Semiconductor Materials and Diodes, Lecture notes of Electronics

Download Semiconductor Materials and Diodes and more Lecture notes Electronics in PDF only on Docsity!

Semiconductor Materials and Diodes

CO2005: Electronics I

 Electrons in the outermost shell are called valence electrons , and the chemical activity of a material is determined primarily by the number of such electrons.

 The valence electrons are shared between atoms, forming what are called covalent bonds.

 Silicon :

At T=0K, each electron is in its lowest possible energy state, so each covalent bonding position is filled. If a small electric field is applied to this material, the electrons will not move. At T=0K, silicon is an insulator , no charge flows through it.

Silicon

 A valence electron that has a certain thermal energy and is adjacent to a hole may move into that position, making it appear as if a positive charge is moving through the semiconductor.

 Two types of charged particles contribute to the current under a small electric field at room temperature:  the negatively charged free electron  the positively charged hole

Current in Semiconductors

 An intrinsic semiconductor is a single-crystal semiconductor material with no other type of atoms within the crystal.

 In an intrinsic semiconductor, the densities of electrons and holes are equal, since the thermally generated electrons and holes are the only source of such charged particles.

 Intrinsic carrier concentration :

the concentration of the free electrons and that of the holes

Intrinsic Semiconductors

3/ 2^2

E g kT ni BT e pi

 (^)      

ni

is theBoltzman' nconstant ( 86 10 eV/ )

istheabsolute temperature( )

is thebandgapenergy (eV)

isaconstant related tothespecific semiconduc tormaterial

6 o

o

k K

T K

E

B

g

^ 

 Adding controlled amounts of certain impurities can greatly increase the concentration, then, the conductivity. The process to add impurities is called doping.

 A desirable impurity is one that enters the crystal lattice and replaces one of the semiconductor atoms. The impurities are from the group III and V elements.

 The materials containing impurity atoms are called extrinsic semiconductors.

 Donor impurity : phosphorus atom (group V element)

 Four of its valence electron are used to satisfy the covalent bond requirements, the fifth valence electron is more loosely bound to the phosphorus atom.  At room temperature, this electron has enough thermal energy to break the bond, thus free to move through the crystal and contribute to the electron current.  The remaining phosphorus atom has a net positive charge, but the atom is immobile in the crystal and cannot contribute to the current.  A semiconductor containing donor impurity atoms is N-type semiconductor.

N-type Semiconductors

 Acceptor impurity : boron atom (group III element)

 Three valence electrons are used to satisfy the covalent bond requirements for three of four nearest silicon atoms. This leaves one bond position open.  At room temperature, adjacent silicon valence electrons have sufficient thermal energy to move into this position, thereby creating a hole.  The boron atom has a net negative charge, but cannot move, and a hole is created that can contribute to a hole current.  A semiconductor containing acceptor impurity atoms is P-type semiconductor.

P-type Semiconductors

 N-type Semiconductor

 Drift velocity of electrons

For low-doped silicon,.

 Drift current density

Drift Current

v (^) dn   nE

 n : electron mobility (cm^2 / V  s )

J (^) n  envdn  en  n E (A/cm 2 )

 n  1350 cm^2 /Vs

:themagnitude of theelectronic charge

:electron concentration

e

n

 The total drift current density is the sum of the electron and hole components.

where is the conductivity of the semiconductor.

 The conductivity can be changed from strongly n-type, n >> p , by donor impurity doping to

strongly p-type, p >> n , by acceptor impurity doping.

Conductivity

J  en  nE  ep  pE   E

  en  n  ep  p



 With diffusion, particles flow from a region of high concentration to a region of lower concentration.

Diffusion Current

:hole diffusion coefficien t

:electron diffusion coefficien t

( ) ,

( )

p

n

n n p p

D

D

dx

dp x J eD dx

dn x J  eD 

 PN Junction: the entire semiconductor material is a single crystal, with one region doped to be p-type and the adjacent region doped to n-type.

 Initially, a large density gradient in both the hole and electron concentration occurs across this junction and causes a diffusion of holes and electrons.

PN Junction

 If no voltage is applied to the pn junction, the diffusion of holes and electrons must eventually cease.

 The direction of the induced electric field will cause the resulting force to repel the diffusion of holes from the p-region and the diffusion of electrons from the n-region.

 Thermal Equilibrium: The force produced by the electric field and the force produced by the density gradient exactly balance.

Thermal Equilibrium

 Reverse Bias: The magnitude of the electric field in the depletion region increases above the thermal equilibrium value.

 This increased electric field holds back the holes in the p-region and the electrons in the n- region, so there is essentially no current across the pn junction, so there is essentially no current across the pn junction.

 When the electric field in the depletion region increases, the number of positive and negative charges also increases.

 With an increasing reverse-bias voltage , depletion width W also increases.

 Junction (depletion layer) capacitance:

Reverse-Biased PN Junction

VT

1 / 2 1



 

  

   bi

R j jo V

V C C