diode    A  diode  operates  as a current rectifier, that is it allows a much larger current to flow in one direction than in the other.  It  can be either a vacuum tube (see under vacuum tubes) or  a semiconductor device. A semiconductor diode, consists of a n-type region adjacent to p-type region, with current leads. p and n regions  have  high  concentrations mobile holes and electrons respectively.  When they are joined, holes will diffuse to the n region  and  electrons  will  diffuse  to the p region. This net charge  transfer produces a potential barrier V_B, which gives rise to  drift  currents  of  holes  and  electrons  in  the  opposite direction. Finally a steady state is reached when

i._e,diffussion = -i_{e, drift}

and i_{h, diffusion} = -i_{h, drift}

If  a  small  potential  V,  is  applied  to the junction with the positive  potential connected to p (forward bias), then junction potential  barrier  V_B is reduced to V_B - V. When V exceeds V_B, the electrons and the holes overcome the barrier potential V_B. This results in the flow of current across the barrier.

With  the  reverse  potential  -V,  the barrier potential is V_B+V.  The  mobile charges are extracted away from the junction. Therefore  in  the reverse bias condition current due to majority carriers  does  not  flow.  However  a  small amount of minority carriers generated by thermal energy flow, giving rise to reverse saturation  current,  I_s.  I_s is strongly temperature dependent. At  about  100^oC, the value of I_s becomes too large for germanium device  to function. This temperature is approximately 200^oC for silicon devices.

The voltage current equation for p-n junction diode is

I = I_s [ exp(eV/kT) -1]

where  k  is  the  Boltzmann  constant,  and  T  the absolute temperature. A practical V-i curve for silicon diode is shown in fig.d12, alongwith the symbol of a diode.

Zener  diode:  Beyond  a limiting value of reverse voltage, the reverse  saturation  current increases abruptly. This voltage at which  breakdown occurs is stable and is reversible. It does not damage  the  device  if the power loss in the diode is limited to the  rating.  The  voltage  remains constant for current ranging from  the knee  of  the breakdown to the maximum established by power  limit  of the diode. Therefore it can be used in circuits to  maintain  a constant voltage drop. Diodes specially designed for  this  purpose  are  called  Zener  diodes.  By  varying the impurity  concentration,  diodes  with  voltage  rating from three to several hundred volts are being manufactured.

Photo diode : If a diode is made very thin and transparent, then light  photons can reach  transition region. If the photon has sufficient  energy  it  can  break the covalent bonds producing electron-hole pairs. If the device is reverse biased, then these carriers  will  cause a current called photocurrent. Solar cells are similar devices.

Light  emitting  diodes : In a forward biased p-n junction, the electrons  and  holes recombine after crossing the junction. If the  gap energy is large, these recombinations will give rise to photons in the visible region. For example a diode made of GaP, has an energy gap of 2.26 eV and radiates green light.