Stephen Hawking communicated using a speech-generating device, which is also called a voice output communication aid (VOCA) or an augmentative and alternative communication (AAC) device. The device generated artificial speech by synthesizing text input typed on a keyboard into spoken words.
Due to his motor neuron disease, Stephen Hawking was almost entirely paralyzed towards the end of his life and could not move any part of his body except for his cheek.