In deriving the diode equation we assumed that the voltage applied to the device appears entirely across the junction. Thus we neglected any voltage drop in the neutral regions or at the external contacts.
- For most devices this is a valid assumption; the doping is usually fairly high, so that the resistivity of each neutral region is low, and the area of a typical diode is large compared with its length.
- However, some devices do exhibit ohmic effects, which cause significant deviation from the expected I- V characteristic.
- We can seldom represent ohmic losses in a diode accurately by including a simple resistance in series with the junction.
- The effects of voltage drops outside the transition region are complicated by the fact that the voltage drop depends on the current, which in turn is dictated by the voltage across the junction.
- For example, if we represent the series resistance of the p and n regions by Rp and Rn, respectively, we can write the junction voltage V as
where Va is the external voltage applied to the device.
- As the current increases, there is an increasing voltage drop in Rp and Rn, and the junction voltage V decreases.
- This reduction in V lowers the level of injection so that the current increases more slowly with increased bias.
- A further complication in calculating the ohmic loss is that the conductivity of each neutral region increases with increasing carrier injection.
- Since the effects of the above equ are most pronounced at high injection levels, this conductivity modulation by the injected excess carriers can reduce Rp and Rn significantly.
- Ohmic losses are purposely avoided in properly designed devices by appropriate choices of doping and geometry.
- Therefore, deviations of the current generally appear only for very high currents, outside the normal operating range of the device.
- Figure shows the forward and reverse current-voltage characteristics of a p-n junction on a semilog scale, both for an ideal Shockley diode as well as for non-ideal devices.
- For an ideal forward-biased diode, we get a straight line on a semilog plot reflecting the exponential dependence of current on voltage.
- On the other hand, taking into account all the second order effects, we see various regions of operation.
- At low current levels, we see the enhanced generation-recombination current, leading to a higher diode ideality factor (n = 2).
- For moderate currents, we get ideal low-level injection and diffusion-limited current (n = 1).
- At higher currents, we get high-level injection and n = 2, while at even higher currents, the ohmic drops in the space charge neutral regions become important.