ELEC 435 ELECTRONICS I
Physical Operation of Diodes
Sec 3.7 of Sedra & Smith
The Forward-Bias Region
Is = the saturation current (constant for a given diode at a given temperature)
VT = k T /q = thermal voltage ( 25 mV at 20 0C)
VT = k T /q = thermal voltage ( 25 mV at 20 0C)
k = Boltzman’s constant = 1.38 x 10 -23 joules/kelvin
T = the absolute temperature in kelvins = 273 + temp in 0C
q = the magnitude of the electron charge = 1.6 X 10 -19 coulomb
n = diode constant (between 1 and 2)
Physical Operation of Diodes
Covalent Bonds are
formed by sharing the
valence electrons (4 in
each silicon atom)
At low temperatures,
all covalent bonds are
intact and no (or very
few) free electrons are
available to conduct
electric current.
Silicon Crystal
• At room temperature, some of the bonds are broken by thermal
ionization and some electrons are freed.
• When a covalent bond is broken, an electron leaves its parent
atom, therefore a positive charge is left in the parent atom.
• An electron from a neighboring atom may be attracted
to this positive charge, leaving its parent atom.
• This action fill up the “hole” that existed in the ionized atom but
creates a new hole in the other atom.
• This means that we
have a positively
charged carrier, or
hole, moving through
the silicon crystal
structure and being
available to conduct
electric current.
• Thermal ionization results in free electrons and holes
in equal numbers and hence equal concentrations.
• In thermal equilibrium, the recombination rate is equal
to the ionization or thermal-generation rate.
• The concentration of free electrons n, which is equal
to the concentration of holes p
n = p = ni
• At room temperature ni = 1.5 X 10 10 carriers/ cm3
• Note that the silicon crystal has about 5 X 1022 atoms/cm3
• Therfore at room temperature, only one of every billion
atoms is ionized !
Doped Semiconductors
• Doped semiconductors are materials in which
carriers of one kind (electrons or holes) predominate
• Doped silicon in which the majority of charge carrier
are electrons is called n type
• Doped silicon in which the majority of charge
carrier are holes is called p type
n type semiconductor
• A Silicon crystal doped by a pentavalent element.
• Each dopant atom donates a free electron.
p type semiconductor
• A Silicon crystal doped by a trivalent element.
• Each dopant atom gives rise to a hole.
The pn junction under open-circuit conditions
Because the concentration of holes is high in the p
region and low in the n region, holes diffuse across the
junction from the p side to the n side
Similarly, electrons diffuse across the junction from the
n side to the p side
These two currents components add together to form the
diffusion current ID, whose direction is from the p side to
the n side
• The holes that diffuse across the junction into the n region
quickly recombine with some of the majority electrons
present there and thus disappear from the scene.
• This recombination process results in the disappearance
of some free electrons from the n-type material
• Since recombination takes place close to the junction,
there will be a region close to the junction that is depleted
of free electrons.
Barrier voltage (Built-in voltage)
• A carrier –depletion region will exist on both sides of the
junction, with the n side of this region positively charged
and the p side negatively charged.
• The charges on both sides of the depletion region cause
an electric field to be established across the region.
• A potential difference results across the depletion region,
with the n side at a positive voltage relative to the p side.
• The voltage drop across the depletion region acts as a
barrier that has to be overcome for holes to diffuse into
the n-region and electron to diffuse into the p-region.
• The larger the barrier voltage, the smaller the number
of carriers that will be able to overcome the barrier and
hence the lower the magnitude of the diffusion current.
• In addition to the current ID due to majority-carrier
diffusion, a component due to minority-carrier drift exists
across the junction.
• Some of the thermally generated holes in the n material
diffuse through the n material to the edge of the depletion
region.
• In the depletion region, these holes experience the
electric field, which sweeps them across that region.
• Similarly, some of the minority thermally generated
electrons in the p material diffuse to the edge of the depletion
region and get swept by the electric field in the depletion
region into the n side.
• These two currents components:
ƒ
electrons moved by drift from p to n
ƒ
and holes moved by drift form n to p
add together to form the drift current Is , whose
direction is from the n side to the p side of the junction
• Because the current Is is carried by thermally generated
minority carries, its value is strongly dependent on temperature
•It is independent of the value of the depletion layer voltage V0
• Under open circuit conditions, no external current
exists, therefore the two opposite currents across the
junction should be equal in magnitude:
Is = I D
• This equilibrium condition is maintained by the barrier
voltage V0
• With no external voltage applied, the built-in voltage
across the pn junction is:
V0 = VT ln (NA ND/ ni2)
NA and ND are the doping concentrations of the p side
and n side of the junction
• Note that V0 depends both on doping concentrations and
on temperature.
• Typically for silicon at room temperature the built- in
voltage, V0, is in the range of 0.6 to 0.8 V