Diffusion ppt

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