Diffusion takes place through higher concentration to lower concentration. Diffusion is the process of movement of charge carriers due to concentration gradient along the semiconductor. The process by which electrons or holes move from region of higher concentration to a region of lower concentration.

1. DEPARTEMENT OF ELECTRONICS AND
COMMUNICATION ENGINEERING
DIFFUSION
UNDER THE GUIDANCE OF PRESENTED BY
DR.K.ANUSUDHA ADIMALLA PRANAY
ASSISTANT PROFESSOR (21304002)

2. CONTENTS
 INTRODUCTION
 STEADY STATE DIFFUSION
 DIFFFUSION MECHANISM
 DIFFUSION PROFILES
 DUAL DIFFUSION PROCESS
 INTRINSIC & EXTRINSIC DIFFUSION
 DIFFUSION IN SILICON
 APPLICATIONS
 REFERENCES

3. INTRODUCTION
 Diffusion is the process of movement of charge
carriers due to concentration gradient along the
semiconductor.
 The process by which electrons or holes move from
region of higher concentration to a region of lower
concentration.

4. INTRODUCTION
 Diffusion is the net movement of molecules from an area
where they are at a higher concentration to areas where
they are at a lower concentration.
 The difference in the concentration of a substance between
two areas is called the concentration gradient .
 Diffusion current density is directly propotional to the
concentration gradient.

5. INTRODUCTION
DIFFUSION CURRENT :
 The current produces due to motion of charge carrier from high
region to lower region.
 It occurs without external field.

6. INTRODUCTION
 Impurity doping is the introduction of controlled amount of impurity
dopants into semiconductors.
 The main goal of doping is changing the electrical properties of
semiconductor.
 Diffusion and ion implantation are the two key method of impurity
doping.

7. STEADY STATE DIFFUSION
 Fick’s second law states that the change in flux, J concentration
with time in a particular region is proportional to the change in the
concentration gradient at that point in the system.
 Under certain conditions the diffusion rate become constant.

8. STEADY STATE DIFFUSION
SINK CONDITIONS:
 State in which concentration in receptor compartment
is maintained at lower level compared to it’s
concentration in donor compartment.
 During diffusion donor compartment act as source and
receptor compartment act as sink.
 Useful for maintaining concentration gradient nearly
constant.

9. FICK’S LAWS OF DIFFUSION
FICK’S FIRST LAW :
 The amount, M, of material flowing through a unit
area, S, of a barrier in unit time, t, is known as the
flux, J
 The flux, in turn, is propotional to the concentration
gradient,dC/dx
FICK’S FIRST
LAW

10. FICK’S LAWS OF DIFFUSION
FICK’S FIRST LAW :
 The negative sign of equation signifies that diffusion
occurs in a direction opposite to that of increasing
concentration.
 That is, diffusion occurs in the direction of decreasing
concentration of diffusant; thus, the flux is always a
positive quantity.
 Diffusion will stop when the concentration gradient
no longer exists(i.e., when dC/dx=0)

11. DIFFUSION MECHANISM
 The open circles represent the host atoms occupying the equilibrium lattice positions.
 The solid dots represent impurity atoms.
 At elevated temperature, the lattice atoms vibrate around the equilibrium lattice sites.
There is a finite probability that a host atom can acquire sufficient energy to leave the
lattice site and to become an interstitial atom thereby creating a vacancy.
 When a neighboring impurity migrates to the vacancy site, the mechanism is called
vacancy diffusion.
 If an interstitial atom moves from one place to another without occupying a lattice site
,the mechanism is interstitial diffusion.

12. DIFFUSION MECHANISM
VACANT DIFFUSION:
 The vacancy diffusion is an atom breaks its bonds and jumps
into neighboring vacant site.
INTERSTITIAL DIFFUSION:
 The interstitual diffusion,solute atoms which are small
enough to occupy interstitial sites diffuse by jumping from
one interstitial site to another. ... Thus interstitial diffusion is
faster than substitutional diffusion by the vacancy
mechanism.

13. DIFFUSION PROFILES
CONSTANT SURFACE CONCENTRATION :
 “infinite source” diffusion
 Initial condition:C(x,0)=0
 Boundary conditions:
C(0,t)=Cs
C(∞,t)=0
 Solution:

14. DIFFUSION PROFILES
CONSTANT TOTAL DOPANT :
 “Limited source” diffusion
 Initial condition:C(X,0)=0
 Boundary conditions:
C(∞,t)=0
 Solution:

15. DUAL DIFFUSON PROCESS
 In VLSI processing, a two-step diffusion sequence is commonly
used, in which a predeposition diffusion layer is formed under a
constant-surfaceconcentration condition and is followed by a drive-
in diffusion or redistribution under a constant- total-dopant condition.
 For most practical cases, the diffusion length √Dt for the
predeposition diffusion is much smaller than that for the drive-in
condition. Hence, the predeposition profile can be treated as a delta
function at the surface.

16. INTRINSIC & EXTRINSIC DIFFUSION
 Diffusion that occurs when the doping concentration is lower than the intrinsic carrier
concentration, ni, at the diffusion temperature is called intrinsic diffusion.
 In this region, the resulting dopant profiles of sequential or simultaneous diffusion of n-
type or p-type impurities can be determined by superposition, that is, the diffusion processes
can be treated independently.
 However, when the dopant concentration exceeds ni (e.g. at 1000oC, ni = 5 x 1018
atoms/cm3), the process becomes extrinsic, and the diffusion coefficients become
concentration dependent

17. DIFFUSION IN SILICON
INTRODUCTION :
 Interstitialcy diffusion results from silicon self-
interstitials displacing substitutional impurities to an
interstitial position - requires the presence of
silicon self-interstitials, the impurity interstitial may
then knock a silicon lattice atom into a self-
interstitial position.

18. DIFFUSION IN SILICON
BASIC CONCEPT :
 The concepts of impurity profile,bacxkground concentration and junction depth
will be used.
 IMPURITY PROFILE :
 The concentration of an impurity versus depth into the silicon.
 BACKGROUND CONCENTRATION :
 An impurity concentration existing in the silicon that an impurity profile is
formed into.
 JUSCTION DEPTH :
 The depth in which the impurity profile concentration is equal to the background
concentration.

19. DIFFUSION IN SILICON
 The preceding three concepts.the impurity profile,C(x) varies with
depth into the silicon, x, the background concentration Cb is shown
at a constant level and the junction depth ,xj is the depth at which
C(x)=Cb

20. APPLICATIONS
 Sliding and rotating parts needs to have hard surfaces.
 These parts are usually machined with low carbon steel as they are
easy to machine.
 Their surfaces is then hardened by carburizing.
 P-n junction: dopant diffusion for semiconductor devices.
 Sputtering,annealing: magnetic materials for hard drives.
 The relese of drug from dosage form is diffusion controlled,such
dosage form is available in market as sustained and controlled
releaseproduct.

21. REFERENCES
Nandita das gupta, textbook of semiconductor devices modelling and
technology,amitava das gupta,page.no.49
www.google.com
http://www.cityu.edu.hk