Introduction to Semiconductors &
Diodes
EE106 Analogue Electronics
Supplementary Notes
F. O’Malley
April 2005
Semiconductor Diodes
• A diode is a very useful non-linear device
• It allows current to flow in one direction
and prevents it flowing in the other
Cathode
Anode
I
Diode Operation
• When a positive voltage is applied across
the diode then current will flow
• The diode is said to be forward biased
• When a negative voltage is applied then the
diode is reverse biased and no current will
flow
Diode Characteristic Curve
Forward Biased Diode
• A positive voltage is applied to circuit so current
will flow
• Diode effectively behaves like a short circuit when
forward biased
1k
Ω
i
+
10V

Atomic Structure of Semiconductors
Atomic Bonding in Silicon
Energy Band Diagrams Comparison
Silicon Energy Band Diagram
• When an electron jumps to
the conduction band, a
vacancy is left in the
valence band within the
crystal (called a hole)
– called an electron-hole pair
• Recombination occurs
when a conduction-band
electron loses energy and
falls back into a hole in
the valence band
Electron – Hole Pairs
Free Electron Drift
Electron Current
• Application of a voltage causes thermally
generated free electrons to move towards
+
ive terminal.
Hole Current
Impoving Conduction in
Semiconductors - Doping
• In an intrinsic semiconductor, there are relatively
few free electrons
– pure semi-conductive materials are neither good
conductors nor good insulators
• Intrinsic semi-conductive materials must be