An LED is a p-n junction with a heavily doped n-type semiconductor(n ) and a lightly doped p-type. The device works if it is forward biased. When p- side of the junction is connected to the positive terminal of a battery and the n side to the negative terminal, the barrier hight gets reduced and the carriers diffuse to the other side of the junction. As the p- side is lightly doped, much fewer number of holes cross over to the n-side than the number of electrons which diffuse to the n-side. Recombination takes place in the depletion region (also called the active layer ) and light is emitted. The emission takes place in random direction. If the width of the p-side is made much thinner than the width of the n-side, the emitted light can escape from the device without getting reabsorbed by the material.