Module 6 : LIGHT EMITTING DIODE (LED)
Lecture : LED - I
   
1.3 LED Materials - Direct and Indirect Band Gaps :
  Various dopants are added to a p-n junction of GaAs to get emission over a wide range of colours. GaAs is preferred over semiconductors like Si and Ge as GaAs is a direct bandgap material.
 

In a direct bandgap material, the bottom of the conduction band lies directly above the top of the valence band. The energy of electrons in the conduction band (with the zero of energy being at the top of the valence band) is

$\displaystyle E_n = \Delta + \frac{p_n^2}{2m} = \Delta + \frac{\hbar^2k^2}{2m_n^\ast}$

where $ k = 2\pi/\lambda$is the wave vector. The energy of holes in the valence band is given by

$\displaystyle E_p = \frac{\hbar^2k^2}{2m_h^\ast}$

\includegraphics{led18a.eps}

GaAs is a direct band gap material with $ \Delta= 1.42$eV. The band structure of GaAs is shown in the figure. Other direct band gap materials are InP, InSb, ZnS, ZnSe etc.

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