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Lecture 2. OUTLINE Semiconductor Basics Reading: Chapter 2. Announcement. Office Hours for tomorrow is cancelled (ONLY for this week) There will be office hours on Friday (2P-3P) (ONLY for this week) Thursday’s class will start at 4P (ONLY for this week). What is a Semiconductor?.

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lecture 2
Lecture 2

OUTLINE

  • Semiconductor Basics

Reading: Chapter 2

announcement
Announcement
  • Office Hours for tomorrow is cancelled

(ONLY for this week)

There will be office hours on Friday (2P-3P)

(ONLY for this week)

Thursday’s class will start at 4P

(ONLY for this week)

what is a semiconductor
What is a Semiconductor?
  • Low resistivity => “conductor”
  • High resistivity => “insulator”
  • Intermediate resistivity => “semiconductor”
    • conductivity lies between that of conductors and insulators
    • generally crystalline in structure for IC devices
      • In recent years, however, non-crystalline semiconductors have become commercially very important

polycrystalline amorphous crystalline

semiconductor materials
Semiconductor Materials

Phosphorus

(P)

Gallium

(Ga)

energy band description
Energy Band Description
  • For current flow, one needs to have electrons in the conduction band or holes in the valence band
  • Completely full or completely empty bands cannot carry current
energy band description1
Energy Band Description

Current due to electron flow and hole flow will add up

silicon
Silicon
  • Atomic density: 5 x 1022 atoms/cm3
  • Si has four valence electrons. Therefore, it can form covalent bonds with four of its nearest neighbors.
  • When temperature goes up, electrons can become free to move about the Si lattice.
electronic properties of si
Electronic Properties of Si

Silicon is a semiconductor material.

  • Pure Si has a relatively high electrical resistivity at room temperature.

 There are 2 types of mobile charge-carriers in Si:

  • Conduction electronsare negatively charged;
  • Holesare positively charged.

 The concentration (#/cm3) of conduction electrons & holes in a semiconductor can be modulated in several ways:

    • by adding special impurity atoms ( dopants )
    • by applying an electric field
    • by changing the temperature
    • by irradiation
electron hole pair generation
Electron-Hole Pair Generation
  • When a conduction electron is thermally generated, a “hole” is also generated.
  • A hole is associated with a positive charge, and is free to move about the Si lattice as well.
carrier concentrations in intrinsic si
Carrier Concentrations in Intrinsic Si
  • The “band-gap energy” Eg is the amount of energy needed to remove an electron from a covalent bond.
  • The concentration of conduction electrons in intrinsic silicon, ni, depends exponentially on Egand the absolute temperature (T):
doping n type
Doping (N type)
  • Si can be “doped” with other elements to change its electrical properties.
  • For example, if Si is doped with phosphorus (P), each P atom can donate a conduction electron, so that the Si lattice has more electrons than holes, i.e. it becomes “N type”:

Notation:

n = conduction electron

concentration

doping p type
Doping (P type)
  • If Si is doped with Boron (B), each B atom can accept an electron (creating a hole), so that the Si lattice has more holes than conduction electrons, i.e. it becomes “P type”:

Notation:

p = hole concentration

terminology
Terminology

donor: impurity atom that increases n

acceptor: impurity atom that increases p

N-type material: contains more electrons than holes

P-type material: contains more holes than electrons

majority carrier: the most abundant carrier

minority carrier: the least abundant carrier

intrinsic semiconductor: n = p = ni

extrinsic semiconductor: doped semiconductor

electron and hole concentrations
Under thermal equilibrium conditions, the product of the conduction-electron density and the hole density is ALWAYS equal to the square of ni:Electron and Hole Concentrations

N-type material

P-type material

dopant compensation
Dopant Compensation
  • An N-type semiconductor can be converted into P-type material by counter-doping it with acceptors such that NA > ND.
  • A compensated semiconductor material has both acceptors and donors.

N-type material

(ND > NA)

P-type material

(NA > ND)

doping
Doping

What is the electron and hole density if you dope Si with Boron to 1018 /cm3 ?

charges in a semiconductor
Charges in a Semiconductor
  • Negative charges:
    • Conduction electrons (density = n)
    • Ionized acceptor atoms (density = NA)
  • Positive charges:
    • Holes (density = p)
    • Ionized donor atoms (density = ND)
  • The net charge density (C/cm3) in a semiconductor is
carrier drift
Carrier Drift
  • The process in which charged particles move because of an electric field is called drift.
  • Charged particles within a semiconductor move with an average velocity proportional to the electric field.
    • The proportionality constant is the carrier mobility.

Hole velocity

Electron velocity

Notation:

mp hole mobility (cm2/V·s)

mn electron mobility (cm2/V·s)

velocity saturation
Velocity Saturation
  • In reality, carrier velocities saturate at an upper limit, called the saturation velocity (vsat).
drift current
Drift Current
  • Drift current is proportional to the carrier velocity and carrier concentration:

Total current Jp,drift= Q/t

Q= total charge contained in the volume shown to the right

t= time taken by Q to cross the volume

Q=qp(in cm3)X Volume=qpAL=qpAvht

 Hole current per unit area (i.e. current density) Jp,drift = qpvh

conductivity and resistivity
Conductivity and Resistivity
  • In a semiconductor, both electrons and holes conduct current:
  • The conductivity of a semiconductor is
    • Unit: mho/cm
  • The resistivity of a semiconductor is
    • Unit: ohm-cm
resistivity example
Resistivity Example
  • Estimate the resistivity of a Si sample doped with phosphorus to a concentration of 1015 cm-3 and boron to a concentration of 6x1017 cm-3.