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. MashiurRahman

2. Contact Rectifying contacts 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. 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.). Thermionic emission

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 10The 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. pnp npn n++ n n ++ = Heavily doped += moderately doped

20. Doping profile n n++

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

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

23. Minority carrier distribution

24. Energy band diagram