When the p-type and the n-type materials are kept in contact with
each other, the junction between them behaves differently from either
side of the material alone. The electrons and holes are close to each
other at the junction. According to coulomb’s law, there is a force
between the negative electrons and the positive holes. When the p-n
junction is formed a few electrons from the n-type diffuse through the
junction and combines with the holes in the p-side to form negative ions
and leaves behind positive ions in the n-side. This results in the
formation of the depletion layer, which acts as the barrier and does not
allow any further flow of electrons from the n region to the p
region.
No Applied Bias(V = 0V)
no-bias
在没有外部电压的情况下，depletion region正常存在。n
type这边主要载流子是电子，但是如果要流向p type，需要克服正电子的吸引，所以只有一小部分能够过去，这个数量和从p
type流过来的电子（少数载流子）数目接近，也就是图12 c 左上的和右下的正好差不多抵消。同理两个也差不多。所以总体电流为0。
In the absence of an applied bias across a semiconductor
diode, the net flow of charge in one direction is zero.
The current that exists under reverse-bias conditions is
called the reverse saturation current and is represented by .
Foward-Bias Condition()
foward-bias
当正向电压的时候，首先depletion region会变薄，因为n
type这边会有大量电子注入，重新和正电子结合，p
type的电子会被“抽走”。当电压加到一定程度，突然大量电子从n涌入p，图12 c
右下的是会不断增大。
is the reverse saturation
current
is the applied forward-bias
voltage across the diode
n is an ideality factor, which is a function of the
operating conditions and physical construction; it has a range between 1
and 2 depending on a wide variety of factors (n =1 will be
assumed throughout this text unless otherwise noted).
k is Boltzmann’s constant =
is the absolute temperature
in kelvins = 273 + the temperature in
q is the magnitude of electronic charge =
diode-characteristics
虚线是理想的曲线，实现是实际的情况。
The actual reverse saturation current of a commercially
available diode will normally be measurably larger than that appearing
as the reverse saturation current in Shockley’s equation.
反向的饱和电流实际会在理想的下方，主要原因是：
– leakage currents
– generation of carriers in the depletion
region
– higher doping levels that result in increased
levels of reverse current
– sensitivity to the intrinsic level of carriers
in the component materials by a squared
factor—double the intrinsic level, and the contribution to the
reverse current could
increase by a factor of four.
– a direct relationship with the junction
area—double the area of the junction, and
the contribution to the reverse current could double. High-power
devices that have
larger junction areas typically have much higher levels of reverse
current.
– temperature sensitivity—for every 5°C increase
in current, the level of reverse sat- uration current in Eq. 1 will
double, whereas a 10°C increase in current will result in
doubling of the actual reverse current of a diode.
The farther an electron is from the nucleus, the higher is
the energy state, and any electron that has left its parent atom has a
higher energy state than any electron in the atomic
structure.
energy-gap
Only specific energy levels can exist for the electrons in the atomic
structure of an isolated atom. The result is a series of gaps between
allowed energy levels where carriers are not permitted.
energy-levels
There is a minimum energy level associated with electrons in the
conduction band and a maximum energy level of electrons bound to the
valence shell of the atom. Between the two is an energy gap that the
electron in the valence band must overcome to become a free carrier.
That energy gap is different for Ge, Si, and GaAs; Ge has the smallest
gap and GaAs the largest gap. In total, this simply means that:
An electron in the valence band of silicon must absorb more
energy than one in the valence band of germanium to become a free
carrier. Similarly, an electron in the valence band of gallium arsenide
must gain more energy than one in silicon or germanium to enter the
conduction band.
Ge devices:
photodetectors sensitive to light
security system sensitive to heat
Si and GaAs:
transistor networks, stability is a high priority
The wider the energy gap, the greater is the possibility of energy
being released in the form of visible (infrared) light waves. For GaAs
the gap is sufficiently large to result in significant light
radiation.
The units of measurement are electron volts (eV). The unit
of measure is appropriate because W (energy) = QV (as
derived from the defining equation for voltage: V =
W/Q). Substituting the charge of one electron and a
potential difference of 1 V results in an energy level referred to as
one electron volt.
As indicated in Fig. 3, silicon has 14 orbiting electrons, germanium
has 32 electrons, gallium has 31 electrons, and arsenic has 33 orbiting
electrons.
For germanium and silicon there are four electrons in the outermost
shell, which are referred to as valence electrons.
Valence electrons are electrons in the outer shells that are not
filled. Because valence electrons have higher energy than electrons in
inner orbits, they are involved in the majority of chemical processes.
They assist us in determining the chemical properties of an element,
such as its valency or how it forms bonds with other elements.
Gallium has three valence electrons and arsenic has five valence
electrons. Atoms that have four valence electrons are called
tetravalent, those with three are called trivalent,
and those with five are called pentavalent.
The term valence is used to indicate that the potential
(ionization potential) required to remove any one of these electrons
from the atomic structure is significantly lower than that required for
any other electron in the structure.
Ionization energy, also called ionization potential, is the energy
necessary to remove an electron from the neutral atom. X + energy → X+ +
e− where X is any atom or molecule capable of being ionized, X + is that
atom or molecule with an electron removed (positive ion), and e − is the
removed electron.
covalent-bonding
In a pure silicon or germanium crystal the four valence electrons of
one atom form a bonding arrangement with four adjoining atoms, as shown
in Fig. 4.
This bonding of atoms, strengthened by the sharing of
electrons, is called covalent bonding.
Because GaAs is a compound semiconductor, there is sharing between
the two different atoms, as shown in Fig. 5. Each atom, gallium or
arsenic, is surrounded by atoms of the complementary type. There is
still a sharing of electrons similar in structure to that of Ge and Si,
but now five electrons are provided by the As atom and three by the Ga
atom.
covalent-binding-gaas
Although the covalent bond will result in a stronger bond between the
valence electrons and their parent atom, it is still possible for the
valence electrons to absorb sufficient kinetic energy from external
natural causes to break the covalent bond and assume the “free” state.
The term free is applied to any electron that has separated
from the fixed lattice structure and is very sensitive to any applied
electric fields such as established by voltage sources or any difference
in potential. The external causes include effects such as light
energy in the form of photons and thermal energy (heat) from the
surrounding medium. At room temperature there are approximately
free carriers in
of intrinsic
silicon material, that is, 15,000,000,000 (15 billion) electrons in a
space smaller than a small sugar cube—an enormous number.
