Semiconductor

[[半导体物理]]

Semiconductors

Characteristics	Conductors导电体	Semiconductors半导体	Insulators绝缘体
Resistivity	10^-5	10^-5~10^6	10^6
Temp-Coefficient	Positive	Negative	Negative
最外层电子数	1-3	4	2*n^2

能带

固体根据导电性可划分为导体、半导体和绝缘体，其划分和其能带电子填充情况有关。在形成固体材料前，孤立原子外围电子受原子核势场的影响，在原子能级上规律排布，形成电子轨道。当原子逐渐靠近形成固体时，原子间电子轨道逐渐交叠，单个电子能级将被分裂成多个电子能级形成能带，原子间距离越近，能级分裂越严重。

从示意图中可以看到，分裂后的能带对应于不同的电子轨道，原子内层电子轨道对应能量低的能带，电子首先填满能量低的能级，被电子填满的能带称为满带或价带，无电子填充的能带称为空带或导带，价带与导带之间无能级分布区域称为禁带。下图为热力学温度为零时三种材料能带示意图。

Types

• An Intrinsic semiconductor is a pure semiconductor crystal in which the electron and hole concentrations are equal. By pure we mean virtually no impurities in the crystal.
• Germanium (Ge) and Silicon (Si) are the most common type of intrinsic semiconductor elements.
• Intrinsic type of semiconductor material is made up of only a single type of element.

Intrinsic

Pure Form of Ge,Si.

Extrinsic

N-type:Pentavalent impurity(五价杂质)

P-type: Trivalent impurity（三价杂质）

特性

• Semiconductor acts like an insulator at Zero Kelvin. On increasing the temperature, it works as a conductor.
• The resistance of semiconductor materials decreases with the increase in temperature and vice-versa.
• Semiconductors are smaller in size and possess less weight. Their resistivity is higher than conductors but lesser than insulators.
• Semiconductors can be modified by doping*(掺杂) to make semiconductor devices suitable for energy conversion, integrated chips, diodes and transistors.

Intrinsic（i-type）

• An Intrinsic semiconductor is a pure semiconductor crystal in which the electron and hole concentrations are equal. By pure we mean virtually no impurities in the crystal.
• Germanium (Ge) and Silicon (Si) are the most common type of intrinsic semiconductor elements.
• Intrinsic type of semiconductor material is made up of only a single type of element.
• They have four valence electrons (tetravalent).
• The number of excited electrons is equal to the number of holes;（n = p）. They are also termed as undoped semiconductors or i-type semiconductors.

Extrinsic

By introducing small amounts of impurities into an otherwise pure Si crystal is called Extrinsic semiconductor.

• The conductivity of semiconductors can be greatly improved by introducing a small number of suitable replacement atoms called Impurities. The process of adding impurity atoms to the pure semiconductor is called Doping.

N-TYPE DOPING (Negative)

When a pure semiconductor is doped by pentavalent impurity（+5价） (Phosphorus P, Arsenic As, Antimony Sb, Bismuth Bi)

n型半导体（n-type semiconductor）又称电子型半导体，因掺有“施主杂质”使其原子的电子密度与空穴密度不平衡，导电的电子密度超过流动的空穴密度（产生超额的电子），导电性则由这些多数载流子为带负电荷的电子来决定。

概而言之，就是在纯净晶体的半导体（本征半导体，如硅晶体或锗晶体）中，掺入少量的五价元素杂质（如磷、锑、砷），由于半导体原子（如硅原子）被杂质原子取代，五价元素的杂质（以磷为例）会和纯半导体作用，磷原子的五个外层电子中的四个与周围的半导体原子形成共价键，多出的一个电子几乎不受束缚，较易成为自由电子。于是，n型半导体就成为了含自由电子浓度较高的半导体，其导电性主要是因为自由电子导电。

P-type （Positive）

When a pure semiconductor is doped with a trivalent impurity （+3价）(Boron B, Aluminium Al, Indium In, Gallium Ga)

p型半导体（p-type semiconductor）又称空穴型半导体，因掺有“受主杂质”使其原子的电子密度与空穴密度不平衡，流动的空穴密度超过导电的电子密度，导电性则由这些多数载流子为带正电荷的空穴（电洞）来决定。

概而言之，就是在纯净晶体的半导体（本征半导体，如硅晶体或锗晶体）中，掺入少量的三价元素杂质（如硼、铝、镓、铟），由于半导体原子（如硅原子）被杂质原子取代，三价元素的杂质（以硼为例）会和纯半导体作用，硼原子的三个外层电子与周围的半导体原子形成共价键的时候，会产生一个“空穴”（电洞），这个空穴可能吸引束缚电子来“填充”，使得硼原子成为带负电的离子。这样，这类半导体由于含有较高浓度的“空穴”（“相当于”正电荷），成为能够导电的物质。

Intrinsic Semiconductor	Extrinsic Semiconductor
Pure semiconductor	Impure semiconductor
Density of electrons = Density of holes	Density of electrons ≠ Density of holes
Low electrical conductivity	High electrical conductivity
Only temperature dependence	Dependence on temperature as well as on the amount of impurity
No impurities	Trivalent impurity, Pentavalent impurity

PN Junction

A pn junction is a type of semiconductor device that is formed by joining together two different types of semiconductors, a p-type semiconductor and an n-type semiconductor.

