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 :: Lec14
Dr. Mashiur Rahman

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

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

4. Ohmic contact
Schotkey Barrier

5. Tunneling barrier
Energy-band diagram of a heavily doped n-semiconductor-to-metal junction
The space charge width in a rectifying metal semiconductor
contact is universally proportional to the square root of the
semiconductor doping. 
The width of the depletion region decrease as the doping
concentration in the semiconductor increases.

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

7. Ohmic contact :: metal –p -semiconductor
(not included in the course)

8. Example 9.7 (page 347)

9. 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

10. 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.).

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

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

13. hole injection from metal
Hole injection from the metal to the
semiconductor
 Recombination in the neutral region.

14. Various metal-semiconductor device structure

16. S. M. Sze : Physics of semiconductor Devices (page 297)

17. Chapter 10
The Bipolar Transistor

18. History
• Bardeen, Brattain & Shockley
– 1948 : Invented the transistor
– 1956 : Received Nobel
• Post war effort to replace vacuum tube.
• They used Germanium: it was possible to
obtain high purity material.

19. Block diagrams and circuit symbols
Bipolar: its operation involves both type of mobile carriers –
electrons & holes.
npn pnp
n++ n n
++ = Heavily doped
+= moderately doped

20. Doping profile
n++ n

21. Conventional ICs
Conventional npn transistor An oxide-isolated npn bipolar transistor
From Muller and Kamins

22. Forward active operating mode
n++ n
1. Electron injected: E → B
2. Create excess concentration of minority carrier
3. Diffuse across the base region: B → C
4. Electric field will swap the electrons into the collector.

23. Minority carrier distribution

24. Energy band diagram