Transcript End 1.4 The Semiconductor Diode

The Science of Electronics:
Analog Devices
Chapter One Electrical Conduction Processes
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Chapter 1 Electrical Conduction Processes

Content
 1.1
Looking Back and Looking Forward
 1.2
Conduction in Homogeneous Materials
 1.3
Junctions and Contacts
 1.4 The
Semiconductor Diode
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1.1 Looking Back and Looking Forward
In the last six chapters, we have introduced basic concepts in network
theory, device modeling, and signal processing. We have discussed five
specific kinds of components: resistors, op-amps, capacitors, inductors,
and transformers, their circuit symbols and terminal characteristics.
In the following chapter, We will begin with a descriptive discussion
of the conduction processes that take place in electronic devices. Then,
as we introduce successively more complex device structures in the
chapters to follow, our descriptive view of device operation will help us
to understand not only the normal network properties of the device, but
also the limitations of device usage in each potential application.
The Main contents of this chapter:
1. The principle of conduction in conductive, semi-conductive
materials.
2. The structure and characteristic of P-N junction.
3. The semiconductor diode and its characteristic.
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1.2 Conduction in Homogeneous Materials (1)
1.2.1 Metals
1. In a metal current is carried by a single type of mobile
charge, the free electron (current carrier 载流子).
2. In metals there is a relatively large density of mobile charge
carriers (approximately 1023). Therefore, metals are good
electrical conductors.
3. For a given metal, the charge-carrier density is fixed (does
not vary with temperature).
4. Over distances large compared to the inter-atomic spacing a
metal is everywhere electrically neutral.
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1.2 Conduction in Homogeneous Materials (2)
1.2.2 Pure Semiconductors
1. Silicon and germanium are two basic pure semiconductors.
2. In semiconductor the current is carried by two distinct type
of mobile charge, the mobile free electron (自由电子) and
mobile hole (空穴).
3. The density of mobile charge carriers in semiconductor is
much smaller than that in metal (1010~1013).
4. As the temperature is increased, the density of mobile charge
carriers is increased.
5. The conduction ability of pure semiconductor is temperature
sensitive.
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1.2 Conduction in Homogeneous Materials (3)
1.2.3 Doped Semiconductors
1. Doping minute quantities of foreign elements, called impurities (杂质), to
the pure semiconductors produces doped semiconductors (掺杂半导体).
2. The relative concentration of free electrons and holes in a semiconductor
may be adjusted by addition of minute quantities of appropriate impurity
elements. Materials in which holes are the majority carrier are called p-type
semiconductors, while those in which free electrons are the majority carrier
are called n-type semiconductors.
3. The difference in charge density between free electrons and holes in a
doped semiconductor is exactly balanced by the charged immobile impurity
ions. Thus, over distances large compared to the inter-atomic spacing, a
semiconductor is electrically neutral.
4. The density of majority carrier in doped semiconductor is mainly
dependent to the doping density.
5. The conduction ability of doped semiconductor is also temperature
sensitive.
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1.2 Conduction in Homogeneous Materials (4)
1.2.4 The v-i Characteristic of the Homogeneous Bar
The volt-ampere characteristic of a homogeneous semiconductor bar is linear
and obeys the Ohm's law, i=G v
Where G is the conductance of the bar, and it can be calculated by
G= (A /L)
A is the cross-sectional area of the bar, L is the length of the bar,  is the
conductivity (电导率)of the material, unit (W-m)-1.
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1.2 Conduction in Homogeneous Materials (5)
For the p-type semiconductor, since the majority charge carrier is holes,
the conductivity can be expressed as
  q(me  n  mh  p)  q  mh  p ( p  n)
For the n-type semiconductor, since the majority charge carrier is electrons,
the conductivity can be expressed as
  q(me  n  mh  p)  q  me  n ( p  n)
Where, q is the electronic charge (1.6× 10-19 coulombs), constants me and mh
are called the mobility (迁移速率) of free electrons and holes in the
semiconductor material (units cm2/Vs), n and p are the concentrations of free
electrons and holes in the semiconductor material.
Therefore the volt-ampere characteristic of homogeneous semiconductor bar is
 L  v
i  q  m  n A v
L
For p-type semiconductor
i  q  mh  p  A
For n-type semiconductor
e
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1.3 Junctions and Contacts (1)
1.3.1 The p-n Junction
When p-type and n-type bar are metallurgically joined, a p-n junction is formed.
In the p-type region holes concentration is higher than that in n-type
region and in the n-type region free electrons concentration is higher than
that in p-type region. Therefore, in the neighborhood of the junction there
must be carrier concentration gradients, which give rise the movement of
charge carriers from higher concentration region to lower concentration
region, holes move from p-type region to n-type region and free electrons go
from n-type region to p-type region.
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1.3 Junctions and Contacts (2)
1.3.2 Diffusion Current 扩散电流
An electric current is defined as the net transport of charge
through a cross-sectional plane.
In an electric field, exerting a force on the individual charge
carriers, then a net charge transport and a corresponding
electric current, drift current漂移电流, is produced.
In a p-n structure, since the presence of the charge carriers
concentration gradients in the interface, the carriers will move
from higher concentration region to lower concentration region,
holes move from p-type region to n-type region and electrons
from n-type region to p-type region. A net charge transport is
formed in the interface, the corresponding current is called
diffusion current.
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1.3 Junctions and Contacts (3)
1.3.3 Charge Transport at the p-n Junction in Equilibrium
The concentration gradients in the interface between p- and
n-type semiconductors produce the charge carriers' diffusion.
When holes diffuse from the p-region, they leave behind an
equal number of immobile, negatively charged acceptor ions.
Similarly, electrons diffusing from the n-region leave behind
positively charged, immobile donor ions. Thus in the vicinity of
the junction the diffusion of holes and electrons results in a
region with excess, immobile negative charge in the p-type
material. and a region with excess. immobile positive charge in
the n-type material. These regions of excess, immobile charge
adjacent to the junction comprise what is called a space-charge
layer (SCL) 空间电荷层.
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1.3 Junctions and Contacts (4)
The space-charge regions (SCR空间电荷区) to each side of the Junctions
are charged with polarity opposite to that of the mobile carriers that have
diffused out of the respective regions. As the diffusion process continues and
the charged regions increase in size, they exert an increasing attractive force
on the majority carriers. This force opposes the diffusive flow.
When the p-n junction is in equilibrium, in the neighborhood of the junction
there are two types of charge transports:
1. Drift 漂移
2. Diffusion 扩散
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1.3 Junctions and Contacts (5)

