Construction and characteristics of Power Diodes
Characteristics of Power Diode:
- Power Diodes of largest power rating are required to conduct several kilo amps of current in the forward direction with very little power loss while blocking several kilo volts in the reverse direction. Large blocking voltage requires wide depletion layer in order to restrict the maximum electric field strength below the “impact ionization” level.
- Space charge density in the depletion layer should also be low in order to yield a wide depletion layer for a given maximum Electric fields strength.
- These two requirements will be satisfied in a lightly doped p-n junction diode of sufficient width to accommodate the required depletion layer. Such a construction, however, will result in a device with high resistively in the forward direction.
- Consequently, the power loss at the required rated current will be unacceptably high. On the other hand if forward resistance (and hence power loss) is reduced by increasing the doping level, reverse break down voltage will reduce.
- This apparent contradiction in the requirements of a power diode is resolved by introducing a lightly doped “drift layer” of required thickness between two heavily doped p and n layers as shown in Fig below shows the circuit symbol.
Construction of Power Diode:
Fig: Diagram of a power; (a) circuit symbol; (b) schematic cross section.
- A lightly doped n- epitaxial layer of specified width (depending on the required break down voltage) and donor atom density (NdD) is grown on a heavily doped n substrate (NdK donor atoms.Cm -3) which acts as the cathode. Finally the p-n junction is formed by defusing a heavily doped (NaA acceptor atoms.Cm-3) p region into the epitaxial layer. This p type region acts as the anode.
- Impurity atom densities in the heavily doped cathode (Ndk .Cm -3) and anode (NaA.Cm -3) are approximately of the same order of magnitude (10 19 Cm -3) while that of the epitaxial layer (also called the drift region) is lower by several orders of magnitude (NdD ≈ 10 14 Cm-3). In a low power diode this drift region is absent.