半导体物理 7.pptVIP

Wen Chang Huang Chapter 7 Chapter 7 MESFET and Related Devices MESFET and related devices Metal-semiconductor contacts MESFET MODFET Introduction MESFET I-V characteristics similar to that of a MOSFET Gate electrode Metal-semiconductor rectifying contact MOS structure (in MOSFET) Source/drain Ohmic contact p-n junction (in MOSFET) Shows negative temperature coefficient at high current level As T?? current? Need more uniform temperature distribution Higher switching speed and higher cutoff frequency MESFET can made from GaAs and InP High electron mobility Introduction (cont.) The basic building block Metal-semiconductor contact Similar to a one-sided abrupt p-n junction Majority carrier device Fast response Two type of metal-semiconductor contacts The rectifying The nonrectifing Metal-semiconductor contacts The first practical semiconductor device Point contact rectifier A metallic whisker pressed against a semiconductor In 1938, Schottky, suggest that The rectifying behavior could arise from a potential barrier as a result of the stable space charge in the semiconductor? Schottky barrier Nonrectifying The contact has a negligible resistance regardless of the polarity of the applied voltage Ohmic contact Perspective view of a metal-semiconductor contact Energy band diagram Isolated metal adjacent to an isolated n-type semiconductor When the metal makes intimate contact with the semiconductor Fermi level of the two material must be equal The vacuum level must be continuous The barrier height qΦBn=qΦm-qχ For p-type semiconductor qΦBp=Eg-(qΦm-qχ) q(ΦBn+ΦBp)=