© 2012 Eric Pop, UIUC ECE 340: Semiconductor Electronics 1
ECE 340 Lecture 37Narrow-base P-N diode
•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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Note: download the “Narrow-Base/BJT”
handout online!
© 2012 Eric Pop, UIUC ECE 340: Semiconductor Electronics 2
• Let’s revisit p-n carrierdistributions:
• Draw “usual” distributions under forward bias (see L23-L24):
• Now if the n-side is shorter than the diffusion length (ℓ < Lp):
© 2012 Eric Pop, UIUC ECE 340: Semiconductor Electronics 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:
/,0( ) px Lnp x p e
,0( ) 1nxp x pL
( ) 0p x
© 2012 Eric Pop, UIUC ECE 340: Semiconductor Electronics 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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