Introduction to Semiconductors & Diodes
EE106 Analogue Electronics Supplementary Notes F. OMalley 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
Anode Cathode
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
+ 10V -
+ i 1k vo -
Reverse Biased Diode
A negative voltage is applied to circuit so NO current will flow Diode effectively behaves like an open circuit when reverse biased
+ i 1k vo -
10V +
Introduction to Semiconductors
Two common types of semi-conductive materials are silicon and germanium
both have four valance electrons
When silicon and germanium atoms combine into molecules to form a solid material, they arrange themselves in a fixed pattern called a crystal
atoms within the crystal structure are held together by covalent bonds (atoms share valence electrons)
An intrinsic crystal is one that has no impurities
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 modified by increasing the free electrons and holes to increase its conductivity and make it useful for electronic devices Doping is the process of adding impurities to intrinsic semi-conductive materials to increase and control conductivity within the material
by adding impurities, n-type and p-type extrinsic semiconductive material can be produced
n-Type Semiconductors
n-type material is formed by adding pentavalent (5 valence electrons) impurity atoms
electrons are called majority carriers in n-type material holes are called minority carriers in n-type material
p-Type Semiconductors
p-type material is formed by adding trivalent (3 valence electrons) impurity atoms
holes are called majority carriers in p-type material electrons are called minority carriers in p-type material
Forming a Diode
Diode is formed from manufacturing p and n type material side by side The arrowhead in the diode symbol points in the direction opposite the electron flow
The anode (A) is the p region The cathode (K) is the n region
The PN Junction Diode
A diode consists of an n and p region separated by a pn junction
the n region has many conduction electrons the p region has many holes
Diffusion of free electrons across junction occurs at formation
PN Junction Diode Forward Bias
Forward bias is the condition that permits current through a diode
the negative terminal of the VBIAS source is connected to the n region, and the positive terminal is connected to the p region
Barrier Potential
The barrier potential, VB, is the amount of voltage required to move electrons through the electric field
At 25C, it is approximately 0.7 V for silicon and 0.3 V for germanium As the junction temperature increases, the barrier potential decreases, and vice versa
Forward Bias
The negative terminal of the bias-voltage source pushes the conduction-band electrons in the n region toward the pn junction, while the positive terminal pushes the holes in the p region toward the pn junction When it overcomes the barrier potential (V B), the external voltage source provides the n region electrons with enough energy to penetrate the depletion region and move through the junction
Reverse Bias
Reverse bias is the condition that prevent current through the diode
the negative terminal of the VBIAS source is connected to the p region, and the positive terminal is connected to the n region
If the external reverse-bias voltage is increased to a large enough value, reverse breakdown occurs
minority conduction-band electrons acquire enough energy from the external source to accelerate toward the positive end of the diode, colliding with atoms and knocking valence electrons into the conduction band (avalanche effect)
Reverse Bias at PN Junction
Diode Applications
Due to this characteristic diodes find many useful applications
Power supplies, voltage regulators Tuning devices in RF (radio frequency) tuned circuits, frequency multiplying devices in RF circuits, mixing devices in RF circuits, switching applications logic decisions in digital circuits. There are also diodes which emit "light", of course these are known as light-emitting-diodes or LED's.
Diode Symbols
Various diodes symbols are shown
Diode Packages
Rectification
Process of converting a AC signal to a DC signal The first step is to use a diode circuit as shown
Half-Wave Rectifier +ive half Observe performance of diode during two half cycle
cycles During +ive half cycle diode is forward biased Diode conducts current
Half Wave Rectifier -ive halfcycle
During -ive half cycle diode is reverse biased Diode does NOT conduct
Have Wave Rectification
Conduction only takes place during positive half of cycle
Output of Have Wave Rectifier
The ive half of the waveform is removed
Average Value of signal The average value of
a waveform is defined by following formula
t+T
1 v av = T
v ( t )dt
t
It is found by Vav averaging the area under the curve over one cycle
w Find the average value of the waveform shown
Example: Average Value
Solution:
t+T
1 v av = T
1 v ( t ) dt = T
v ( t )dt
0
Example Solution T
v ( t ) dt = 100 T
v av =
1 T
2 sin t dt T
T 2 2 cos t T 0
100 T
T 2 T 100 2 cos t = 2 T 2 0
2 T 2 cos 0 cos T 2 T 100 = ( 1 1 ) = 100 = 31 . 8 V 2
100 = 2
31.8V
Example
Sketch the output of the following circuit
Peak Inverse Voltage PIV
This rating of the diode tells what the maximum reverse voltage the diode can/must withstand
Non-ideal Diode Consideration
For precise design applications we must take account of voltage drop across diode
Diode Limiters & Clampers
