1. Junctions 10 Surface ~ simplest one; junction between vacuum/surface
Metal/Metal
Metal/Semiconductor
Schottky contact ~ blocking barrier
Ohmic contact ~ low resistance
Semiconductor/Semiconductor
Homojunction ~ junctions are composed of the same material
Heterojunction ~ each side is composed of different material
Iso-type ~ types are same to both sides <n-types or p-types>
Anisotype <p-type and n-type, vise versa>
M/I/M, MIS, SIS, MOS

2. Surfaces Chemisorption of oxygen on n-type Semiconductor surface
Termination of the periodic potential  localized states at the surface
Chemisorption of oxygen on n-type
Semiconductor surface Ec EF Ev
Surface state
O2-
chemisorption
bending of band gives E +
separation of charge - d -
negatively charged surface state
positively charged depletion region
As more O2- are formed, the energy bands bend at the surface of the semiconductor because of the local field that is built up.

3. Metal/Metal B A B A E contact potential
Transfer of electron until EF are the same for both of metals
A potential difference known as the contact potential is set up between two metals.
EF has to be the same in thermal equilibrium for whole system
Although an internal field exists, no potential can be measured in an external circuit connecting the two metals together.

4. Metal/Semiconductor: Schottky Barrier
≡ electron
affinity Eb
- - ++ Wd
Blocking contact
Blocking contact if for n-type and for p-type
A flow of electrons from the semiconductor to the metal in order to equalize the Fermi energies in the two materials.
An internal field is developed in the semiconductor.
: diffusion potential, built-in potential
: Energy barrier

5. Metal/Semiconductor: Schottky Barrier
The width of the depletion region
Ionized donor density = ND+

6. Metal/Semiconductor: Schottky Barrier
In the depletion region Wd, potential change
If we include the effects of an applied voltage

7. Metal/Semiconductor: Schottky Barrier
depletion region
conductor
dielectric
nearly conductor
slope
→ One method to measure

8. Metal/Semiconductor: Schottky Barrier
P-type semiconductors
- - Eb Wd
Schottky barrier of hole
If semiconductor is p-type,
e- transferred from M → S
h+ transferred from S → M
In the depletion region (negatively charged depletion region),

9. Metal/Semiconductor: Schottky BarrierJ V
JV characteristics for Schottky barrier (n-type semiconductor)
Thermionic emission +

10. Metal/Semiconductor: Schottky Barrier
Metal + forward
Metal – reverse J
Φapp J0
Basic equation for current in junction
≡ J0 , not depend on the Φapp

11. Metal/Semiconductor: Schottky Barrier
Rectifying
Φapp
logJ
-J0 T

12. Junction vocabulary Depletion layer Depletion layer width
Accumulation layer ~ carrier having added to the materials
Band bending
Surface states
Contact potential ~ difference in work function
Electron affinity ~
Junction capacitance
Rectifying
I-V for junction
forward bias
reverse bias

13. Junction formation
Fermi level alignment throughout whole system in thermal equilibrium
Schottky barrier
n-type semiconductor
transfer e- from semiconductor to metal
p-type semiconductor
transfer e- from metal to semiconductor
Diffusion potential
Δ(work function)

14. Junction formation For n-type, Eb J J0 Φapp Metal – reverse++
Metal + forward
Metal – reverse J
Φapp J0
for thermionic emission

15. Ohmic contact n-type: p-type:
The accumulation layer in the semiconductor serves as a ready reservoir of electrons for conduction in the material available at the contact, and thus application of an electric field measures only the conductivity of the semiconductor. J V R
n-type:
p-type:
(opposite of Schottky barrier)

16. Ohmic contact
Accumulation region
(reservoir of e-) e-
n-semiconductor
metal
Transfer of e- from metal to semiconductor
Electrons flow without any barrier contact does not make any change in J.
Drift current = diffusion current

17. Ohmic contact: electric potential

18. Ohmic contact - + n-type
Virtual cathode
Extra electrons (source of carriers)
Ohmic contacts when electric field is applied.
For sufficiently high applied voltage, however, injection of electrons from the contact may dominate carriers in the material, charge neutrality in the material is violated, and one enters into a new regime of space charge limited current with
Critical voltage of SCLC
Transit time through the material  dielectric relaxation time

