Q.Why is the transistor called as current controlled device?

A bipolar junction transistor (BJT) cannot amplify, switch, or oscillate on its own — it must first be given steady DC voltages. Transistor biasing is the application of a DC voltage of correct polarity and suitable magnitude across the transistor's terminals so that the device works in the region required for its job: the active region for amplification, or the saturation and cutoff regions for switching. A transistor with no DC source connected to its terminals is an unbiased transistor; it carries no useful current and is never used in practice.

Biasing fixes the transistor's zero-signal DC operating conditions — the base current $I_B$, collector current $I_C$, collector-to-emitter voltage $V_{CE}$ and base-to-emitter voltage $V_{BE}$. Because the output collector current of a BJT is set by the small input base current through $I_C = \beta I_B$, the BJT is a current-controlled device, and the base current is exactly the quantity a biasing circuit establishes.

Good biasing must meet three requirements: place the operating point near the centre of the active region (so an applied signal does not push it into saturation or cutoff), keep $I_C$ stable against temperature changes, and keep the operating point fixed when one transistor is replaced by another of the same type. These ideas underpin the whole of the Karnataka 2nd PUC Electronics chapter on transistor biasing.