Let’s revisit the problem of the transistor that we worked on in the last proble

Question

Let’s revisit the problem of the transistor that we worked on in the last problem set. In the previous problem set, we calculated the exact value of the electric field in between two finite, square parallel plates by direct integration. However, in a typical transistor the dimensions of the plates L are much larger than the separation d, so we can effectively treat each plate as infinite (L ? ?), as long as we are interested in the field in the middle of the plates (not near the edges).

(a) Let’s approximate the transistor as two infinite plates, with a positive charge density +? on the top plate and a negative charge density ?? on the bottom plate, separated by a distance d. What is the electric field in between the plates? What is the electric field outside the plates?

(b) In the last problem set, Question 3a, we obtained an expression for the exact value of the electric field precisely in the midpoint between the two plates. Assuming that the typical dimensions of a transistor are L = 5 × 10^?8 m and d = 1.5 × 10^?9 m, compare our approximate expression for the electric field from (a), assuming infinite plates, versus the exact expression from the last problem set, Eapprox/Eexact

Explanation / Answer

here,

Electric field between the plates is constant and = (sigma1/e0 + sigma 2/e0)

where sigma is surface charge density

eo is 8.85 e -12

so

part A: Enet = 0

part B :

Enet out side the plate = signma/2 eo for each plate out

needed sigma from prev set of question

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