Bipolar Junction Transistors (1) Dr. Wojciech Jadwisienczak EE314.
EE314 Basic EE II The Devices: Diode. EE314 Basic EE II Chapter 10: Diodes 1.Semiconductors 2.Doping...
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Transcript of EE314 Basic EE II The Devices: Diode. EE314 Basic EE II Chapter 10: Diodes 1.Semiconductors 2.Doping...
EE314 Basic EE II
The Devices: Diode
EE314 Basic EE II
Chapter 10: Diodes
1.Semiconductors2.Doping concept3.n & p-type semiconductors 4.Si diode5.Forward & reversed bias6.Examples7.Diode Characteristic
http://www.amazon.com/Engineer-Training-Reference-Michael-Lindeburg/dp/0912045566
Engineer-In-Training Reference Manual
EE314 Basic EE II
Outline
Motivation and Goals Semiconductor Basics Diode Structure Operation
» Static model
EE314 Basic EE II
Composed of 3 Basic particles: Protons, Electrons & Neutrons.
An Atom requires balance, an equal No. of Protons & Electrons.
When an atom has one more particle (protons or electrons) it acquires a charge:
+ Ion Ion has more Protons Protons than Electrons,- Ion Ion has more ElectronsElectrons than Protons.
Atom
EE314 Basic EE II
What do we know about an atomic structure?
EE314 Basic EE II
Semiconductor Basics I
Electrons in intrinsic (pure) Silicon
» covalently bonded to atoms
» “juggled” between neighbors
» thermally activated: density eT
» move around the lattice, if free
» leave a positively charged `hole’ behind
http://www.masstech.org/cleanenergy/solar_info/images/crystal.gif
EE314 Basic EE II
Semiconductor Basics II Two types of intrinsic carriers
» Electrons (ni) and holes (pi)
» In an intrinsic (no doping) material, ni=pi
» At 300K, ni=pi is low (1010cm-3)
» Use doping to improve conductivity
EE314 Basic EE II
Semiconductor Basics III Extrinsic carriers
» Also two types of dopants (donors or acceptors)– Donors bring electron (n-type) and become ive ions
– Acceptors bring holes (p-type) and become ive ions
» Substantially higher densities (1015cm-3)» Majority and minority carriers
– if n>>p (n-type) electrons majority and holes minority
– Random recombination and thermal generation
EE314 Basic EE II
ConductorConductor;Has loosely bound electrons in its outer or Valence ring, they are easily displaced.
InsulatorInsulator;Has tightly bound electrons in its outer or Valence ring, they cannot be easily displaced.
SemiconductorSemiconductor;Has at least 4 electrons in the outer or Valence ring, it is neither a conductor nor an insulator. In its pure state it makes a better insulator than conductor. 4 electrons allows easy bonding w/ other materials.
Conduction
EE314 Basic EE II
Semiconductor Basics I
Electrons in intrinsic (pure) Silicon» covalently bonded to atoms» “juggled” between neighbors» thermally activated: density eT
» move around the lattice, if free
» leave a positively charged `hole’ behind
EE314 Basic EE II
Semiconductor Basics II Two types of intrinsic carriers
» Electrons (ni) and holes (pi)
» In an intrinsic (no doping) material, ni=pi
» At 300K, ni=pi is low (1010cm-3)
» Use doping to improve conductivity
Extrinsic carriers» Also two types of dopants (donors or acceptors)
– Donors bring electron (n-type) and become ive ions
– Acceptors bring holes (p-type) and become ive ions
» Substantially higher densities (1015cm-3)» Majority and minority carriers
– if n>>p (n-type) electrons majority and holes minority
– Random recombination and thermal generation
EE314 Basic EE II
The Diode
p
n
B ASiO 2
Al
Cross section of pn-junction in an IC process
P-type region
doped with acceptor impurities (boron)
N-type region
doped with donor impurities (phosphorus, arsenic)
EE314 Basic EE II
The Diode
A
B
n
p
A
B
Al
One-dimensionalrepresentation diode symbol
The pn region is assumed to be thin (step or abrupt junction)
Different concentrations of electrons (and holes) of the p and n-type regions cause a concentration gradient at the boundary
Simplified structure
EE314 Basic EE II
•Concentration Gradient causes electrons to diffuse from n to p, and holes to diffuse from p to n
•This produces immobile ions in the vicinity of the boundary
•Region at the junction with the charged ions is called the depletion region or space-charge region
•Charges create electric field that attracts the carriers, causing them to drift
•Drift counteracts diffusion causing equilibrium ( Idrift = -Idiffusion )
Depletion Region
hole diffusionelectron diffusion
p n
hole driftelectron drift
EE314 Basic EE II
Depletion Regionhole diffusion
electron diffusion
p n
hole driftelectron drift
ChargeDensity
Distancex+
-
ElectricalxField
x
PotentialV
W2-W1
(a) Current flow.
(b) Charge density.
(c) Electric field.
(d) Electrostaticpotential.
•Zero bias conditions
•p more heavily doped than n (NA > NB)
•Electric field gives rise to potential difference in the junction, known as the built-in potential
EE314 Basic EE II
Forward Bias
hole diffusionelectron diffusion
p n
hole driftelectron drift
+ -
•Applied potential lowers the potential barrier, Idiffusion > I drift
•Mobile carriers drift through the dep. region into neutral regions
•become excess minority carriers and diffuse towards terminals
EE314 Basic EE II
Reverse Bias
hole diffusionelectron diffusion
p n
hole driftelectron drift
- +
•Applied potential increases the potential barrier
•Diffusion current is reduced
•Diode works in the reverse bias with a very small drift current
EE314 Basic EE II
Diode Current
Ideal diode equation: