The zener breakdown process is intrinsically noisy, and has a white noise spectrum. If you bias a bipolar transistor so that the base junction goes into breakdown, you can get a nice noisy signal. In practice you need a high impedance bias, so that you don't melt the transistor, and you normally need to select the transistor to give nice noise (if you know what I mean). To turn voltage noise into a random digital level, you need to AC couple into a comparitor.