An optical modulator in silicon based on a diode structure has been operated in both forward and reverse bias. This modulator achieves near state-of-the-art performance in both modes, thereby making this device idea for comparing the two modes of operation. In reverse bias, the device has a V[pi]L of 4.9 V-cm and a bandwidth of 26GHz. In forward bias, the device is very sensitive, a V[pi]L a slow as 0.0025 V-cm has been achieved, but the bandwidth is only 100 MHz. A ndw geometyr for a reverse-bias device is proposed, and it is predicted to achieve a V[pi]L of 0.5V.cm.