"In Figure 1, the band-gap is made by associating transistors Q4 and Q5 together with the emitter resistor R4. The area parameter on both devices indicates that Q4 is “equivalent” to three transistors in parallel whereas Q5 is
made of six paralleled transistors. Otherwise stated, the emitter size of the transistor Q5 is twice that of transistor Q4 given respective area parameters of 6 and 3. Therefore, not only the current densities J in their emitters are
linked (J4 = 6J5 ), but their saturation currents I S is also affected by this relationship..."
The equation 2 is very easy to follow.