Ch t 12
Chapter
12. El
Electrical
ti lP
Properties
ti
• Semiconductivity
Semiconductivity
12 10 Intrinsic semiconduction
12.10
Intrinsic conductivity
σ = nqμ e + pqμ h
= ni q ( μ e + qμ h )
Ex 12.1)
Doping
Extrinsic
E
t i i semiconductors
i
d t
(b
(both
th n- and
d p-type)
t
) are produced
d
d ffrom
materials that are initially of extremely high purity, commonly
having total impurity contents on the order of 10-7 at%.
Controlled concentrations of specific donors or acceptors are then
intentionally added, using various techniques. Such an alloying in
semiconducting materials is termed “doping”.
12.11 Extrinsic semiconduction
n-Type Extrinsic semiconduction
σ ≅ nqμ e
p-Type Extrinsic semiconduction
σ ≅ pqμ h
12.12 The temperature dependence of carrier concentration
intrinsic
extrinsic
12.13 Factors that affect carrier mobility
the major scattering processes in semiconductors.
There are three important mechanisms:
The first (and least important one) is scattering at crystal defects like dislocations or (unwanted) impurity atoms.
Since we consider only "perfect” semiconductors at this point, and since most economically important semiconductors
are pretty perfect in this respect, we do not have to look into this mechanism here. However, we have to keep an
open mind because semiconductors with a high density of lattice defects are coming into their own (e.g. GaN or
CuInSe2) and we should be aware that the mobilities in these semiconductors might be impaired by these defects.
Second, we have the scattering at wanted impurity atoms, in other word at the (ionized) doping atoms.
atoms
This is a major scattering process which leads to decreasing mobilities with increasing doping concentration.
The relation, however, is non-linear and the influence is most pronounced for larger doping concentration, say
beyond 1017 cm–3 for Si. Examples for the relation between doping and mobilities can be found in the illustration.
Third we have scattering at phonons - the other important process. Phonons are an expression of the thermally
stimulated lattice vibrations and such strongly dependent on temperature. This part must scale with the density
of phonons, i.e. it must increase with increasing temperature. It is thus not surprising that it dominates at high
temperatures (while scattering at dopant atoms may dominate at low temperatures).
Ex 12.2) calculate the electrical conductivity of intrinsic silicon at 423K
Design 12.1) p-type Si, 50(Ωm)-1
Design 12.1) p-type Si, 50(Ωm)-1
12.13 The Hall effect
A result of the phenomenon whereby a magnetic field applied
Perpendicular to the direction of motion of a charged particle
Exerts a force on the particle perpendicular to both the magnetic
Field and the particle motion directions.
12.15 Semiconductor devices
Forward bias (electron + hole -> energy)
Reverse bias
Solar cell
LED
Organic light-emitting diode
Transistor
Junction
i Transistor
i
1947 William Shockley,
1947,
Shockley John Bardeen
Bardeen, Walter Brattain
Metal Oxide Semiconductor Field Effect Transistor
(MOSFET)
Integrated Circuit
AMLCD Backplane
Memory (DRAM)