Lecture on Introduction of Semiconductor at North South University as the undergraduate course (ETE411) ======================= Dr. Mashiur Rahman Assistant Professor Dept. of Electrical Engineering and Computer Science North South University, Dhaka, Bangladesh http://mashiur.biggani.org

1. ETE411 :: Lec13
Dr. Mashiur Rahman

2. Chapter 9 :: Metal-semiconductor junction
• Contact
– Semiconductor devices must make contact with
the outside world.
P-n junction
?

3. History
• 1874- Braun : dependence of the total resistance
on the polarity of the applied voltage and on the
detailed surface conditions.
• 1904 : Point contact rectifier found in practical
applications.
• 1938 – Schottky: potential barrier could arise
from stable space charges in the semiconductor
alone without the presence of a chemical layer.

4. Contact
1. Rectifying contacts (Schottky Barrier Diode)
2. Nonrectifying contacts (Ohmic contact)

5. Energy Band diagram of metal and semiconductor
Before contact After contact
VB0
Фm = metal work function
Фs = Semiconductor work function
ФB0= ideal barrier height of the semiconductor contact
χ= electron affinity
Ideal energy-band diagram of a metal-n-semiconductor junction for Фm > Фs

6. Schottky barrier
ФB0= (Фm- χ)
Vbi= ФB0-Фn
Mechanism : due to the flow of majority carrier electron
(Thermionic emission of majority currier)
S. M. Sze : Physics of semiconductor Devices (page 255)

7. Space charge region width :: W
Vbi - Va Forward bias
The space charge width in a rectifying metal
semiconductor contact is universally proportional to the
square root of the semiconductor doping.

8. Measured Schottky-barrier heights (at 300 K)

9. S. M. Sze : Physics of semiconductor Devices

10. Not included in the course
P-type semiconductor

12. χ

13. Reverse bias and forward bias
Reverse bias forward bias

14. Comparison
Schottky Barrier Diode & pn junction Diode
1. Magnitudes of the reverse-saturation
current density.
2. Switching characteristics

15. Magnitudes of the reverse-saturation current density
• Pn juction: determined by the diffusion of
minority current.  generation current
• Schottky barrier diode : determined by
thermonic emission of majority carriers over a
potential barrier.

16. Turn on voltage
• Frequency response.
• Schottky diodes can be used in fast-switching
application.

17. Contact
1. Rectifying contacts
2. Nonrectifying contacts (Ohmic contact)

18. Nonrectifying contacts (Ohmic contact)
Before contact After contact
Metal-n-semiconductor junction for Фm < Фs

19. Voltage applied
Positive voltage applied to the metal Positive voltage applied to the semiconductor

20. Example 9.7 (page 347)

21. Transport mechanisms
Forward bias
S. M. Sze : Physics of semiconductor
Devices (page 254)
Transport mechanisms at metal–semiconductor junctions. (1)
Thermionic emission (‘above’ the barrier) (2) tunneling (‘through’ the
barrier), (3) recombination in the depletion layer, (4) hole injection from
metal

22. Thermionic emission
Transport of electrons from the semiconductor over the
potential barrier into the metal.  Dominent process for
Schottky diodes with moderately doped semicondutor (Si
with ND ≤1017cm-3) operated at moderate temperature (room
temp.).

23. tunneling (‘through’ the barrier)
Quantum-mechanical tunneling of electrons through the
barrier (important for heavily doped semiconductors and
responsible for most ohmic contacts).

24. recombination in the depletion layer
Recombination in the space-charge region
 identical to the recombination process in a p-n junction.

25. Not included in the course
hole injection from metal
Hole injection from the metal to the
semiconductor
 Recombination in the neutral region.