A transistor is used in the circuit to switch on or switch off the LED. If some part of the light emitted at the upper surface of the led enters the bottom surface, then the gold layer reflects the light back to the upward direction, hence the gold film increases the efficiency of the LED. The gold film also acts as a reflective layer in the LED structure. The p-type layer is covered with the thin metallic layer which provides the anode connections, and the N-type region is coated with the thin gold layer, which provides the cathode connection. If the P region is placed below the n region, then the light emitted from the LED may not be clearly observable, which is why the p-type layer is always placed above the n-type region.

The P-layer is deposited above the N layer because the electron-hole pair recombination takes place towards the p region, hence if the p region is the upward surface of the LED, the light (photons) emitted due to the electron-hole pair recombination can be clearly observed from the surface. The N-type region is heavily doped as compared to the P-type region in the LED structure. The middle layer, i.e., the active region, consists of both the holes and electrons. As the name suggests, the P-region consists of a large number of holes, and the N-region consists of a large number of electrons. The top layer is a P-type region, the bottom layer is the n-type region, and the central layer is called the active region of the LED. The LED structure consists of three semiconductor layers deposited over the substrate. Electric Current or Voltage Specifications

How the led works