Transcript MSE-630 Lecture 4 Semiconductor theories

Junctions and
Semiconductors
Theories and practical devices
Metallic and Semiconductor
Junctions
Each conductor has a unique quantity of energy that is required to
free an electron. This energy is called the work function energy,
qF. Values for qF for various metals are given in the table below:
In figure A, two dissimilar metals are not
in contact. When they come into
contact as shown in B, their respective
Fermi energies must equilibrate
throughout. To do this, electrons
transfer from B to the lower unfilled
levels of A until the level of the electron
“sea” in both metals is equal. This
causes A to become negatively
charged, and B to become positively
charged. The resulting potential is
called the contact potential (qVC)
The Contact Potential, qVC, is equal to the
difference in the respective work functions:
qVC = EF(B)-EF(A), or qVC=qF(A)-qF(B)
Although a potential exists, no work can be extracted, because
when we attach leads, (i.e., metal C) the sum of the work
functions is zero:
qFnet = q(FC-FA)+(FA-FB)+FB-FC)] = 0
If we join an “n” type
region, which has excess
negative charges, with a
“p” type region, i.e. a “p-n”
junction” a charged region
develops a the interface:
Electrons and holes recombine at
the interface, depleting the
available electrons in the n
material making it more positive,
and holes in the p material,
making it more negative. This
creates a built-in electric field that
discourages further transfer across
the interface.
Electron band diagrams are a way to visualize what happens at a p-n
junction, using the following rules:
1. The Fermi level must be at the same level on both sides of the junction
when there is no applied field
2. Far from the junctions, the materials inherent electrical structure exists
3. He bands are bent, or curved, where the built-in electric fields exist at the
junction
4. A potential energy step, qVo, due to contact potential Vo, develops at the
junction. It is equal in magnitude to EF(n)-EF(p) or, equivalently qF(p)qF(n)
5. Externally applied electric potentials displace the relative positions of EF
and the band edges by amounts over and above those produced by the
above rules.
If we apply a “reverse bias”, as depicted in A above and in figure B
on the left, the barrier at the junction increases to q(Vo+V), thus
increasing the barrier to current flow. If we apply a forward bias, as
in B above and C, left, we annihilate EHPs and have a positive
current flow
The Junction Equation
Forward biasing causes current to flow; reverse
biasing causes it to stop flowing in a p-n
junction. This can be represented in the
following equation:
j = jR[exp (eV/kT)-1]
V is positive for forward biasing, and negative
for reverse biasing.
jR is the reverse biased current
DIODES are p-n junctions that
act as rectifiers, allowing current
to pass only in one direction
Tunnel Diodes
Tunnel diodes act as oscillators.
Current increases up to Vp,
decreases between Vp and Vc, then
increases beyond Vc
As shown at left, the Fermi level
exists in the conduction zone of the
n-type material and the valence
zone of the p-type material. When a
biasing voltage is applied, the
electrons jump, or “tunnel” across
the forbidden gap at the junction.
When Vp<V<Vc, the gap widens
and tunneling becomes small.
Then, when V>Vc, tunneling can
begin again.
Zener diodes
Zener diodes are used to
regulate voltages in circuits.
When the voltage becomes
sufficiently large (10-1000V,
depending on doping level), it
reaches a “limiting” or “break
down” voltage, and current is
shunted to ground.
Transistors: triodes, npn and pnp
junctions
Current flow in a transistor vary with
emitter voltage, Ve:
I = Ioexp(Ve/B)
Where Io and B are constants
MOS-FET
Numerous transistors, resistors
and electronic elements can be
incorporated on a single wafer
of semiconductor material,
thus creating the integrated
circuit
Thermocouples and other thermoelectric devices
If opposite ends of a metal bar
are maintained at different
temperatures, electrons will flow
from the hot end to the cold end.
As electrons pile up at the cold
end, it creates a net
electromotive force, or Seebeck
voltage, opposing further
charge transfer.
Seebeck voltage varies
sensitively with temperature,
and are 1000-fold larger in
semiconductors than in metals.
Using semiconductors used to
measure changes in voltage are
called thermistors, and can very
accurately measure temperature
Other devices that measure
temperature or have
thermoelectric properties include
thermocouples, Peltier effect and
thermoelectric refrigerators.
When metals with different work
functions are joined, they generate a
voltage that varies with temperature.
Thermocouples accurately measure
temperature by measuring this
voltage difference.
When current flows through a junction,
heat may be generated or absorbed,
depending on current polarity. This
enables the design of electric
refrigerators