19. p-n junction - homojunctions
p-type
n-type h+ e-
Field free p-type
Field free n-type Wp Wn
W = Wp+ Wn
Similar to two Schottky barriers

20. p-n junction - homojunctions
When we apply Φapp,
Φapp = 0
Φapp = Φapp
Minority np y y
Majority carrier
Majority
Minority x x pn
Majority carriers ~ e- in n-type, h+ in p-type
Minority carriers ~ h+ in n-type, e- in p-type

21. p-n junction - homojunctions
Hole density in n-type material
Diffusion length for hole
Lifetime of hole
Diffusion controlled flow

22. p-n junction - homojunctions
Φapp J0
Same form as the J vs. Φapp dependence of a Schottky barrier, but the pre-exponential reverse saturation current J0 has a different definition.

23. Applications of p-n junctions
Rectifier
Because of asymmetry in J-V curve
Amplifier
Applying small signal to get large signal
source
collector
larger
modulator
(small)
p-n-p type transistor

24. Application of p-n junctions
p-n-p type transistor
n-type base is very narrow, only a fraction of a hole diffusion length; the collector current is nearly equal to the emitter current
V2 > V IV2 > IV1  Small V1 results in large emitter collector current
FET
(High resistance)
IV1
IV2
large current
collector
source
Small current

25. Application of p-n junctions
Photodetector: operate under reverse bias
Increase of current by light - +
light
electron/hole pair

26. Application of p-n junctions
Phototransistor
Photoexcitation is used to produce an electron-hole pair with a diffusion length of the p-n2 junction.
The electron is collected but the hole remains in the base until it diffuses out or until recombination occurs.
Many electrons transit per photo-excited hole flow before the hole is lost out of the base across the junction
forward
reverse
light

27. Application of p-n junctions
Solar cell: no applied bias - +
light
electron/hole pair
Open circuit voltage O
Short circuit current
The band gap of about 1.4 eV proves to be optimal for solar energy conversion application.

28. Application of p-n junctions
Light emitter: Forward biased p-n junction
Direct band gap materials are favored for this application.
GaAsP (R), InGaN (B), GaN (B)
By coating a blue LED with a phosphor material, a portion of the blue light can be converted to green, yellow and red light to get white light.
Laser possible - +
light
electron/hole pair + -
LED, Laser

29. Application of p-n junctions
Tunnel diode
A p-n junction formed between a degenerate p-type material and a degenerate n-type material.
The maximum current flows when the occupied states in the n-type material are energetically exactly equal to the unoccupied states in the p- type material.

30. Semiconductor-semiconductor junctions: Heterojunctions
A positive value of or a negative value of implies a spike impeding the transport of electrons or holes, respectively.
An isotope heterojunction between two n-type materials with the same electron density, but showing a discontinuity at the interface because of a difference in electron affinities.

31. Semiconductor-semiconductor junctions: Heterojunctions
Energy band diagrams for p-n heterojunctions. The materials in (a) and (b) have the same band gaps, but in (a) the p-type material has a smaller electron affinity than the n-type material, whereas in (b) the situation is reversed.

32. Quantum wells and superlattices
Avalanche photodiode
Avalanche photodiode using superlattices with electron ionization but no hole ionization to minimize noise.
Noise is caused by feedback between slight fluctuations in the gain due to multiplication by one carrier as these are then amplified by changes in the multiplication due to the other carrier.

33. Quantum wells and superlattices
Tunneling
If the allowed levels in the quantum well formed by the low band gap material are limited to discrete values, however, resonant tunneling can occur only for specific electron energies negative resistance regions
In superlattices with relatively thick barriers, sequential tunneling can occur in which electrons tunnel from the ground state in one well to an excited state in the next, where they then relax to the ground state and continue the process.

34. Quantum wells and superlattices
Effective mass filter and formation of a miniband
A superlattice can act as an effective mass filter, making it possible to localize holes in a superlattice, for example, while permitting tunneling transport of electrons.
Photoexcited holes remain localized in the quantum wells since their tunneling probability is small because of their larger effective mass.
The transport of electrons by tunneling through the thin barriers can be described as the formation of a miniband.