EE 30357: Semiconductors II: Devices Lecture Note

1
EE 30357 Semiconductors II DevicesLecture Note
10 (02/04/09)Review of other semiconductor
junctions and MOSFETGrace Xing

• Outline
• Previously reviewed everything about p-n
  junctions
• Semiconductor heterojunctions
• Metal-semiconductor junctions
• Schottky diodes
• Ohmic contacts
• Band diagram construction golden rules (again)

2
Three types of heterojunctions Type I
(straddling), in which the wide-gap energy
overlaps that of the narrow gap, is the most
important Type II (staggered) and Type III
(broken gap). Figure 6.7
Type I straddling Type III broken
gap Type II - staggered
Error in AA The graphs for type II and III are
switched
Concept Graph
3
The most essential concept in semiconductor
devices is B.D., B.D., B.D. (BD band diagram)

• Golden rules in constructing a band diagram at
  equilibrium using EAM
• Always start with the charge neutral band diagram
  (put it on the side)
• Fermi level is flat
• All other energy levels relative to EF in the
  bulk semiconductor (far from the interface) are
  the same
• Energy levels (EC and EV) at the interface do not
  change their relations
• Slopes of band bending (e (1/q) (dE/dx) ) at the
  interface related to dielectric constants ?1 ?2
  (?1 e1 ?2 e2 )

Concept Graph
4
Electron affinity model ? construction of band
diagram
Kink due to different dielectric constants ?1
?2 ?1E1 ?2E2 Note Electric field (slope of
band bending)
P(big p) - n (small n) type I
SB
Concept Graph - Equation
5
The most essential fundamentals in electrical
engineering Q, e, V, E
DP on parallel plate capacitors
Variation on parallel plate capacitors
Q
Q
E-field
E-field
V
V
qVbi-qVa
qVbi- qVa
(electron) Energy
(electron) Energy
6
Energy band offset for a GaAsGe heterojunction
(a) as predicted by the electron affinity model
and (b) experimentally measured. Figure 6.11

• In this class, unless specifically stated, we
  will assume the Electron Affinity Model or use
  the experimentally determined values if known.
• Often, the observed effects are explained by
  interfacial states or thin oxide layer between
  two materials, instead of the tunneling induced
  dipole.

7
Equilibrium energy band diagram of an arbitrary
Type I Np heterojunction as predicted by the
electron affinity model. Electrons from the
valence band of semiconductor B can tunnel a
short distance into the forbidden gap of A, thus
creating an interfacial dipole. Figure 6.12
DP on parallel plate capacitors
Q
E-field
V
Energy
8
Equilibrium energy band diagram within a few
nanometers of the interface of the heterojunction
of Figure 6.12. The circles indicate the band
discontinuities predicted by the electron
affinity model the squares indicate the
influence of the tunneling-induced
dipoles. Figure 6.13
Expand this region
9
Equilibrium diagram of the Np Type I
heterojunction considered in Figures 6.12 and
6.13. The lateral scale is reduced by a factor of
102 to 103 so that the indicated discontinuities
appear in Evac, EC, and EV. The circles and black
lines are for the electron affinity model. The
squares and colored lines present the result
including the tunneling-induced dipoles. Figure
6.14
Expand this region
10
Schematics of a crystal showing (a) dangling
bonds at the surfaces and (b) passivation of the
bonds by atomic hydrogen. Figure 6.15
11
Effect of surface states on the energy band
diagram. (a) Under the neutrality condition there
are empty states at the surface at lower
energies than electrons in the n-type
semiconductor. (b) At equilibrium the transfer of
electrons into the surface states results in a
surface potential fs. The resultant net surface
state charge per unit area is designated
QSS. Figure 6.16

• Surface potential or surface band bending is
  commonly observed in all semiconductors.
• Generally speaking, n-type surface bends up and
  p-type surface bends down.
• Surface Fermi level pinning referring to the
  Fermi level at the surface is fixed at a certain
  energy level regardless of the doping conditions

12
Energy band diagram for an Nn heterojunction
between silicon and germanium, based on the
electron affinity model. The case of neutrality
is shown in (a), where the Fermi levels for the
Si, Ge, and interface are indicated. The
equilibrium case is shown in (b), where all three
Fermi levels have been aligned. Figure 6.17
13
Surface Fermi level being pinned means no matter
what metal we use (i.e. different work function)
on the semiconductor, the Schottky barrier height
is the SAME!
14
Energy band diagram as predicted by the electron
affinity model for an Aln-Si metal semiconductor
junction (a) Neutrality (b) equilibrium. The
predicted barrier of 0.10 eV from metal to
semiconductor is much less than the experimental
value of about 0.7 eV. A more refined model is
required. Figure 6.18
Based on EAM
15
(a) The neutrality diagram for the Aln-Si
Schottky barrier diode including the
tunneling-induced dipole effect. (b) The
equilibrium energy band diagram for an Aln-Si
Schottky barrier diode. Figure 6.19

• Again, this observed effect is often explained
  by an existing thin oxide layer between the metal
  and the semiconductor, instead of the tunneling
  induced dipole.

16
Energy band diagrams for a metaln-semiconductor
Schottky barrier. (a) For forward bias, electrons
flow from semiconductor to metal. (b) For
reverse bias, only a small leakage current flows.
(c) For the first-order model, the
metal-semiconductor barrier (EB(0) EC(x 0) -
Efm) is independent of applied voltage. Figure
6.20
17
A Schottky barrier diode made with a p-type
semiconductor. (a) Equilibrium (b) forward bias
(c) reverse bias. Figure 6.21
Error in A A P.335, bottom equation in the
exponential term, Eg(0) should be EB(0).
18
Comparison of the I-Va characteristics of a
Schottky diode and a pn junction diode. The scale
for the reverse characteristic is compressed
compared with the scale for forward bias. Figure
6.22

• Key words for Schottky diodes
• Thermionic emission current
• Majority carrier device
• (i.e. electrons carry current in an n-
  Schottky, holes in p-Schottky)

19
Low-resistance metal-semiconductor contacts using
degenerate surface layers. Metal-nn contact (a)
and metal-pp contact (b). The Schottky barrier
is thin enough to permit tunneling. Figure 6.23
Ohmic contacts almost all realistic ohmic
contacts are made in this fashion
20
Schematic of an npn homojunction transistor
indicating the low-resistance contacts. They are
the base ppmetal contact, the emitter
nmetal contact, and the collector nnmetal
contact. Figure 6.24
21
Schottky contact
22
Schottky contact
23
(No Transcript)
24
Ohmic contact to n-type
Ohmic contact to p-type