We devised a PAM4 driver IC that can compensate for the nonlinearity of the extinction curve of an electroabsorption modulator (EAM). The driver was designed and fabricated by using our InP HBT technology (ft = 290 GHz, fmax = 320 GHz). The driver can adjust the modulation voltage swing for each intensity level to obtain clear eye openings in the optical domain. The results showed clear eye openings of the optical output waveform when compensating for the nonlinearity. To evaluate the effect of compensating for the nonlinearity quantitatively, the bit error rate (BER) was measured. A minimum receiver sensitivity of -9 dBm was observed when compensating at a BER of 2 × 10-4, while a minimum receiver sensitivity was not observed at the BER without compensating. Our PAM4 driver can provide clear 56-Gb/s optical eye openings compensating for the nonlinearity of the extinction curve of an EAM.