Electromagnetism Lecture#6 Instructor: Engr. Muhammad Mateen Yaqoob

Guidelines about Final Term Exam Course Contents: ◦Lecture # 4 to Lecture # 6 ◦Also includes the assignments Pattern of Final Paper: ◦Total Marks = 45 marks ◦Paper will be from the slides ◦Short Questions (one-two line answer) = 30 marks [15x2] ◦Numerical = 15 marks [3 separate ones] ◦Get yourself prepared MATEEN YAQOOB DEPARTMENT OF COMPUTER SCIENCE

Categories of Solids There are three categories of solids, based on their conducting properties: ◦conductors ◦semiconductors ◦insulators MATEEN YAQOOB DEPARTMENT OF COMPUTER SCIENCE

Band Theory of Solids In order to account for decreasing resistivity with increasing temperature as well as other properties of semiconductors, a new theory known as the band theory is introduced. The essential feature of the band theory is that the allowed energy states for electrons are nearly continuous over certain ranges, called energy bands, with forbidden energy gaps between the bands. MATEEN YAQOOB DEPARTMENT OF COMPUTER SCIENCE

Valence band: Band occupied by the outermost electrons Conduction: Lowest band with unoccupied states Conductor: Valence band partially filled (half full) Cu. or Conduction band overlaps the valence band MATEEN YAQOOB DEPARTMENT OF COMPUTER SCIENCE

Band Theory and Conductivity Band theory helps us understand what makes a conductor, insulator, or semiconductor. 1) Good conductors like copper can be understood using the free electron 2) It is also possible to make a conductor using a material with its highest band filled, in which case no electron in that band can be considered free. 3) If this filled band overlaps with the next higher band, however (so that effectively there is no gap between these two bands) then an applied electric field can make an electron from the filled band jump to the higher level. This allows conduction to take place, although typically with slightly higher resistance than in normal metals. Such materials are known as semimetals. MATEEN YAQOOB DEPARTMENT OF COMPUTER SCIENCE

Valence and Conduction Bands The band structures of insulators and semiconductors resemble each other qualitatively. Normally there exists in both insulators and semiconductors a filled energy band (referred to as the valence band) separated from the next higher band (referred to as the conduction band) by an energy gap. If this gap is at least several electron volts, the material is an insulator. It is too difficult for an applied field to overcome that large an energy gap, and thermal excitations lack the energy to promote sufficient numbers of electrons to the conduction band. MATEEN YAQOOB DEPARTMENT OF COMPUTER SCIENCE

For energy gaps smaller than about 1 electron volt, it is possible for enough electrons to be excited thermally into the conduction band, so that an applied electric field can produce a modest current. The result is a semiconductor. Smaller energy gaps create semiconductors MATEEN YAQOOB DEPARTMENT OF COMPUTER SCIENCE

Holes and Intrinsic Semiconductors When electrons move into the conduction band, they leave behind vacancies in the valence band. These vacancies are called holes. Because holes represent the absence of negative charges, it is useful to think of them as positive charges. Whereas the electrons move in a direction opposite to the applied electric field, the holes move in the direction of the electric field. A semiconductor in which there is a balance between the number of electrons in the conduction band and the number of holes in the valence band is called an intrinsic semiconductor. Examples of intrinsic semiconductors include pure carbon and germanium. MATEEN YAQOOB DEPARTMENT OF COMPUTER SCIENCE

Impurity Semiconductor It is possible to fine-tune a semiconductor’s properties by adding a small amount of another material, called a dopant, to the semiconductor creating what is a called an impurity semiconductor. As an example, silicon has four electrons in its outermost shell (this corresponds to the valence band) and arsenic has five. Thus while four of arsenic’s outer-shell electrons participate in covalent bonding with its nearest neighbors (just as another silicon atom would), the fifth electron is very weakly bound. It takes only about 0.05 eV to move this extra electron into the conduction band. The effect is that adding only a small amount of arsenic to silicon greatly increases the electrical conductivity. MATEEN YAQOOB DEPARTMENT OF COMPUTER SCIENCE

Extra weakly bound valence electron from As lies in an energy level close to the empty conduction band. These levels donate electrons to the conduction band. MATEEN YAQOOB DEPARTMENT OF COMPUTER SCIENCE

n-type Semiconductor The addition of arsenic to silicon creates what is known as an n-type semiconductor (n for negative), because it is the electrons close to the conduction band that will eventually carry electrical current. The new arsenic energy levels just below the conduction band are called donor levels because an electron there is easily donated to the conduction band. MATEEN YAQOOB DEPARTMENT OF COMPUTER SCIENCE

