Chap 4.
M-S Contact
u Workfunction
u M-S Contacts Formation
u Biases of Contacts
MOS Device Physics and Designs 1 Instructor: Pei-Wen Li
Chap. 4 Dept. of E. E. NCU
M-S Contacts
u The metal-semiconductor (MS) contacts play an important role
of one kind or another in all solid-state devices
– Ohmic contact: is a critical link between the semiconductor
and the outside world.
– Schottky contact: ~ p+/n diode.
I I
Ohmic contact Schottky contact
V V
MOS Device Physics and Designs 2 Instructor: Pei-Wen Li
Chap. 4 Dept. of E. E. NCU
Ideal MS Contacts
u Vacuum level: carriers with energy above vacuum level Eo
would be completely free itself from the material.
u Work function: the energy difference between the vacuum level
and the Fermi level. φ ≡ Eo – EF
That is the energy needed for an valence electron to jump into
vacuum. ( to free itself from the binding of nuecli)
MOS Device Physics and Designs 3 Instructor: Pei-Wen Li
Chap. 4 Dept. of E. E. NCU
Work Function
u The work function is variant for different materials.
u In addition, χ : electron affinity ≡ Eo – EC, is an invariant
fundamental property of the specified semiconductor.
u Recall φ ≡ Eo – EF = χ +(EC – EF)FB
Constant for a Changes with the doping
specific material condition of a specific material
MOS Device Physics and Designs 4 Instructor: Pei-Wen Li
Chap. 4 Dept. of E. E. NCU
Formation of MS Contacts
u Once contact is formed and
reaches equilibrium conditions,
the fermi-level of two materials
should be aligned.
u φB = φM - χS : surface potential
energy barrier encountered by
electrons with E = EF in metal.
MOS Device Physics and Designs 5 Instructor: Pei-Wen Li
Chap. 4 Dept. of E. E. NCU
For φM > φS Bias of MS Contacts
⇒The potential barrier seen by the
electrons in the semiconductor reduced
VA ≥ 0 V
⇒a net electron flow from semiconductor
to metal
⇒The potential barrier seen by the
electrons in the semiconductor increased
VA < 0 V
⇒electrons in n-Si are blocked
⇒only some electrons in Metal could
surmount the potential barrier φB.
⇒Reverse current is “Small”
“Rectifying Contacts”
MOS Device Physics and Designs 6 Instructor: Pei-Wen Li
Chap. 4 Dept. of E. E. NCU
Electrical Nature of Ideal MS Contacts
MOS Device Physics and Designs 7 Instructor: Pei-Wen Li
Chap. 4 Dept. of E. E. NCU
Schottky Diode
u Built-in voltage, Vbi = (1/q)[φB – (EC – EF)FB]
MOS Device Physics and Designs 8 Instructor: Pei-Wen Li
Chap. 4 Dept. of E. E. NCU
I-V Characteristics
MOS Device Physics and Designs 9 Instructor: Pei-Wen Li
Chap. 4 Dept. of E. E. NCU
I-V Characteristics
MOS Device Physics and Designs 10 Instructor: Pei-Wen Li
Chap. 4 Dept. of E. E. NCU
Ohmic Contacts
MOS Device Physics and Designs 11 Instructor: Pei-Wen Li
Chap. 4 Dept. of E. E. NCU
