Power Diode under Forward Bias Conditions
Forward Bias condition:
Fig: Characteristics of a forward biased power Diode; (a) Excess free carrier density distribution; (b) i-v characteristics.
- The introduction of a lightly doped drift region in the p-n structure of a diode boosts its blocking voltage capacity. It may appear that this lightly doped drift region will offer high resistance during forward conduction. However, the effective resistance of this region in the ON state is much less than the apparent ohmic resistance calculated on the basis of the geometric size and the thermal equilibrium carrier densities.
- This is due to substantial injection of excess carriers from both the p and the n regions in the drift region.
- As the metallurgical p n- junction becomes forward biased there will be injection of excess p type carrier into the n- side. At low level of injections (i.e δp << nno) all excess p type carriers recombine with n type carriers in the n- drift region.
- However at high level of injection (i.e large forward current density) the excess p type carrier density distribution reaches the n- n junction and attracts electron from the n cathode.
- This leads to electron injection into the drift region across the n- n junction with carrier densities δn = δp. This mechanism is called “double injection”.
- Excess p and n type carriers defuse and recombine inside the drift region. If the width of the drift region is less than the diffusion length of carries the spatial distribution of excess carrier density in the drift region will be fairly flat and several orders of magnitude higher than the thermal equilibrium carrier density of this region.
- Conductivity of the drift region will be greatly enhanced as a consequence (also called conductivity modulation). The voltage dropt across a forward conducting power diode has two components
Vak = Vj VRD
Where, Vj is the drop across the p n- junction and can be calculated from the equation above for a given forward current jF.
- The component VRD is due to ohmic drop mostly in the drift region. Detailed calculation shows
VRD ∞ JF WD
Where JF is the forward current density in the diode and WD is the width of the drift region.
Vak = Vj RON IF
The ohmic drop makes the forward i-v characteristic of a power diode more linear.