Lecture6 - 11/26/2010 11/26/2010 2 LECTURE6:MOSFET EECS170A

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11/26/2010 1 LECTURE 6: MOSFET EECS 170A 11/26/2010 1 Metal Semiconductor/ Metal Insulator Semiconductor junctions Previous lectures mainly focused on homo junctions made of two same type of semiconductors (Si Si or Ger Ge, etc). Recent low power intergraded circuits heavily rely on low power semiconductor devices that uses metal semiconductor or metal insulator semiconductor junctions. In particular we will focus on physics of MOSFETs. To start let’s discover the basic properties of metal semiconductor and metal oxide semiconductor junctions. 11/26/2010 2 As we discussed in the 2 nd week of our lecture, semiconductors, metals and insulators are categorized based on their bandgap energy that determines their electrical property. METAL E G ~0 E C , E V , E F E C E V SC E C E V Insulator Metal Semiconductor junctions (1) Let’s start our discussion with Metal Semiconductor junctions Since we are dealing with two different materials we need to have a common reference plane in energy domain. Vacuum level is the commonly used reference plane in metals and in semiconductors (also in heterojunctions) Vacuum level (E 0 ) describes the energy required for electron to leave the atom and to become completely free. The energy difference between the vacuum level and the Fermi energy is called the work function of the material ( φ ) . It is a distinct parameter for each metal Energy difference between the conduction band and the E at the surface is 11/26/2010 3 0 called the electron affinity ( χ ) of the SC, and it is a distinct parameter of a SC which is constant . E 0 METAL E FM E C E V E F M S Metal Semiconductor junctions (2) For instance Relative magnitude of work functions ( φ ) in metals and in semiconductors 11/26/2010 4 () 3.66 4.0 5.15 4.03 4.07 M eV eV Mg Si Ni Ge GaAs determine the property of the junction and how it will operate METAL E FM E C E V E FS E 0 M S
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11/26/2010 2 Metal Semiconductor junctions (3) For instance, let’s consider a metal semiconductor junction formed by an n type SC where φ M > φ S After the junction is formed, the electrons start to transfer from the semiconductor to the metal to create a thermal equilibrium at which the Fermi energy will be constant The flow of electrons will create a depletion layer at the surface of SC that will create a depletion layer and potential barrier to prevent flow of electrons The transfer of electron will continue until it reaches the thermal equilibrium that net flow of carriers is 0 11/26/2010 5 that net flow of carriers is 0. Equilibrium potential barrier: METAL E FM E C E V E FS E 0 M S METAL E FM E C E V E FS equilibrium E 0 E i B BM  E i Metal Semiconductor junctions (4) The same argument can be made for a metal semiconductor junction formed by an n type SC where φ M < φ S After the junction is formed, the electrons start to transfer from the metal to
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This note was uploaded on 12/13/2010 for the course ELECTRICAL EECS 170A taught by Professor Ozdalboyraz during the Fall '10 term at UC Irvine.

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Lecture6 - 11/26/2010 11/26/2010 2 LECTURE6:MOSFET EECS170A

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