The General Electric BD-4  (P/N 152-0070-00) is a 0.1 mA (@90 mV) Germanium back diode. It has a specified rise time of 0.4 ps.

Characteristics of a BD4 back diode

The BD4 is used in the HF-sync circuit of the 7S14 and trigger circuit of the late 5T1A.

Some instruments using part BD4

Instrument Manufacturer Class Model Description Introduced
5T1A Tektronix Plug-in 5T1A Timing plugin 1963
7S14 Tektronix Plug-in 7S14 1 GHz dual-trace, delayed-sweep sampler 1974

Links