α cut-off frequency of a bipolar junction transistor increases

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α cut-off frequency of a bipolar junction transistor increases with the—

1. Increase in base width
2. Increase in emitter width
3. Increase in the collector width
4. Decrease in the base width

Correct Option: A

We know that,

f_α = f_T = h_fe.	1	…(i)
	2πRC

where, f_α = cut-off frequency of a BJT.
h_fe = Gain factor
Capacitance C is given as

C =	∈₀A	…(ii)
	d

Where
∈₀ = free space permittivity
C = capacitance.
A = Area
d = deplection width
from the relation (ii) it is clear that with increase in base width C decreases which increases the f_α i.e. α-cut off frequency.
Hence alternative (A) is the correct answer.