5非晶硅材料物理

大学本科必修课：《薄膜晶体管原理及应用》第5讲

非晶硅材料物理

任课教师：董承远

上海交通大学电子工程系

2015‐3‐30

Solids

(cont) Solids (cont.)

Structure Crystal

Crystal (cont.)

Structure(cont)

Structure(cont) Crystal (cont.)

Imperfections in Crystals p y

Impurities in Crystals

Crystal

Bonding

Energy Band in Single Crystal

Energy Band in Single Crystal (cont.)

Influence of Defect on EB

Influence of Doping on EB

p g

Definition of Amorphous State

Properties of Amorphous State p p

of Amorphous State Instability

Description of Amorphous Structure

p Localization in Amorphous State

Localization in Amorphous State (cont.)

p

Energy Band in Amorphous Semiconductor

Crystal Structure of c‐Si

Exercise: The diameter of a silicon atom is d=0.235 nm, under the assumption that silicon atoms are hard spheres that just touch each th i th ili t l D t i th t l l tti t t other in the silicon crystal. Determine the crystal‐lattice constant (width of the unit cell, a).

Covalent Bond in c‐Si

Si

Si

Si

Si

Energy Band in c‐Si

Doping in c‐Si

Conduction in c‐Si

Exercise: P-type silicon has resistivity of 0.5 Ω·cm. Assuming μn=1450 cm2/V·s and μp=500 cm2/V·s, find the hole and electron concentrations. (Electronic Charge q=1.6×1019C)

qτ

in c Photoconductivity ‐Si

Photoconductivity in c‐Si (cont.)

Exercise: Please calculate the wave-length limit to induce

34 photoconductivity effect in c-Si. (Plank’s Constant h=6.63×10-34 J·s, Speed of light in Vacuum c=3×1010cm/s)

Why Amorphous Silicon in TFTs?

•In an AMFPD incorporating TFTs, the substrate is usually a borosilicate glass and the area often greater than 1000 cm;

devices cover an2

•The thin film semiconductor in a TFT must be deposited on top of the substrate, usually from a gas phase precursor that can be easily distributed uniformly over a l arge area;

•Of the materials, a‐Si:H currently dominates as the semiconductor material of

Benefits & Weakpoints of a‐Si TFTs •Good film deposition uniformity •Simple device structure

•Low leakage current

•High yield

•Low mobility y

•Instability

Structure of a‐Si:H

※The structure of four-fold-coordinated amorphous semiconductors consists

p

of randomly arranged tetrahedra of atoms.The bonding is generally covalent, thus this structure is referred to as a random covalent network.Bond distances are close to those of the corresponding crystalline materials

p g y

(generally<±1%deviation).The major change is in the bond angle (distortions typically<±10°relative to the tetrahedral109°28’).