Download Chapter 1 file - e

Chapter 1 Semiconductor Diode
• types of materials: conductor, insulator and semiconductor
Conductor:
• allow current to flow
• have large number of free electrons
• one to three valence electrons in the outer shell
• examples:
• the best conductor:
• copper atom
Insulator:
• poor conductors of electric current
• have small number of free electrons
• more than four valence electrons in the outer shell
• examples:
Semiconductor:
• conductivity level between conductors and insulators
• fewer free electrons compared to conductors
• four valence electrons in the outer shell
• basis for construction of electronic devices: diode, transistor and integrated
circuit
• atomic structure (Bohr model)
Types of semiconductor materials
• single crystal: with a repetitive crystal structure; i.e. silicon (Si) and germanium
(Ge)
• compound: constructed from two or more semiconductor materials of different
atomic structure; i.e. gallium arsenide (GaAs) and cadmium sulfide (CdS)
• most commonly used:
Diode
• is a 2-terminal device
• constructed by joining n-type and p-type materials
• free electrons in the n-type material migrate across the p-n junction to the ptype material (electron flow)
• holes in the p-type material migrate across the p-n junction to the n-type
material (current flow)
• as a result, a depletion region is formed around the p-n junction
• the forces between the opposite charges at the junction form an electrostatic
field
• energy must be supplied to break the forces between the opposite charges
• the amount of energy (potential difference) needed is known as knee voltage
(VK or VD)
• Si: VD =
; Ge: VD =
Biasing the p-n junction (diode)
1. No biasing (VS = 0 or VS < VD)
; GaAs: VD =
2. Forward Bias (VS > VD)
• external voltage is applied across the p-n junction in the same polarity as the
p-type and n-type materials
• electrons (and holes) are pushed toward the p-n junction to neutralize the
positive ions (and negative ions)
• these causes the width of the depletion region to decrease
• electrons and holes have sufficient energy to cross the p-n junction
• when the supplied voltage exceeds the knee voltage, the current will rise
exponentially
• Shockley (Diode) equation,
I D  I S (e
VD
nVT
IS : reverse saturation current
VD :
n : 1 (varies between 1 -2 )
VT : thermal voltage = 26 mV
Example 1
 1)
3. Reverse Bias
• external voltage is applied across the p-n junction in the opposite polarity
• holes in the p-type material attracted toward the negative terminal
• electrons in the n-type material attracted toward the positive terminal
• these causes the width of the depletion region to widen
• if the reverse bias voltage is very high, the diode breaks down and the reverse
current increases tremendously
• this maximum voltage is called breakdown voltage, VZ
• the maximum reverse bias voltage that can b applied before entering zener
region is called peak inverse voltage (PIV rating @ PRV rating)
Characteristics
• diode ideally conducts current in one direction
Temperature Effects
• as temperature increases it adds energy to the diode
Temperature Effects
• the required forward bias voltage is reduced
• the amount of reverse current and maximum reverse bias voltage increases
• Ge is more sensitive to temperature variations than Si
Resistance Level
1. Static resistance (DC)
R D = V D / ID
Example 2
Si diode has the following characteristic. Determine the DC resistance levels at:
a) ID = 2 mA
b) ID = 20 mA
c) VD = - 10 V
Resistance Level
2. Dynamic resistance (AC)
26mV
rd 
 rB
ID
• the resistance depends on ID (through the diode)
• voltage across the diode is fairly constant (26mV for 25C)
• rB (resistance of semiconductor material & contact resistance); ranges from a
0.1  for high power devices to 2  for low power, general purpose diodes. In
some cases rB can be ignored.
26mV
rd 
ID
• reverse bias region: The resistance is infinite. The diode acts like an open.
Example 3
Si diode has the following characteristic. Determine the DC resistance levels at:
a) ID = 2 mA
b) ID = 20 mA
c) ID = 1 µA
Specification Sheet
•
Specification Sheet
1. VF, forward voltage at a specific current and temperature
2. IF, maximum forward current at a specific temperature
3. IR, maximum reverse current at a specific temperature
4. PIV or PRV or V(BR), maximum reverse voltage at a specific temperature
5. Power Dissipation, maximum power dissipated at a specific temperature
6. C, Capacitance levels in reverse bias
7. trr, reverse recovery time
8. Temperatures, operating and storage temperature ranges