Silicon p+-i-n+ diode Mach-Zehnder electrooptic modulators having an ultra-compact length of 100 to 200 µm are presented. These devices exhibit high modulation efficiency, with a Vπ·L figure of merit of 0.36 V-mm. Optical modulation at data rates up to 10 Gb/s is demonstrated with low RF power consumption of only 5 pJ/bit.