一塊半導體晶體一側摻雜成p型半導體，另一側摻雜成n型半導體，中間二者相連的接觸面间有一个过渡层，稱為pn结、p-n结、pn接面（p-n junction）。pn结是電子技術中許多元件，例如半導體二極管、雙極性晶體管的物质基础。

• A pn junction is an interface or a boundary between two semiconductor material types, namely the p-type and the n-type, inside a semiconductor.
• There is an discontinuity between the p- and n-regions, which we also call the metallurgical junction (M).
• The fixed (immobile) ionized donors and the free electrons (in the conduction band, CB) in the n-region and fixed ionized acceptors and holes (in the valence band, VB) in the p-region.
• Due to the hole concentration gradient from the p-side, holes diffuse toward the right.
• The electron concentration gradient drives the electrons by diffusion toward the left.
• Holes diffusing and entering the n-side recombine with the electrons in the n-side near the junction.
• Electrons diffusing and entering the p-side recombine with holes in the p-side near the junction.
• Consequently the junction region becomes depleted of free carriers (electron or hole) in comparison with the bulk p- and n-regions far away from the junction.

原理

There are two types of electron flow:

• - The flow due to diffusion ( P➔N)
• - The flow due to electric filed (P《—N)

The depletion region offer resistance to the electric current because consist of mainly immobile ions (there is no charge carriers).

三种状态

Total three biasing conditions for the pn junction, and this is based on the voltage applied:

• Zerobias: No external voltage is applied to the p-n junction.
• Forward bias: The positive terminal of the voltage potential is connected to the p-type while the negative terminal is connected to the n-type.
• Reverse bias: The negative terminal of the voltage potential is connected to the p-type and the positive is connected to the n-type.

Forward bias

施加在p區的電壓高於n區的電壓，称为正向偏置（forward bias）

二极管可以导通正向大电流

• A battery is connected across a PN junction so that the +ive terminal is attached to p-side and the -ive terminal to n-side (Forward-biased).
• In forward biased, the built-in electric field at the pn junction and the applied electric field are in opposite directions. This results in a less resistive and thinner depletion region.
• Regions outside the depletion width have high conductivities due to plenty of majority carriers in the bulk, in comparison with the depletion region in which there are mainly immobile ions.
• The depletion region resistance becomes negligible when the applied voltage is large.
• At the voltage > built-in potential, the resistance of the depletion region becomes negligible, and the current flows.

Reverse bias

施加在n區的電壓高於p區的電壓，这种状态称为pn结反向偏置（reverse bias）

多数载流子扩散过pn结的势垒增大，pn结的电阻变大，宏观看二极管成为绝缘体。

• When the p-type is connected to the battery’s negative terminal and the n-type is connected to the positive side, the pn junction is reverse biased.
• In this case, the built-in electric field and the applied electric field are in the same direction.
• When the two fields are added, the resultant electric field is in the same direction as the built-in electric field, creating a more resistive, thicker depletion region.

Applications

Products/components made
Bipolar transistors
Diodes
Field-effect transistors
Integrated circuits
Light-emitting diodes (LEDs)
Metal-oxide semiconductor field-effect transistors (MOSFETs)
Silicon-controlled rectifiers

MOSFET（Metal Oxide Silicon Field Effect Transistors）

used to switch or amplify voltages in circuits. It is a voltage-controlled device and is constructed by three terminals.

The terminals of MOSFET are named as follows:

• Source (S)
• Gate (G)
• Drain (D)
• Body (B)

Structure

• The p-type semiconductor forms the base of the MOSFET.
• The two types of the base are highly doped with an n-type impurity which is marked as n+ in the diagram.
• The layer of the substrate is coated with a layer of silicon dioxide for insulation.
• A thin insulated metallic plate is kept on top of the silicon dioxide.
• The gate terminal is brought out from the thin metallic plate.
• A DC circuit is then formed by connecting a voltage source between these two n-type regions.

N-channel MOSFET

MOSFETs can be built as either NMOS or PMOS transistors, depending on the polarities of the bulk, source and drain regions

The N-channel MOSFETs are abbreviated as NMOS and are represented as figure as shown in Figure.

The majority of carriers in NMOS devices are electrons.

CMOS

CMOS是Complementary Metal Oxide Semiconductor（互补金属氧化物半导体）的缩写。它是指制造大规模集成电路芯片用的一种技术或用这种技术制造出来的芯片，是电脑主板上的一块可读写的RAM芯片。因为可读写的特性，所以在电脑主板上用来保存BIOS设置完电脑硬件参数后的数据，这个芯片仅仅是用来存放数据的