Basic experimental results obtained with a GaAs/AlGaAs double‐heterojunction bipolar transistor (DHBT) waveguide structure carrier‐injected optical intensity modulator/switch are fully described. Typical common‐emitter dc current gain obtained with a 0.25‐&mgr;m‐thick base/waveguide was ∼40, and the switching times have been determined as 1.5 ns. Optical on/off modulation ratios of up to 2.1:1 have been demonstrated at an input base current of as small as 3 mA for a device with emitter/waveguide width of 7 &mgr;m and length of 190 &mgr;m. Design and analysis of the optical switching characteristics expected for a reflection‐type X‐crossing optical switch of DHBT waveguide structure are also presented on the basis of these experimental results.