Question

The diffusion of collector impurities in npn transistor should be small because,

(a) No additional diffusion or masking steps required

(b) Bandwidth is controlled by lateral diffusion of p-type impurity

(c) Collector need not be kept at negative potential

(d) None of the mentioned

This question was addressed to me in an online quiz.

I'd like to ask this question from Active and Passive Components of IC in division IC Fabrication of Linear Integrated Circuits

Answer 1

Answer: (b) Bandwidth is controlled by lateral diffusion of p-type impurity

In npn transistors, the diffusion of collector impurities should be small because the bandwidth of the transistor is directly controlled by the lateral diffusion of p-type (base) impurities. A small collector impurity diffusion ensures minimum diffusion capacitance and maintains high-frequency response capability.