半导体物理学半导体 (37).pdf

《半导体物理学半导体 (37).pdf》由会员分享，可在线阅读，更多相关《半导体物理学半导体 (37).pdf（15页珍藏版）》请在淘文阁 - 分享文档赚钱的网站上搜索。

1、Metal-Semiconductor Ohmic Contacts（金半欧姆接触）Ideal nonrectifying barrier01Tunneling barrier02Specific contact resistance0304Characteristics of Ohmic Contactsn Any contact between semiconductor device or integrated circuit with outside is achieved through ohmic contact.n Ohmic Contacts:a junction with v

2、ery low contact resistance,forming current on either side of metal or semiconductor,not rectifying contact.n Two kinds:Nonrectifying Contact,Ohmic contact based on tunneling effect.n Schottky contact considers the ideal situation of metal to n-type semiconductor contact in the case of m sn Ohmic con

3、tact considers the ideal situation in the case of m sn To achieve thermal equilibrium,electrons flow from metal to semiconductor,which makes the surface of semiconductor more close to n-type.The surface charge density is formed by the excess electron charge existing on the surface of n-type semicond

4、uctor.Ideal Nonrectifying Barrier m sIdeal Nonrectifying BarrierFigure 9.11 Ideal energy-band diagram(a)before contact and(b)after contact for a metal-n-type semiconductor junction for m sIdeal Nonrectifying BarrierEbin When a positive voltage is applied to metal,electrons easily flow to the directi

5、on of low potential,from semiconductor to metal.n When a positive voltage is applied to semiconductor,electrons can easily pass through the barrier and flow from metal to semiconductor.This junction is an ohmic contact.Figure 9.12 Ideal energy-band diagram of a metal-n-type semiconductor ohmic conta

6、ct(a)with a positive voltage applied to the metal and(b)with a positive voltage applied to the semiconductor.EEm sFigure 9.13 ideal nonrectifying contact between metal and p-type semiconductor.(a)before contact and(b)after contact for a metalp-type semiconductor junction for m sEbin After contact fo

7、rmation,electrons flow from semiconductor to metal to realize hot electron emission,leaving empty states in semiconductor,that is,holes.n Excessive hole accumulation on the surface makes the semiconductor p-type deeper,and electrons flow easily from the metal to the empty state in the semiconductor.

8、n This charge transfer corresponds to the flow of holes from the semiconductor into the metal.Holes flowing from a metal to a semiconductor is also ohmic contact.Metal to p-type Semiconductor Ohmic Contacts9Metal-Semiconductor ContactRectifying ContactOhmic Contact Ohmic Contact m sRectifying Contac

9、tMetal-n type semiconductorMetal-p type semiconductor m sDue to the influence of surface state,good ohmic contact cannot be formed both for the metal of ms contacting with p-type.01()BngEee()BnmWithout surface stateInfluence of surface stateWith severe surface statesn The space charge width of the m

10、etal semiconductor contact is inversely proportional to the square root of the semiconductor doping concentration,and the depletion layer width decreases with the increased doping concentration.n The tunneling effect increases with the increase of doping concentration.Tunneling BarrierFigure 9.14 En

11、ergy-band diagram of a heavily doped n-semiconductor-tometal junction.A figure of merit of ohmic contacts is the specific contact resistance Rc For a rectifying contact,the current-voltage relation is：The thermionic emission current is dominant in this junction.The specific contact resistance isRc d

12、ecreases rapidly as the barrier height decreasingSpecific Contact ResistanceFor a metal-semiconductor junction with a high impurity doping concentration,the tunneling process will dominate.It is a very strong function of semiconductor dopingSpecific Contact Resistancen Figure shows theoretical and e

13、xperimental values of Rc as a function of semiconductor doping.n For lower doping concentrations,the Rc values are dependent on the barrier heights and become almost independent of the doping.n For doping concentrations greater than approximately 1019cm-3,the tunneling process dominates and Rc shows

14、 the exponential dependence on Nd.Specific Contact Resistancen Ohmic contact:Forming a good ohmic contact requires a low barrier.However,the realization of ohmic contact in actual production is not easy.It will be more difficult to achieve good ohmic contact on metals with wide energy band.n Heavy d

15、oping at the surface:The surface heavily doped semiconductor must form ohmic contact based on the tunneling effect.The formation of tunnel junction is realized by diffusion,ion implantation or growth of an epitaxial layer.n The doping concentration of semiconductor surface is limited by the solid solubility of impurities.For n-type GaAs,the solid solubility of impurities is about 51019cm-3.M-S and heavily doped ohmic contact