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	…(1)
	2πRC

Where, f_α = cut-off frequency of a BJT
h_fe = Gain factor
Capacitance C is given by

C =	∈₀A	…(2)
	d

Where,
ε_e = Free space permittivity
C = Capacitance.
A = Area
d = Depletion width
From the relation (2) 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 choice