Electronic & Semiconductors Devices

Electronic & Semiconductors Devices

• SEMICONDUCTOR
  
  A semiconductor is a substance which has almost filled valence band and nearly empty conduction band with a small energy (= 1ev) separating the two.
  A pure semiconductor is called intrinsic semiconductor. It has thermally generated current carriers.
  (i) In an intrinsic semiconductor, the number of free electrons (in conduction band) is exactly equal to the number of hole (in valence band)
  n_e = n_n = n_i
  
  where n_e = number density of free electrons in conduction band
  n_h = number density of holes in valence band
  n_i = number density of intrinsic carriers (free electrons or holes).
  
  (ii) The number of free electrons or holes in an intrinsic semiconductor is given by;
  
  n_e = n_h = AT^3/2
  where A = constant independent of temperature
  
  k = Boltzmann constant
  
  E_g = energy gap of 0 K
  
  T = absolute temperature

• n- type semiconductor
  
  When a small amount of pentavalent impurity is added to a pure semiconductor, it is known as n-type semiconductor. Energy band description.
  
  For an n-type semiconductor
  
  n_e n_h =
• p- type semiconductor
  When a small amount of trivalent impurity is added to a pure semiconductor, it is known as p-type semiconducto.
  
• Electrical Conductivity of Semiconductor
  
  Current density, J =
• Intrinsic semiconductor: In an intrinsic semiconductor, the number of electrons is equal to the number of holes i.e. n_e = n_h = n_i (say).
  Electrical conductivity = n_ie (_e + _h)
  
  (1)A practical p - type semiconductor Now nhis approximately equal to Na( =number density of acceptor atoms).
  
  (2)A practical n-type semiconductor Now neis approximately equal to N_d( =number density of donor atoms). Therefore,
  
  Resistivity =
  
  Electrical conductivity, = e n_ee= eN_de.
• When external voltage applied to the junction is in such a direction that it cancels the potential barrier, thus permitting current flow, it is called forward biasing.
• When the external voltage applied to the junction is in such a direction that potential barrier, is increased, it is called reverse biasing
• Breakdown voltage is the reverse voltage at which pn junction breaks down with sudden rise in reverse current.
• Knee voltage is the forward voltage at which the current through the junction starts to increase rapidly.
• When PN junction diode rectifies half of the ac wave, it is called half wave rectifier.
• Full wave rectifier; rectifies both halves of ac input signal.
  
  
  Transistors
  
  
  • Transistor: The name of this electric device is derived from it's fundamental action transfer resistor.
    It is consists of three main regions.
    
    Emitter: It is more heavily doped than any of the other regions because its main function is to supply majority charge carriers to the base.
    
    Base: It forms the middle layer; it is very thin and very lightly doped.
    
    Collector:Its main function is to collect majority charge carriers. This region is made larger than the emitter because it has to dissipate much greater power.
  • Transistor Biasing :
    For the normal operation of a transistor;
    1.Emitter-base junction is always forward biased and,
    2.Collector base junction is always reverse biased.
  • Transistor as an amplifier :
    A device which increases the amplitude of the input signal is called amplifier.
    
    Performance of Transistor Amplifier
    
    The performance of a transistor amplifier depends upon input resistance, output resistance, effective collector load, current gain, voltage gain and power gain. As common emitter connection is universally adopted, therefore, we shall explain these terms with reference to this mode of connection.
    
    (1) Output characteristics: It is the curve between collector current I_C and collector-emitter voltage V_CE at constant base current I_B.
    
    (2) Output resistance: It is the ratio of change in collector-emitter voltage (V_CE) to the change in collector current (I_C) at constant I_B i.e.
    
    Output resistance, at constant I_B.
    
    (3) Effective collector load : It is the total load as seen by the a.c. collector current.
    In case of a single stage amplifier, the effective collector load is the parallel combination of R_C and R₀.
    
    Effective collector load, R_AC = R_C || R₀
    =
    
    
    It follows, therefore, that for a single stage amplifier, effective load is equal to collector resistance R_C.
    
    (4) Current gain : It is the ratio of change in collector current (I_C) to the change in base current (I_B) i.e.
    
    
    
    The value of ranges from 20 to 500.
    
    
    
    (5) Voltage gain: It is the ratio of change in output voltage (V_CE) to the change in input voltage (V_BE) i.e.
    
    Voltage gain A_V =
    
    =
    
    =
    For single stage, R_AC = R_C. However, for multistage, where R_i is the input resistance of the next stage
    Note. R_AC/ R_i is called resistance gain
    
    Voltage gain, A_V = b × resistance gain.
    
    
  • Relation between and :
    
    We have and a =
    
    But, I_B = I_E - I_C
    
    or,
    
    or,
    
    And