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ECE 340 Lecture 37 Narrow-base P-N diode

ECE 340 Lecture 37 Narrow-base P-N diode. Note: download the “Narrow-Base/BJT” handout online!. One last touch-up before we get to bipolar transistors Let’s recall some math: What is a typical minority carrier diffusion length in Si? How does it compare to modern device lengths?.

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ECE 340 Lecture 37 Narrow-base P-N diode

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  1. ECE 340 Lecture 37Narrow-base P-N diode Note: download the “Narrow-Base/BJT” handout online! • One last touch-up before we get to bipolar transistors • Let’s recall some math: • What is a typical minority carrier diffusion length in Si? • How does it compare to modern device lengths? (what if 0 < x << 1?)

  2. Let’s revisit p-n carrier distributions: • Draw “usual” distributions under forward bias (see L23-L24): • Now if the n-side is shorter than the diffusion length (ℓ < Lp):

  3. Remember, the (metal) contacts at the ends of the p-n junction can be thought of as infinite source/sink of carriers • So instead of the “long” (ℓ > Lp) exponentially decaying: • We have the “narrow” or “short” ℓ < Lp linear approximation: • What is the physical meaning of the diffusion length Lp? • Note the diode is now too narrow (short) for any hole recombination in the n-region. So, all recombination happens at the contact which sets a boundary condition for our excess minority carrier concentration:

  4. Total injected (stored) minority charge at forward bias is the area under the “triangle”: • Easy to write the hole diffusion current for “narrow” diode: • Compare with “long” diode hole diffusion current from L23: • Total diode current if: • It’s a p+/n (NA >> ND) diode • It’s a p/n (NA ~ ND) diode

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