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INTRINSIC AND EXTRINSIC SEMICONDUCTORS, :star: When lattice defects or…

- - - - Conductivity
  - - - p-type
      - n-type
    - - T > 0K
        
        Some electrons in VB receive enough thermal energy to be excited across band gap and into CB, leaving behind holes.
        
        Adding heat (even at room temperature, T~300K) allows some bonds to break and produces free electrons ie. allows flow of electrons.
        
        Produces electron-hole pairs (EHP): when CB electron and VB hole are created by excitation of VB electron to CB.
        
        Can act as a conductor (allows current flow).
        
        As T increases, greater probability that an energy level in CB is occupied with electrons.:
        
        An equal probability that hole is present in VB.
        
        In a steady state whereby carrier concentration is maintained, a recombination of EHPs occur.
        
        As T increases, number of free electrons and holes created increases exponentially.
        
        Temperature influences conductivity of semiconductor.
      - T = 0K
        
        All electrons are in valence band (VB) because there is not enough thermal energy to excite electrons to conduction band (CB).
        
        CB is empty.
        
        Exists as an insulator (no current flow).
        
        All electrons are tightly shared with neighboring atoms eg. Si or Ge.
        
        All electrons are bound in covalent bonds thus no free charge carriers available for conduction / current flow.
      - :star: Equal charge carriers n = p , where n: number of electrons in CB [m^-3] and p: number of holes in VB [m^-3]
        :star: Energy gap Eg controls conductivity.
    - - :<3: Drift : Charge particle motion under the influence of an electric field
        :<3: Diffusion : Particle motion due to concentration or temperature gradient.
        :<3: Recombination : Electron in CB recombine with hole in VB hence elimination of EHPs (corresponds with Generation : Electron in VB jumps to CB, leaving a hole behind hence generation of EHPs).
  - - - :star: They are known as extrinsic semiconductors with alterations to the original Fermi level of the materials. :star: The Matthiesen rule is applicable, in which the total resistivity of the material is now inclusive due to electron scattering from impurity atoms.
      - GROUP 3 AS DOPANTS
        
        :star: This will yield p-type semiconductors which has a smaller band gap, hence lesser energy is required.
        :star: Its new Fermi level is closer to the valence band and is known as the acceptor level due to its capability of accepting excited electrons, thus creating holes in the valence band.
        :star: Carrier type : holes
      - GROUP 5 AS DOPANTS
        
        :star: This will yield n-type semiconductors which has a smaller band gap, hence lesser energy is required.
        :star: Its new Fermi level is closer to conduction band and is known as the donor level due to its capability of donating electrons into the conduction band.
        :star: Carrier type : electrons
      - MODERATE LEVEL OF DOPING
        
        :star: This yields a non-degenerate semiconductor in which the dopant atoms are well separated from each other.
        :star: This forms discrete energy levels either atop the valence band or below the conduction band.
      - HIGH LEVEL OF DOPING
        
        :star: This yields a degenerate semiconductor.
        :star: This forms the impurity levels (donor/acceptor) within the either the conduction band or the valence band.
      - INTERFACE BETWEEN N-TYPE AND P-TYPE SEMICONDUCTORS
        
        :star: This yields a pn-junction
        :star: This will cause the system to not be in equilibrium.
        :star: The formation of depletion region takes place in order for the system to reach equilibrium, in which excess electrons of n-region diffuse across the junction and cancel out the holes of the p-region. This leads to the n-region being positively charged and the p-region negatively charged.
        :star: Creation of barrier voltage causes carrier drift in the opposite direction to the carrier diffusion.
        :star: Process continues until carrier drift = carrier diffusion
        :star: By the end, an unbiased diode is obtained.
      - SUPPLYING EXTERNAL ENERGY TO THE PN-JUNCTION
        
        :star: This yields either a forward biased or reverse biased diode
        
        NEGATIVE VOLTAGE ON N-SIDE
        
        :star: This yields a forward biased diode.
        :star: This setup promotes diffusion.
        :star: Barrier potential is reduced leading to easier current flow.
        
        NEGATIVE VOLTAGE ON P-SIDE
        
        :star: This yields a reverse biased diode.
        :star: This setup restricts diffusion.
        :star: Barrier potential is increased leading to a high resistive path which then prevents current flow.
    - - :star: Also known as pure/intrinsic semiconductor.
        :star: Number of holes,h is the same a number of electrons; n=p.
        :star: occur naturally in some material, i.e: Si and Ge
        :star: can also prepared by alloying different elemens, i.e: GaAs and LnSb
        
        ELECTRICAL CONDUCTIVITY
        
        :star: Current conductivity is influenced by the electron density in the conduction band
        :star: At temperature, T > 0K, there will be probability of electrons escaping from lattice due to excitation. Thus, current flow can be observed when voltage is applied across the semiconductor.
      - PREPARATION OF SEMICONDUCTOR
        
        :star: Intrinsic semiconductor can be prepared through the bulk crystal growth method.
        :star: The aim of bulk crystal growth is to obtain a large single crystal boule with as few defects as possible.
        
        :sparkles: The raw material must be refined before it can be used for crystal growth.
        :sparkles: The refinement process includes
        
        Some crystal growth technique that can produce intrinsic semiconductors are
        
        Bridgman-Stickbarger Technique
        
        Float Zone Crystal Growth
        
        Liquid Encapsulated Czochralski (LEC) Method
        
        Czochralski(CZ) Method
        
        Reduction with carbon
        
        Purification
        
        Fractioning
        
        1 more item...
- - - - 1) High-electron-mobility transistor (HEMT)
        
        2) Heterojunction bi-polar transistors (HBT)
        
        3) Multi-Quantum Well Lasers
    - - 1) Infrared Light Emitting Diodes (IR-LED)
        
        2) Conductometric Gas Sensor
        
        3) Optical Gas Sensor
  - - - Group IV alloys:
        SiGe, SiGeC, GeSn, SiGeSn, GeC
- - - - :star: As T increases, more donor electrons jump the Ed gap.
        :star: Eventually all impurities enter CB (donor exhaustion state).
        
        :star: At high T, intrinsic semiconductor becomes important.
- - - - :<3: The place where the fourth electron should be: hole
        
        :<3: At T = 0K, impurity level are empty of electrons.
        :<3: At low temperature(T~50K), enough thermal energy is available to excite electrons from valence to impurity level, leaving holes in VB.
        
        :<3: Since impurity level "accepts" electrons from VB, it is called acceptor level.
        
        :<3: At room temperature, semiconductor have a hole concentration, p0 much greater than electron concentration, n0 in CB.
- - - - :<3: Not tightly bounded with host atom, extra electron only need small amount of energy, Ed to excite it into CB.
        
        :<3: Ed now controls conductivity, compared to Eg previously.
        
        :<3: At room temperature, semiconductor have significant number of donors n0>>(ni,p0)