ANNOUNCEMENTS

1
Lecture 13

• ANNOUNCEMENTS
• Quiz 2 next Friday (2/23) will cover the
  following
• carrier action (drift, diffusion, R-G)
• continuity minority-carrier diffusion eqns
• MS contacts (electrostatics, I-V
  characteristics)
• Review session will be held Friday 2/16 at
  1230PM
• No office hour or coffee hour today ?

• OUTLINE
• Metal-semiconductor contacts (cont.)
• practical ohmic contacts
• small-signal capacitance
• Introduction to pn junction diodes
• Reading Finish Ch. 14, Start Ch. 5

2
Practical Ohmic Contact

• In practice, most M-S contacts are rectifying
• To achieve a contact which conducts easily in
  both directions, we dope the semiconductor very
  heavily
• ? W is so narrow that carriers can tunnel
  directly through the barrier

3
Band Diagram for VA?0
Equilibrium Band Diagram
q(Vbi-VA)
qVbi?FBn
EFM
EFM
Ec, EFS
Ec, EFS
Ev
Ev
4
Specific Contact Resistivity, rc

• Unit W-cm2
• rc is the resistance of a 1 cm2 contact
• For a practical ohmic contact,
• ? want small FB, large ND for small contact
  resistance

5
Approaches to Lowering fB

• Image-force barrier lowering

Df
qfBo
EF
EC
n Si
metal
? Very high active dopant concentration desired

• Band-gap reduction
• strain
• germanium incorporation

A. Yagishita et al. (UC-Berkeley), 2003 SSDM
Extended Abstracts, p. 708
M. C. Ozturk et al. (NCSU), 2002 IEDM Technical
Digest, p. 375
6
Voltage Drop across an Ohmic Contact

• Ideally, Rcontact is very small, so little
  voltage is dropped across the ohmic contact, i.e.
  VA ?0V
• equilibrium conditions prevail

7
Review MS-Contact Charge Distribution

• In a Schottky contact, charge is stored on either
  side of the MS junction
• The applied bias VA modulates this charge

8
Schottky Diode Small-Signal Capacitance

• If an a.c. voltage va is applied in series with
  the d.c. bias VA, the charge stored in the
  Schottky contact will be modulated at the
  frequency of the a.c. voltage
• displacement current will flow

9
Using C-V Data to Determine FB

Once Vbi and ND are known, FBn can be determined
10
Summary
Ec
Ec
EF
EF
Ev
Ev
Ec
Ec
EF
EF
Ev
Ev
Since it is difficult to achieve small FB,
practical ohmic contacts are achieved with heavy
doping
Ec
EF
Ec
EF
Ev
Ev
Charge storage in an MS junction ? small-signal
capacitance
11
pn Junctions

I
V
I
N
P
V
Reverse bias
Forward bias
diode
symbol
12
Terminology
Doping Profile
13
Idealized Junctions