Recombination and Generation in the Transition Region
In analyzing the p-n junction, we have assumed that recombination and thermal generation of carriers occur primarily in the neutral p and n regions, outside
the transition region. In this model, forward current in the diode is carried by recombination of excess minority carriers injected into each neutral region by the junction.
Recombination and Generation in the Transition Region:
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When a junction is forward biased, the transition region contains excess carriers of both types, which are in transit from one side of the junction to the
other. - Unless the width of the transition region W is very small compared with the carrier diffusion lengths Ln and Lp, significant recombination can take place within W.
- An accurate calculation of this recombination current is complicated by the fact that the recombination rate, which depends on the carrier concentrations, varies with position within the transition region.
- Analysis of the recombination kinetics shows that the current due to recombination within W is proportional to nt and increases with forward bias according to approximately exp(qV/2kT).
- On the other hand, current due to recombination in the neutral regions is proportional pn and np and therefore to ni2/Nd and ni2/Na, and increases according to exp(qV/kT).
- The diode equation can be modified to include this effect by including the parameter n
where n varies between 1 and 2, depending on the material and temperature. Since n determines the departure from the ideal diode characteristic, it is often called the ideality factor.
The ratio of the two currents
becomes small for wide band gap materials, low temperatures (small ni,), and for low voltage. Thus the forward current for low injection in a Si diode is likely to be dominated by recombination in the transition region, while a Ge diode may follow the usual diode equation.
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Just as recombination within W can affect the forward characteristics, the reverse current through a junction can be influenced by carrier generation in
the transition region. - We found that the reverse saturation current can be accounted for by the thermal generation of EHPs within a diffusion length of either side of the transition region.
- The generated minority carriers diffuse to the transition region, where they are swept to the other side of the junction by the electric field (Fig. given below).
- However, carrier generation can take place within the transition region itself.
- If W is small compared with Ln or Lp, band-to-band generation of EHPs within the transition region is not important compared with generation in the neutral regions.
- However, the lack of free carriers within the space charge of the transition region can create a current due to the net generation of carriers by emission from recombination centers.