1. A p-n junction is not a device
5.7 Metal-Semiconductor Junctions p n
A p-n junction is not a device
A p-n junction diode p n
Difference: a p-n junction also has two Metal-Semiconductor (MS) junctions
MS junction:
can be either rectifying (Schottky Barrier) or nonrectifying (Ohmic)
A rectifying MS junction is similar to a p-n junction in many ways
Applications: high speed rectification, ohmic contact

2. Energy Band Relation for Rectifying Contacts (equilibrium)
5.7.1 Schottky Barriers
Energy Band Relation for Rectifying Contacts (equilibrium)
CASE 1: Metal/n-type semiconductor ; m> s
This MS junction is similar to a p+n junction
depletion region width
junction capacitance
However, no forward bias hole injection
qm metal work function
qs semiconductor work function
q electron affinity
q(m-s) =qV0 energy barrier prevents electron diffusion from semiconductor to metal
q(m-) =qB energy barrier for electron injection from metal to semiconductor

3. Figure 5—40
A Schottky barrier formed by contacting an n-type semiconductor with a metal having a larger work function: (a) band diagrams for the metal and the semiconductor before joining; (b) equilibrium band diagram for the junction.

4. Figure 5—41
Schottky barrier between a p-type semiconductor and a metal having a smaller work function: (a) band diagrams before joining; (b) band diagram for the junction at equilibrium.

5. Energy Band Relation for Rectifying Contacts (under bias)
CASE 1: Metal/n-type semiconductor ; m> s Forward bias is defined as Metal +; semiconductor-
The barrier q(m-) =qB
is unaffected by bias
Reverse saturation current I0
I0 exp(-qB/kT)
Forward current is due to the injection
of majority carrier
No storage delay time, high speed.
Rectifying! Schottky barrier diode

6. Figure 5—42
Effects of forward and reverse bias on the junction of Fig. 5—40: (a) forward bias; (b) reverse bias; (c) typical current-voltage characteristic.

7. 5.7.3 Ohmic Contact (Non-rectifying Metal/semiconductor contact)
CASE 2: Metal/n-type semiconductor ; m< s
Ohmic contact: linear I-V characteristics in both biasing directions; negligible contact
resistance.
No depletion region is formed

8. Figure 5—43
Ohmic metal–semiconductor contacts: (a) Fm < Fs for an n-type semiconductor, and (b) the equilibrium band diagram for the junction; (c) Fm < Fs for a p-type semiconductor, and (d) the junction at equilibrium.

9. 5.7.4 Typical Schottky Barriers
ie. Real Schottky Barriers
For a real semiconductor surface, we may have
Surface states
Interfacial layers
Other effects
Fermi level pinning

10. 6.3 Metal-Semiconductor FET
The channel control by depletion discussed above for a JFET can be accomplished by the use of
a reverse-biased Schottky barrier instead of a p-n junction. The resulting device is called a MESFET
Why MESFET?
Simplicity (No diffusion)
Small gate length
Why GaAs or III-V compounds?
High electron mobility, high speed applications
High temperature, high power operations
Band gap engineering
GaAs MESFET formed on an n-typeGaAs layer grown
epitaxially on a semi-insulating GaAs substrate

11. Figure 6—7
GaAs MESFET formed on an n-type GaAs layer grown epitaxially on a semi-insulating substrate. Common metals for the Schottky gate in GaAs are AI or alloys of Ti, W, and Au. The ohmic source and drain contacts may be an alloy of Au and Ge. In this example the device is isolated from others on the same chip by etching through the n region to the semi-insulating substrate.

12. The High Electron Mobility Transistor (HEMT)
We want high transconductance gmhigh channel conductivityincrease the doping of the channel
and thus the carrier concentration
Problems of this approach:impurity scattering; degradation of mobility
We need Band gap engineering of III-V compounds !
Figure 6—8
(a) Simplified view of modulation doping, showing only the conduction band. Electrons in the donor-doped AIGaAs fall into the GaAs potential well and become trapped. As a result, the undoped GaAs becomes n-type, without the scattering by ionized donors which is typical of bulk n-type material. (b) Use of a single AIGaAs/GaAs heterojunction to trap electrons in the undoped GaAs. The thin sheet of charge due to free electrons at the interface forms a two-dimensional electron gas (2-DEG), which can be exploited in HEMT devices.