Ga has only three electrons and creates a hole in one of the bonds. As electrons move into the hole the hole moves driving electric current Impurity creates empty energy levels just above the filled valence band MATEEN YAQOOB DEPARTMENT OF COMPUTER SCIENCE

Intrinsic Semiconductor Elemental or pure semiconductors have equal numbers of holes and electrons ◦Depends on temperature, type, and size. Compound Semiconductors can be formed from two (or more) elements (e.g., GaAs) MATEEN YAQOOB DEPARTMENT OF COMPUTER SCIENCE

Extrinsic Semiconductors A pure semiconductors where a small amount of another element is added to replace atoms in the lattice (doping). ◦The aim is to produce an excess of either electrons (n-type) or holes (p-type) ◦Typical doping concentrations are one part in ten million ◦Doping must be uniform throughout the lattice so that charges do not accumulate MATEEN YAQOOB DEPARTMENT OF COMPUTER SCIENCE

N-Type and P-Type One valence electron too many (n-type) ◦Arsenic, antimony, bismuth, phosphorus One valence electron too few (p-type) ◦Aluminum, indium, gallium, boron MATEEN YAQOOB DEPARTMENT OF COMPUTER SCIENCE

At a pn junction holes diffuse from the p side MATEEN YAQOOB DEPARTMENT OF COMPUTER SCIENCE

The PN Junction Diode Start with a P and N type material. Note that there are excess negatives in the n-type and excess positives in the p-type Merge the two – some of the negatives migrate over to the p-type, filling in the holes. The yellow region is called the depletion zone. More positive than rest of N More negative than rest of P MATEEN YAQOOB DEPARTMENT OF COMPUTER SCIENCE

Biasing the Junction Apply a voltage as indicated. The free charge carriers (negative charges in the N material and positive charges in the P material) are attracted to the ends of the crystal. No charge flows across the junction and the depletion zone grows. This is called reverse bias. Switch polarity. Now the negative charges are driven toward the junction in the N material and the positive charges also are driven toward the junction in the P material. The depletion zone shrinks and will disappear if the voltage exceeds a threshold. This is called forward bias. MATEEN YAQOOB DEPARTMENT OF COMPUTER SCIENCE

Diode Circuit Symbols Reverse BiasForward Bias N material (cathode) P material (anode) MATEEN YAQOOB DEPARTMENT OF COMPUTER SCIENCE

Types of Diodes Rectifier Diode ◦Used in power supplies Signal Diode ◦Used in switches, detectors, mixers, etc. Zener Diode ◦Voltage regulation – operated reverse bias in the avalanche region Reference Diode ◦Used like zener for voltage regulation MATEEN YAQOOB DEPARTMENT OF COMPUTER SCIENCE

Rectification Conversion of ac to dc. ◦Many devices (transistors) are unidirectional current devices ◦DC required for proper operation. MATEEN YAQOOB DEPARTMENT OF COMPUTER SCIENCE

Half Wave Rectifier MATEEN YAQOOB DEPARTMENT OF COMPUTER SCIENCE

Full Wave Rectifier MATEEN YAQOOB DEPARTMENT OF COMPUTER SCIENCE

Light Emitting Diodes (LEDs) When forward biased, electrons from the N-type material may recombine with holes in the P-type material. ◦System energy is decreased ◦Excess energy emitted as light ◦Indium gallium nitride (InGaN) semiconductors have been used to make colored LEDs ◦Stop lights ◦Progress toward white LEDs is promising MATEEN YAQOOB DEPARTMENT OF COMPUTER SCIENCE

Laser Diodes N material P material Highly Reflective Mirror Partially Reflective Mirror Used in forward bias. Electrons move into depletion zone and recombine with holes, producing light (like an LED). More electron-hole recombinations can be stimulated by this photon, producing more photons at the same wavelength. The mirrors reflect the photons back and forth through the depletion zone, stimulating more photon at each pass. Eventually, the beam passes out of the right hand mirror. MATEEN YAQOOB DEPARTMENT OF COMPUTER SCIENCE

Laser Diode Application Used as CD and DVD detector Laser Diode Photodiode Also used as bar code readers, laser pointers, fiber optics, etc. MATEEN YAQOOB DEPARTMENT OF COMPUTER SCIENCE

LCD Liquid Crystal Display ◦State between solid and liquid ◦Requires only a little heat to change material into a liquid ◦Used as thermometers or mood rings ◦Electric currents are used to orient the crystals in predictable ways ◦The liquid crystals polarize the light (either internal light source or external) to give light and dark areas on the display MATEEN YAQOOB DEPARTMENT OF COMPUTER SCIENCE