Upload
mashiur
View
635
Download
1
Embed Size (px)
DESCRIPTION
Lecture on Introduction of Semiconductor at North South University as the undergraduate course (ETE411)=======================Dr. Mashiur RahmanAssistant ProfessorDept. of Electrical Engineering and Computer ScienceNorth South University, Dhaka, Bangladeshhttp://mashiur.biggani.org
Citation preview
ETE411 :: Lec13
Dr. Mashiur Rahman
Chapter 9 :: Metal-semiconductor junction
• Contact– Semiconductor devices must make contact with
the outside world.P-n junction
?
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.
Contact
1. Rectifying contacts (Schottky Barrier Diode)2. Nonrectifying contacts (Ohmic contact)
Before contact After contact
Energy Band diagram of metal and semiconductor
Ideal energy-band diagram of a metal-n-semiconductor junction for Фm > Фs
Фm = metal work functionФs = Semiconductor work functionФB0= ideal barrier height of the semiconductor contact χ= electron affinity
VB0
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)
Space charge region width :: W
The space charge width in a rectifying metal semiconductor contact is universally proportional to the square root of the semiconductor doping.
Vbi - VaForward bias
Measured Schottky-barrier heights (at 300 K)
S. M. Sze : Physics of semiconductor Devices
P-type semiconductorNot included in the course
χ
Reverse bias and forward bias
Reverse bias forward bias
Comparison Schottky Barrier Diode & pn junction Diode
1. Magnitudes of the reverse-saturation current density.
2. Switching characteristics
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.
Turn on voltage
• Frequency response. • Schottky diodes can be used in fast-switching
application.
Contact
1. Rectifying contacts2. Nonrectifying contacts (Ohmic contact)
Nonrectifying contacts (Ohmic contact)
Before contact After contact
Metal-n-semiconductor junction for Фm < Фs
Voltage applied
Positive voltage applied to the metal Positive voltage applied to the semiconductor
Example 9.7 (page 347)
Transport mechanisms
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
Forward bias
S. M. Sze : Physics of semiconductor Devices (page 254)
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
tunneling (‘through’ the barrier)
Quantum-mechanical tunneling of electrons through the barrier (important for heavily doped semiconductors and responsible for most ohmic contacts).
recombination in the depletion layer
Recombination in the space-charge region identical to the recombination process in a p-n junction.
hole injection from metal
Hole injection from the metal to the semiconductor Recombination in the neutral region.
Not included in the course