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EdrawMindChapter 7 Contact between Metals and Semiconductors
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Chapter 7 Contact between Metals and Semiconductors
This is a mind map about the contact between metals and semiconductors in Chapter 7. The main contents include metal-semiconductor contact and its energy band diagram, metal-semiconductor contact rectification theory, minority carrier injection and ohmic contact, Schottky potential barrier diode.
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- One Hundred Years of Solitude Character Relationship Chart
One Hundred Years of Solitude is the masterpiece of Gabriel Garcia Marquez. Reading this book begins with making sense of the characters' relationships, which are centered on the Buendía family and tells the story of the family's prosperity and decline, internal relationships and political struggles, self-mixing and rebirth over the course of a hundred years.
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Chapter 7 Contact between Metals and Semiconductors
- One Hundred Years of Solitude Character Relationship Chart
One Hundred Years of Solitude is the masterpiece of Gabriel Garcia Marquez. Reading this book begins with making sense of the characters' relationships, which are centered on the Buendía family and tells the story of the family's prosperity and decline, internal relationships and political struggles, self-mixing and rebirth over the course of a hundred years.
- One Hundred Years of Solitude Character Relationship Chart
One Hundred Years of Solitude is the masterpiece of Gabriel Garcia Marquez. Reading this book begins with making sense of the characters' relationships, which are centered on the Buendía family and tells the story of the family's prosperity and decline, internal relationships and political struggles, self-mixing and rebirth over the course of a hundred years.
- Project Management Process Template
Project management is the process of applying specialized knowledge, skills, tools, and methods to project activities so that the project can achieve or exceed the set needs and expectations within the constraints of limited resources. This diagram provides a comprehensive overview of the 8 components of the project management process and can be used as a generic template for direct application.
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- Outline
Chapter VII Contact between metals and semiconductors
Metal-semiconductor contact and its energy band diagram
Metal and semiconductor work function W
Def of work function: the difference between the energy E0 of stationary electrons in vacuum and the EF energy of metal; used to characterize the ability of an object to bind electrons. The larger the metal work function Wm, the stronger the metal's binding to electrons; and it changes periodically with the increase of atomic number.
E0 is called the vacuum energy level, which symbolizes the surface energy level of the object. The Fermi level of a semiconductor is related to the doping concentration, so its work function is also closely related to the impurity concentration!
expression
Metal:
semiconductor:
contact potential difference
Phenomenon: When a metal comes into contact with a semiconductor, due to the different work functions of the two, the electrons in them will migrate to the side with a larger work function, causing the energy band of the semiconductor to bend. (Example of n-type semiconductor Wm>Ws)
barrier height
Metal:
semiconductor:
Barrier and anti-blocker layers:
Wm>Ws: Electrons flow from semiconductor to metal, The surface energy brings bending, Form an electronic barrier, Form n-type barrier layer and p-type anti-blocking layer Wm<Ws: Electrons flow from metal to semiconductor, The surface energy band bends downward, Form a hole barrier, Form n-type anti-blocking layer, p-type blocking layer
Effect of surface states on contact barrier
Surface state: The periodic structure of the crystal terminates suddenly at the surface. Each atom in the surface layer will have an unpaired electron, that is, an unsaturated bond. This bond is called a dangling bond, and the corresponding electron energy states (introducing energy levels in the forbidden band) are surface states.
Donor surface state: The surface below qΦ0 is electrically neutral when occupied by electrons, and becomes positive after releasing electrons.
Acceptor surface state: The surface above qΦ0 is electrically neutral when empty, and becomes negatively charged after accepting electrons.
If there is a surface state in an n-type semiconductor, the Fermi level is higher than qΦ0, and a positive space charge region appears on the surface, forming an electronic barrier. The barrier height qVD just makes the negative charge on the surface state equal to the positive charge in the barrier area.
Even if the surface state is not in contact with the metal, the surface will form a potential barrier. If the surface state density is very large, pinned occurs at high surface state density.
Without surface states, the work function of a semiconductor is:
There are surface states, and the work function of the semiconductor is:
When the surface state density of a semiconductor is very high, the influence of metal contact can be shielded, so that the barrier height within the semiconductor has almost nothing to do with the work function of the metal, which is determined by the surface properties of the semiconductor.
