The base current (IB) plays a key role in the transistor since its discovery (16 December 1947, Bardeen and
Brattain). It separates the low impedance input (emitter) from the high impedance output (collector), thus yielding a
“transfer resistor.” Recently, III-V semiconductor material has been fabricated as a heterojunction bipolar transistor
(HBT) which can operate as a high speed device. The HBT can be modified and operated as a three-port light-emitting
device (an electrical input, electrical output, and a third port optical output) by incorporating one or more quantum wells
in the base region, thus becoming a heterojunction bipolar light-emitting transistor (LET). In the present work, we have
designed different sizes of emitter diameter DE and base diameter DB of InGaP/GaAs LETs in aperture layout design.
Through different layout designs, the LETs exhibit different electrical current gain (β= IC/IB) and optical light output due to different carrier recombination processes in the transistor base region. By reducing the lateral emitter size from 18 to 13 μm, β increases due to the higher injection current densities and better confinement of the radiative recombination in the base region. Moreover, β increases when reducing the base diameter from 27 to 22 μm with fixed emitter diameter. The effective carrier recombination lifetime, τrec, can be estimated from dc analysis and rf measurement (small-signal modulation).We have obtained multi-GHz spontaneous light modulation of LETs, and the device performance is closely related to different layout designs with different device parasitics.