(i) Derivation of drift velocity:
In absence of field, electrons move randomly. On applying field $E$:
Force: $F = eE$
Acceleration: $a = eE/m$
Extra velocity gained in relaxation time $\tau$:
$$v_d = a\tau = \frac{eE\tau}{m}$$

(ii) Effect of temperature:
As temperature increases, lattice vibrations increase, causing more frequent collisions, so $\tau$ decreases.
Since $v_d = eE\tau/m$ and $\tau$ decreases:
$$v_d \downarrow \text{ as } T \uparrow$$
Drift velocity decreases with increase in temperature.