the science of electronics: analog devices

Chapter One Electrical Conduction ProcessesChapter One Electrical Conduction Processes

The Science of Electronics: The Science of Electronics: Analog DevicesAnalog Devices

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Chapter 1Chapter 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


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.


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.



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.



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.



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 vWhere 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 (-m)-1.



For the p-type semiconductor, since the majority charge carrier is holes, the conductivity can be expressed as

For the n-type semiconductor, since the majority charge carrier is electrons, the conductivity can be expressed as

Where, q is the electronic charge (1.6× 10-19 coulombs), constants e and h

are called the mobility ( 迁移速率 ) of free electrons and holes in the

semiconductor material (units cm2/Vs), n and p are the concentrations of

free electrons and holes in the semiconductor material.

( ) ( )e h hq n p q p p n

( ) ( )e h eq n p q n p n

Therefore the volt-ampere characteristic of homogeneous semiconductor bar is

For p-type semiconductor

For n-type semiconductor

hAi q p vL

eAi q n vL


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.



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.



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 .



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



1. Drift 漂移The mobile holes and free electrons are forced by the build-up 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 p-type and n-type regions are neutral and homogeneous, unaffected by the presence of the junction.



1.3.4 The v-i Characteristic of the p-n junction

1. Forward bias

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.

The current increases rapidly with applied forward bias voltage.



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.



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.



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.


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


1.4 The Semiconductor Diode (2)

1.4.2 The Exponential DiodeA theoretical analysis of the p-n junction structure yields a single equation, the characteristic of a p-n junction or a diode, as follows

( 1)qv

kTSi I e

26

v

mVSi I e (Forward) Si I (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.



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.



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:

,maxDv i P

The right figure shows the safe operating region of a diode.



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.



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



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



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.


Chapter 1 Electrical Conduction Processes

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the science of electronics: analog devices

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