The P-N junction resistance is known as forward resistance.

ii). Reverse Bias

• Definition : N-type semiconductor material is connected to the positive terminal of a battery and the P-type semiconductor material is connected to the negative

terminal (Figure 14).

Figure 14 : Reverse Bias

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• Circuit Operation : - Electron from N-Type will be pushed to the positive supply.

The depletion region will be larger.

Depletion Region

Figure 15 : Depletion Region Of Reverse Bias

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- The resistance will increase. We call it as Reverse Resistance.

- So the current cannot flow through the combination region.

- Leakage current / Reverse current is a minority current in the device.

- Electrons from the P type will pushed by the voltage bias towards the combination region and then crossing the region. So, it will produce a small current flow.

- Its value is depends on the temperature. If the temperature is low, the current’s value will be low too.

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1.2.4 Breakdown when P-N junction is reverse biased.

- If the P-N junction is supplied with extremely high -reverse bias, it will distract the covalent bond.

- Electron will be pushed to positive terminal and will free as current carrier. The free electron will hit others bond.

- The reverse current will flow and the value is suddenly increase. This level is called as Breakdown Voltage. This will cause the PN junction to burn.

- To avoid this problem, the maximum reverse bias voltage given cannot be larger than the breakdown voltage limit.

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