CBSE Class 12 · 2006 · All India · SemiconductorsLong Answer

The base controls the number of majority carriers passing from the emitter to the collector, which makes transistor action possible.
It is made thin and lightly doped so that very few of the injected carriers recombine in the base, allowing most (~95-98%) to reach the collector and give a large collector current.
Circuit: connect the n-p-n transistor in CE configuration with $V_{BB}$ (through a resistor) biasing the base and $V_{CC}$ biasing the collector through $R_C$, with meters for $I_B$, $V_{BE}$, $I_C$, $V_{CE}$.
Input characteristic: $I_B$ versus $V_{BE}$ at fixed $V_{CE}$. Output characteristic: $I_C$ versus $V_{CE}$ for several fixed $I_B$.
From the output characteristics, at a fixed $V_{CE}$,

$\boxed{\beta ={\left(\frac{\Delta I_C}{\Delta I_B}\right)}_{V_{CE}}.}$