Semiconductor Device Modeling and Characterization – EE5342 Lecture 3 – Spring 2011 Professor...
-
Upload
berniece-rose -
Category
Documents
-
view
217 -
download
0
Transcript of Semiconductor Device Modeling and Characterization – EE5342 Lecture 3 – Spring 2011 Professor...
Semiconductor Device Modeling and
Characterization – EE5342 Lecture 3 – Spring 2011
Professor Ronald L. [email protected]
http://www.uta.edu/ronc/
©rlc L03 24Jan2011
2
Web Pages
* Bring the following to the first class
• R. L. Carter’s web page– www.uta.edu/ronc/
• EE 5342 web page and syllabus– http://www.uta.edu/ronc/5342/
syllabus.htm• University and College Ethics Policieswww.uta.edu/studentaffairs/conduct/www.uta.edu/ee/COE%20Ethics%20Statement%20Fall%2007.pdf
©rlc L03 24Jan2011
3
First Assignment
• e-mail to [email protected]– In the body of the message include
subscribe EE5342 • This will subscribe you to the
EE5342 list. Will receive all EE5342 messages
• If you have any questions, send to [email protected], with EE5342 in subject line.
©rlc L03 24Jan2011
4
Second Assignment
• e-mail to [email protected]– In the body of the message include
subscribe EE5342 • This will subscribe you to the
EE5342 list. Will receive all EE5342 messages
• If you have any questions, send to [email protected], with EE5342 in subject line.
©rlc L03 24Jan2011
5
Schrodinger Equation
• Separation of variables givesY(x,t) = y(x)• f(t)
• The time-independent part of the Schrodinger equation for a single particle with KE = E and PE = V.
2
2
280
x
x
mE V x x
h2 ( )
©rlc L03 24Jan2011
6
K-P Potential Function*
©rlc L03 24Jan2011
7
K-P Static Wavefunctions• Inside the ions, 0 < x < a
y(x) = A exp(jbx) + B exp (-jbx) b = [8p2mE/h]1/2
• Between ions region, a < x < (a + b) = L y(x) = C exp(ax) + D exp (-ax) a = [8p2m(Vo-E)/h2]1/2
©rlc L03 24Jan2011
8
K-P Impulse Solution• Limiting case of Vo-> inf. and b ->
0, while a2b = 2P/a is finite• In this way a2b2 = 2Pb/a < 1, giving
sinh(ab) ~ ab and cosh(ab) ~ 1• The solution is expressed by
P sin(ba)/(ba) + cos(ba) = cos(ka)
• Allowed values of LHS bounded by +1
• k = free electron wave # = 2p/l
©rlc L03 24Jan2011
9
K-P Solutions*
P sin(ba)/(ba) + cos(ba) vs. ba
xx
©rlc L03 24Jan2011
10
K-P E(k) Relationship*
©rlc L03 24Jan2011
11
Analogy: a nearly-free X electron model
• Solutions can be displaced by ka = 2np
• Allowed and forbidden energies• Infinite well approximation by
replacing the free electron mass with an “effective” mass (noting E = p2/2m = h2k2/2m) of
1
2
2
2
2
4
k
Ehm
©rlc L03 24Jan2011
12
Silicon BandStructure**• Indirect Bandgap• Curvature (hence
m*) is function of direction and band. [100] is x-dir, [111] is cube diagonal
• Eg = 1.17-aT2/(T+b) a = 4.73E-4 eV/K b = 636K
©rlc L03 24Jan2011
13
Generalizationsand Conclusions• The symm. of the crystal struct.
gives “allowed” and “forbidden” energies (sim to pass- and stop-band)
• The curvature at band-edge (where k = (n+1)p) gives an “effective” mass.