1. Drift 漂移
The mobile holes and free electrons are forced by the buildup electric field (内建电场). Once the current carriers enter
the SCR, the holes drift to p-type region, and the electrons
drift towards to n-type region.

2. Diffusion 扩散
The mobile holes and free electrons are forced by
concentration gradient. The holes diffuse from p-type region
to n-type region, and the electrons diffuse from n-type
region to p-type region.
Notice: the drift and the diffusion are localized in the
neighborhood of the junction. Far from the junction the ptype and n-type regions are neutral and homogeneous,
unaffected by the presence of the junction.
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1.3 Junctions and Contacts (6)
1.3.4 The v-i Characteristic of the p-n junction
1. Forward bias
The current increases
rapidly with applied
forward bias voltage.
The applied electric field by the voltage source is opposite to the build-up
electric field of the p-n junction, then the width of the space-charge region
becomes narrower as the figure.
A forward-bias voltage weakens the field in the space-charge layer, allowing
diffusion of majority carriers across the junction to the side where they are
in the minority. Thus, under forward-bias conditions, the concentrations of
minority carriers near the junction increase substantially. This increase in
minority carrier concentration near a forward-biased junction, by diffusion
across the junction, is called injection of minority carriers.
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1.3 Junctions and Contacts (7)
2. Reverse bias
The applied electric field by the voltage source strengthens the electric field in
the SCL, then the width of the space-charge region becomes wider as the figure.
As the field increases, it opposes the diffusion of majority carriers so strongly
that the diffusive components of charge transport are virtually stopped. That is,
the field is directed so as to hold majority carriers in their respective neutral
regions and to prevent their diffusion across the SCL. Also, the field direction is
such that it attracts minority carriers from their respective neutral regions and
moves them by drift across the SCL, a much smaller negative value of i, or
reverse current, flows through the p-n junction.
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1.3 Junctions and Contacts (8)
Although increasing the reverse voltage produces an
accompanying increase in the electric field in the SCL, the reverse
current is limited by the supply of minority carriers in the neutral
regions. Thus, once the field has reached the point that it extracts all
the minority carriers that the neutral regions can supply, the current
becomes independent of further increases in the field strength. This
constant reverse current is called the reverse saturation current of the
p-n junction, since the value of current reaches a maximum or
saturation value as the reverse voltage is increased.
In summary, the p-n junction supports significant current flow in
the forward direction (from p- to n-type material), but permits only a
very small current in the reverse direction. Typical forward currents
are in the range of mA to well over 1A, depending on the size of the
structure and its power dissipation capability. On the other hand,
reverse currents are about six orders of magnitude less, lying in the
nA to mA range.
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1.3 Junctions and Contacts (9)
1.3.5 Ohmic Contacts and Schottky Barriers
In fabricating a semiconductor device for use in a circuit, it is
necessary to attach metal contacts to the semiconductor. When
metal contacts to the semiconductor, another mode of junction is
created. It is possible to classify metal-semiconductor contacts into
two rough categories: Ohmic contacts and Schottky barriers.
1. Ohmic Contact 欧姆结
Ohmic contact is a junction with a v-i characteristic that is
perfectly linear, being the electrical equivalent of a homogeneous
bar (an ideal resistor).
2. Schottky Barrier 肖特基栅
Schottky barrier is a junction with a space-charge layer in the
semiconductor in the vicinity of the metal-semiconductor junction.
Its v-i characteristic is very similar to that of p-n junction.
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1.4 The Semiconductor Diode (1)半导体二极管
1.4.1 Structure and Circuit Symbol
The structure of a semiconductor diode is exactly a p-n
junction with two terminals. The terminal contacted to the p
region is called “Anode”(阳极), and the terminal to the n
region is called “Cathode”(阴极).
The structure of a semiconductor diode
The circuit symbol of
a semiconductor diode
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1.4 The Semiconductor Diode (2)

