Ultrahigh-speed compact flash analog-to-digital converters (ADCs) using resonant-tunneling diodes (RTDs) have been designed to demonstrate a high potential of RTD circuits. Novel multi-input subtraction gates are introduced to the encoder to obtain a compact circuit configuration. By assuming 0.1-µm InP-based RTD/HEMT technology, circuit simulations of 4-bit 10-GHz flash ADCs are carried out. It is found that the device counts of the ADC with an 8-input gate are one third that of the ADC with 4-input gates. This leads to a reduction in the power dissipation by 50%. In addition, bandwidths of more than 20 GHz have been obtained for 4-bit and 5-bit ADCs at a sampling frequency of 10 GHz.