©rlc L03 24Jan2011
14
Silicon Covalent Bond (2D Repr)
• Each Si atom has 4 nearest neighbors
• Si atom: 4 valence elec and 4+ ion core
• 8 bond sites / atom
• All bond sites filled
• Bonding electrons shared 50/50
_ = Bonding electron
©rlc L03 24Jan2011
15
Si Energy BandStructure at 0 K
• Every valence site is occupied by an electron
• No electrons allowed in band gap
• No electrons with enough energy to populate the conduction band
©rlc L03 24Jan2011
16
Si Bond ModelAbove Zero Kelvin
• Enough therm energy ~kT(k=8.62E-5eV/K) to break some bonds
• Free electron and broken bond separate
• One electron for every “hole” (absent electron of broken bond)
©rlc L03 24Jan2011
17
Band Model forthermal carriers• Thermal energy
~kT generates electron-hole pairs
• At 300K Eg(Si) = 1.124 eV >> kT = 25.86
meV,Nc = 2.8E19/cm3
> Nv = 1.04E19/cm3
>> ni = 1.45E10/cm3
©rlc L03 24Jan2011
18
Donor: cond. electr.due to phosphorous
• P atom: 5 valence elec and 5+ ion core
• 5th valence electr has no avail bond
• Each extra free el, -q, has one +q ion
• # P atoms = # free elect, so neutral
• H atom-like orbits
©rlc L03 24Jan2011
19
Bohr model H atom-like orbits at donor• Electron (-q) rev. around proton
(+q)• Coulomb force,
F=q2/4peSieo,q=1.6E-19 Coul, eSi=11.7, eo=8.854E-14 Fd/cm
• Quantization L = mvr = nh/2p• En= -(Z2m*q4)/[8(eoeSi)2h2n2] ~-
40meV• rn= [n2(eoeSi)h2]/[Zpm*q2] ~ 2 nm
for Z=1, m*~mo/2, n=1, ground state
©rlc L03 24Jan2011
20
Band Model fordonor electrons• Ionization energy
of donor Ei = Ec-Ed ~ 40 meV
• Since Ec-Ed ~ kT, all donors are ionized, so ND ~ n
• Electron “freeze-out” when kT is too small
©rlc L03 24Jan2011
21
Acceptor: Holedue to boron
• B atom: 3 valence elec and 3+ ion core
• 4th bond site has no avail el (=> hole)
• Each hole, adds --q, has one -q ion
• #B atoms = #holes, so neutral
• H atom-like orbits
©rlc L03 24Jan2011
22
Hole orbits andacceptor states• Similar to free electrons and donor
sites, there are hole orbits at acceptor sites
• The ionization energy of these states is EA - EV ~ 40 meV, so NA ~ p and there is a hole “freeze-out” at low temperatures
©rlc L03 24Jan2011
23
Impurity Levels in Si: EG = 1,124 meV• Phosphorous, P: EC - ED = 44 meV
• Arsenic, As: EC - ED = 49 meV
• Boron, B: EA - EV = 45 meV
• Aluminum, Al: EA - EV = 57 meV
• Gallium, Ga: EA - EV = 65meV
• Gold, Au: EA - EV = 584 meV EC - ED = 774 meV
©rlc L03 24Jan2011
Semiconductor Electronics - concepts thus far• Conduction and Valence states
due to symmetry of lattice• “Free-elec.” dynamics near band
edge• Band Gap
– direct or indirect– effective mass in curvature
• Thermal carrier generation• Chemical carrier gen
(donors/accept)
24
©rlc L03 24Jan2011
25
References *Fundamentals of Semiconductor Theory and
Device Physics, by Shyh Wang, Prentice Hall, 1989.
**Semiconductor Physics & Devices, by Donald A. Neamen, 2nd ed., Irwin, Chicago.
M&K = Device Electronics for Integrated Circuits, 3rd ed., by Richard S. Muller, Theodore I. Kamins, and Mansun Chan, John Wiley and Sons, New York, 2003.
• 1Device Electronics for Integrated Circuits, 2 ed., by Muller and Kamins, Wiley, New York, 1986.
• 2Physics of Semiconductor Devices, by S. M. Sze, Wiley, New York, 1981.