Metal-semiconductor contact rectification theory
Qualitative analysis
When there is an external voltage, the surface potential is (Vs)0 + V. When forward bias is applied, V > 0, and when reverse bias is applied, V < 0. (Forward bias refers to the metal side potential being positive)
Effect on n-type barrier layer
V>0
V<0
The direction of change of the barrier height mainly depends on the moving direction of the carriers after applying the bias voltage. When electrons accumulate, the barrier height decreases.
Quantitative analysis
diffusion theory (for barrier height>>electron mean free path)
For n-type barrier layers, when the width of the barrier is much larger than the mean free path of electrons, electrons will have to collide multiple times through the barrier area. This barrier is called a thick barrier; both drift and diffusion need to be considered
in conclusion:
Thermionic emission theory: (for barrier height<<electron mean free path)
When the n-type barrier layer is very thin, the electron mean free path is much larger than the barrier width. It is the barrier height that matters, not the barrier width.
in conclusion:
JsT has nothing to do with the applied voltage and is more dependent on temperature So it is called thermionic emission
The influence of mirror force and tunnel effect
Reason for research: The actual volt-ampere characteristics of gold-half contact rectifiers are far different from the theoretical results.
Cause Analysis:
The influence of mirror power
The concept of mirror charge: An electron outside the metal should induce a positive charge on the metal surface, and the electron should be attracted by the positive charge. If the distance of an electron from the metal surface is Like charge.
Mirror power:
Effect: The mirror force moves the top of the potential barrier inward and causes a decrease in the potential barrier q ΔΦ. At equilibrium, q ΔΦ is small and can be ignored. The amount of potential barrier reduction caused by the image force increases slowly with the increase of reverse voltage. When the reverse voltage is high, the reduction of the potential barrier becomes obvious, and the influence of the image force becomes important.
The x value corresponding to the maximum value of the potential barrier:
The impact of the tunnel effect
The concept of tunnel effect: electrons with energy lower than the top of the barrier have a certain probability of passing through the barrier. The probability of penetration is related to the electron energy and barrier thickness. For electrons of a certain energy, there is a critical barrier thickness xc. If xd > xc, then electrons cannot pass through the potential barrier at all; if xd < xc, then the potential barrier is completely transparent to electrons, that is, the potential barrier is lowered. When the reverse voltage is higher, the reduction of the potential barrier is obvious.
The potential barrier caused by the tunnel effect is reduced to
Mirror force and tunnel effect have a significant impact on reverse characteristics 1) Causes the barrier height to decrease and increase the reverse current. 2) The greater the reverse voltage, the more significant the potential barrier decreases, and the greater the reverse current.
Minority carrier injection and ohmic contact
Injection of minority carriers
When a forward bias is applied, the potential barrier is lowered and minority carrier diffusion dominates, forming a minority carrier flow from the outside to the inside, with the same direction as the majority carrier flow. Therefore, part of the forward current is carried by minority carriers.
size
First, it depends on the concentration of minority carriers in the barrier layer. If the barrier is high enough, the concentration of holes close to the contact surface will be high. With an external voltage, the contribution of the minority carrier current will increase.
It also depends on the efficiency of hole diffusion into the semiconductor. The higher the diffusion efficiency, the greater the contribution of minority carriers to the current.
When a positive voltage is applied, the ratio of the minority carrier current to the total current is called the minority carrier injection ratio γ, γ=Jp/J=Jp/(Jn Jp)
Ohmic contact
Def: Metal-semiconductor contact does not produce obvious additional impedance, and does not significantly change the equilibrium carrier concentration inside the semiconductor; the contact resistance is much smaller than that of semiconductor samples or devices, and no voltage drop is generated, which does not affect the I-V characteristics. .
Implementation method: Dope one side of the semiconductor to form a metal-n n or metal-p p structure, and use the principle of tunneling effect to create ohmic contacts on the semiconductor.
Main application anti-blocking layer
Schottky Barrier Diode
Def: Diode made using the rectifying contact properties of metal-semiconductor
Comparison with pn junction diodes
Same as - unidirectional conductivity
different
The forward current of the pn junction is the current formed by non-equilibrium minority carrier diffusion, which has a significant charge storage effect; the forward current of the Schottky barrier diode is mainly formed by the majority carriers of the semiconductor entering the metal. It is a multi-carrier device and has no accumulation. , so the high frequency characteristics are better;
The barrier area of a Schottky barrier diode only exists on one side of the semiconductor
The forward conduction voltage of Schottky diodes is low, generally about 0.3V, and the pn junction is generally 0.7V.
Application: clamping diodes (increase circuit speed), etc.