Solid State Electronics ECE-1109 Md. Ebtidaul Karim Lecturer,ECE,KUET

Energy Curve in Classical Mechanics Total Energy, 𝐸= 𝑝 2 2𝑚 + 𝐸 0 Kinetic Energy Potential Energy

Energy Curve in Classical Mechanics Fig 1: Energy Band

Energy Band In solid-state physics, the electronic band structure of a solid describes the range of energies that an electron within the solid may have. Range of energies electron may not have is known as band gap. Fermi level is the highest energy state occupied by electrons in a material at absolute zero temperature. As the temperature is increased, electrons start to exist in higher energy states too.

Energy Band At T=0k, electrons occupy the lowest energy state. Thus all the state in valance band will be full and all states in conduction band will be empty. The bottom of conduction band is called 𝐸 𝑐 and the top of valance band is called 𝐸 𝑣 . Band gap energy 𝐸 𝑔 =( 𝐸 𝑐 - 𝐸 𝑣 ) Physically 𝐸 𝑔 defined as the energy required to break a bond to free an electron to the conduction band.

Energy Band For insulator, 𝐸 𝑔 >5𝑒𝑉 For Semiconductor, 𝐸 𝑔 =0.67 eV (Ge) 𝐸 𝑔 =1.1 eV (Si)

Semiconductors Solid-state electronics means semiconductor electronics; electronic equipment using semiconductor devices such as semiconductor diodes, transistors, and integrated circuits. Now what is semiconductor?? Semiconductors are a special class of elements having a conductivity between that of a good conductor and that of a insulator. It has four valance electrons in its outer orbit. In pure Si or Ge crystal four valence electron of one atom form a bonding arrangement with adjoining atoms.

Semiconductors

Semiconductors Fig 3: Covalent bond in semiconducotr

Semiconductors Semiconductors have negative temperature co-efficient but conductors have positive temperature co-efficient. Why ?? When we rise temperature of a semiconductor more electrons from the valance band moves up to the conduction band. Which means the number of free electron increases, resistance decreases. But when we rise the temperature of the conductor, resistance increases. Because in a conductor there are lots of free electron. So when we apply heat electrons movement increases. It leads to more collision among the electrons.

Semiconductors Semiconductor Intrinsic Semiconductor: The semiconductor which is refined to make as pure as possible is known as intrinsic semiconductor. Extrinsic Semiconductor: The semiconductor which is subjected to doping process is known as extrinsic semiconductor. The process of adding impurity in a semiconductor material is known as doping process. Intrinsic n-type p-type Extrinsic

Semiconductors n-type: n-type semiconductors are those in which pentavalent impurity is added. Example: As, Sb etc. Conductivity of semiconductor increases as we add pentavalent impurity. Because it adds more free carrier. Fig 4: n-type semiconductor

Semiconductors N-type Conductivity: If we apply voltage to a n-type material terminal then free electrons will move toward positive terminal of the voltage. As a result current will flow. And because the impurity which constitutes n-type material gives high number of free electron, it is known as donor impurity or donor atom.

Semiconductors P-type: Trivalent impurity is added to a semiconductor crystal to produce p-type material. Example: B, Ga, In etc. When this type of impurity is added then its valence electron can form three covalent bond in crystal. But another one remains void and cannot make any covalent bond in semiconductor. This void is known as hole.

Semiconductors P-type Conductivity: If we apply voltage to a p-type material terminal then holes will move toward negative terminal of the voltage. As a result electrons will flow at opposite direction. Hole And because the impurity which constitutes p-type material gives high number of holes, it is known as acceptor impurity or acceptor atom.

Effect of Impurity on Energy Level A discrete energy level (acceptor energy level) appears in the forbidden band with an 𝐸 𝑔 significantly less than that of the intrinsic material A discrete energy level (donor energy level) appears in the forbidden band with an 𝐸 𝑔 significantly less than that of the intrinsic material. As a result conductivity increases.

Charges On N-type and P-type Material P-type and N-type material are electronically neutral. Why ?? The terms n- and p-type doped do only refer to the majority charge carriers but p and n type materials are NOT positively and negatively charged. An n-type material by itself has mainly negative charge carriers (electrons) which are able to move freely, but it is still neutral because the fixed donor atoms, having donated electrons, are positive. Similarly p-type material by itself has mainly positive charge carrier (holes) which are able to move relatively freely, but it is still neutral because the fixed acceptor atoms, having accepted electrons, are negative.

Electron vs Hole Flow

Majority and Minority Carrier

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