Semiconductors. Silicon crystal Types of semiconductors Intrinsic semiconductors Extrinsic semiconductors www.bzupages.com

Semiconductors Materials that are neither conductor nor insulator. Element with 4 valance electrons is the best semiconductor. They contains free electrons. Element with electrical properties between those of conductors and insulators. www.bzupages.com

Difference between semiconductor and conductor is because of the existence of holes, present in semiconductors. For example; Germanium Silicon www.bzupages.com

Silicon crystals As an insulator: As a conductor: Perfect insulator at 25°C 8 valance electrons, covalently bonded with the neighboring atoms. As a conductor: act as conductor at ambient temperature (above -273°C) An electron gains energy, creating a vacancy in valence shell (hole), and move to bigger orbit. Hence, become a free electron. www.bzupages.com

Merging of free electron and a hole. Lifetime: Recombination: Merging of free electron and a hole. Lifetime: Amount of time for the creation and disappearance of free electron www.bzupages.com

Intrinsic semiconductor Pure semiconductors. Because of thermal energy, equal numbers of electrons & holes are present. These are often called as ‘carriers’ because they carry charge from one part to another. These holes & electrons move in opposite direction. www.bzupages.com

Extrinsic semiconductor Doped semiconductor is called extrinsic semiconductor. Doping: A way to increase conductivity of semiconductor. Adding impurity atoms to intrinsic crystal to alter its conductivity. www.bzupages.com

Increasing free electrons: Adding pantavalent atoms, e.g; Arsenic, Antimony, Phosphorus They are also called donar impurities,as they donate an extra electron. Greater the impurity added, greater will be the conductivity. Lightly-doped semi- conductors has high resistance. Heavily-doped semi- conductors has low www.bzupages.com

Adding trivalent impurity, e.g; Aluminium, Boron,Gallium. Increasing the holes: Adding trivalent impurity, e.g; Aluminium, Boron,Gallium. Also called acceptor atom, because hole can accept a free electron. www.bzupages.com

>N-TYPE SEMICONDUCTOR >P-TYPE SEMICONDUCTOR >THE UNBIASED DIODE >PN-JUNCTION >DEPLETION LAYER www.bzupages.com

DEFRENCE BETWEEN INTRINSIC AND EXTRINSIC SEMICONDUCTORS www.bzupages.com

n-Type semiconductor 2 types of Extrinsic semiconductors Doped with pantavalent impurity ‘n’ stands for negative Majority carriers are free electrons Minority carriers are the holes www.bzupages.com

p-Type semiconductor Doped with trivalent impurity ‘p’ stands for positive Majority carriers are holes Minority carriers are free electrons www.bzupages.com

Unbiased Diode Diode explains, two electrodes It is electrically neutral Also called junction diode or pn-crystal www.bzupages.com

pn-junction Border between p-type & n-type semiconductors. Lead to inventions like, diode, transistors, & integrated circuits. www.bzupages.com

The depletion layer Free electrons diffuses from n-type to p-type, converting the pentavalent atom in ‘n’ into positive ion and trivlent in ‘p’ into negative ion Each pair of negative and positive ions at the junction is called Dipole This depreted region around the junction, having no charges but only ions, is called Depletion region www.bzupages.com

>BARRIER POTENTIAL & TEMPRATURE >FORWARD BIAS >REVERSE BIAS BEENISH JAHANGIR Roll#07-04 >BARRIER POTENTIAL & TEMPRATURE >FORWARD BIAS >REVERSE BIAS >CURRENT IN REVERSE BIAS www.bzupages.com

Barrier Potential Electric field between positive and negative ions Act as a barrier and causes no diffusion of free electrons from ‘n’ to ‘p’ The electric field between ions is equivalent to difference of potential At 25°C the Barrier potential for; Germanium Diode is 0.3V Silicon Diode is 0.7V www.bzupages.com

BARRIER POTENTIAL &TEMPERATURE Junction temperature >Temperature inside the diode Ambient temperature >Temperature outside the diode Conducting diode>when Junction temperature is greater then Ambient temperature >Less barrier potential at higher temperature A barrier potential of a silicon diode decreases by 2mV for each degree celsius rise. V/T=-2mV/°C www.bzupages.com

Forward bias DC source across diode n-type connected to negative terminal of the battery p-type connected to positive terminal of the www.bzupages.com

Conduction of current is in forward direction because of flow of free electron No current if DC voltage is less then barrier potential Continuous current if DC voltage is greater then barrier potential www.bzupages.com

Reverse bias n-type connected to positive terminal of the battery p-type connected to negative terminal of the negative terminal attracts the holes in ‘p’ Positive terminal attracts the free electrons in ‘n’ www.bzupages.com

Depletion layer gets wider (because of ions created around the junction ) Greater the reverse voltage greater will be the potential www.bzupages.com

Current in reverse biased semiconductor Saturation current >A very small current because of minority carriers and electron- hole pair inside the depletion layer because of thermal energy.. Surface- leakage current >Caused by surface impurity and imperfection in the crystal structure. Approximately ZERO current. www.bzupages.com

>ENERGY BAND IN INTRINSIC SEMICONDUCTOR >N-TYPE ENERGY BAND >ENERGY LEVELS >ENERGY BANDS >ENERGY BAND IN INTRINSIC SEMICONDUCTOR >N-TYPE ENERGY BAND >P-TYPE ENERGY BAND www.bzupages.com

ENERGY LEVELS Higher energy in larger orbit Electron gain energy when lifted to larger orbit Which has more potential after lifting This Energy is provided by heat ,light and voltage Falling electron radiate light Electron lose energy in form of heat ,light, radiation when falls to lower orbit www.bzupages.com

ENERGY BANDS Electron of an isolated silcon atom are bound to the neucleus having distinct energy level In a solid, energy level of these isolated atom splits into sub-levels called discrete states These states are so close that they appear as a continuous energy band www.bzupages.com

A range of energy states between two consective FORBIDDEN GAP A range of energy states between two consective permissible energy bands which cannot be occupied by electron VALANCE BAND The energy band in the outer most shell of an atom occupying free electron It is either completely or partially filled but never empty Conduction is because of movement of holes www.bzupages.com

CONDUCTION BAND Above the valance band Electron move freely May be empty or partially filled Conduction is because of movement of free electron Bands below valance band take no part in conduction process because they are completely filled www.bzupages.com

ENERGY BAND IN INTRINSIC SEMICONDUCTOR Partially filled conduction & valance band A narrow forbidden gap of order 1eV between conduction band & valance band www.bzupages.com

N-TYPE ENERGY BAND P-TYPE ENERGY BAND www.bzupages.com

COMPARISON www.bzupages.com