• Presence of two heavily doped n-type regions (n+) in the p-type material
  Source and drain

• n-channel MOS (NMOS) transistor
  (Positive value of V_G and V_D)
  More frequently used
              Higher mobility of electrons - fast response
  
• Similar arrangement using an n-type substrate: p-channel MOS (PMOS) transistor
  (Negative value of V_G and V_D)
  More easily fabricated than NMOS

NMOS Operation:

• Drain region with the surrounding substrate form p-n junction
  Source grounded: V_S = 0
  V_D has a small positive value

• V_G = 0
  i_D ~ 0
  p-n junction reversed biased

• V_G very small
  Region 'R' is depleted
  p-n junction reversed biased
  i_D ~ 0

• V_G increased
  Region 'R' is inverted
  Layer containing mobile electrons is formed
       Inversion layer (channel)
      Connects the drain to the source
       i_D > 0

• Threshold voltage V_T ... smallest value of V_G necessary to produce a channel
  Usually V_T corresponds to the value of V_G needed for i_D = 1 A
  (may range from a fraction of a volt to several volts)