1.4.2 The Exponential Diode
A theoretical analysis of the p-n junction structure yields a
single equation, the characteristic of a p-n junction or a
diode, as follows
i  I S (e
q
v
kT
 1)
i  IS e
v
26 mV
(Forward)
i   I S (Reverse)
Where, IS is the reverse saturation current, q is the electronic charge
(1.6×10-19coulombs), k is Boltzmann’s constant (1.38×10-23 joules/K 波尔茨
曼常数), and T is the absolute temperature (degrees Kelvin). The quantity
kT/q has the dimension of a voltage, at 300°K it is 25.8mV. When
v>>25.8mV, the characteristic becomes exponential Forward bias. When
v<-25.8mV, the characteristic becomes constant  Reverse bias.
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1.4 The Semiconductor Diode (3)
The threshold voltage of the diode is the voltage at which the
current appears to depart significantly from zero. For the
germanium diode the threshold voltage is 0.2~0.3V, and for the
silicon diode the threshold voltage is 0.6~0.7V.
Generally, the saturation current of the germanium diode is
much larger than that of the silicon diode.
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1.4 The Semiconductor Diode (4)
1.4.3 Maximum Power Dissipation
The archenemy of electrical component is excessive hearing. In
resistive elements, the power dissipated in the element is converted
to heat, which raises the temperature of the element above its
surroundings. The maximum temperature that a device can
withstand coupled with its ability to transfer the generated heat to
the ambient sets a limit on the maximum power dissipation for the
device.
The maximum allowable power dissipation PD, max limits the
maximum product of voltage and current in the device:
v  i  PD ,max
The right figure shows the safe
operating region of a diode.
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1.4 The Semiconductor Diode (5)
1.4.4 Diode Voltage Limitations
The increases in the reverse voltage of a diode is accompanied by
a small reverse saturation current and an increasing electric field
in the space-charge layer (SCL).
1. Avalanche breakdown (雪崩击穿)
As the electric field in the SCL increases, so does the velocity of
the reverse-current mobile carriers crossing the SCL. At some
point these carriers attain sufficient speed so that through
collisions they knock additional electrons from the covalent bonds
in the SCL, producing both a free electron and a hole. These new
carriers add to the reverse current and may themselves produce
still more mobile electrons and holes through additional collisions.
This process, called avalanche multiplication, produces a very
rapid increase in the reverse current. This phenomenon is called
avalanche breakdown.
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1.4 The Semiconductor Diode (6)
2. Zener breakdown (齐纳击穿)
The electric field in the SCL becomes so strong that it can
dislodge electrons directly from their covalent bonds. And
produces the same result as avalanche multiplication.
Generally, Zener breakdown is dominant in diodes that
breakdown below 6V, and avalanche breakdown is dominant in
diodes that break down above 6V.
The reverse-bias voltage that the diode will withstand before
reverse breakdown occurs is called the maximum reverse
blocking voltage of the diode, denoted VR.
The reverse breakdown does not mean the
destruction of the diodes. Diodes may be operated
in and out of the break-down region with no
irreversible changes, provided that the maximum
power dissipation limits are not exceeded
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1.4 The Semiconductor Diode (7)
1.4.5 Zener Diodes 稳压二极管
In the reverse breakdown region a diode has the property
that the voltage is nearly independent of the current. Thus if
a constant voltage at some point in the circuit is required,
one can employ a diode operating in the reverse breakdown
region. Diodes intended for this mode of operation are
called voltage reference diodes or Zener diodes. And the
voltage at which breakdown occurs is called the Zener
voltage, denoted VZ.
The symbol of the Zener diode is
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1.4 The Semiconductor Diode (8)
The v-i characteristic of Zener diode:
Manufacturers often specify a minimum reverse current, IZmin,
at which the voltage reference diode is to be operated to insure
that the breakdown mechanism is well established. The
maximum current limit, IZmax, is set by the maximum
permissible power dissipation.
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Chapter 1 Electrical Conduction Processes
End of Chapter One