Heterojunction phototransistors (HPTs) with scaling emitters have a higher optical gain compared to HPTs with normal emitters.However,to quantitatively describe the relationship between the emitter-absorber area ratio (Ae/Aa) and the performance of HPTs,and to find the optimum value of Ae/Aa for the geometric structure design,we develop an analytical model for the optical gain of HPTs.Moreover,five devices with different Ae/Aa are fabricated to verify the numerical analysis result.As is expected,the measurement result is in good agreement with the analysis model,both of them confirmed that devices with a smaller Ae/Aa exhibit higher optical gain.The device with area ratio of 0.0625 has the highest optical gain,which is two orders of magnitude larger than that of the device with area ratio of 1 at 3 V.However,the dark current of the device with the area ratio of 0.0625 is forty times higher than that of the device with the area ratio of 1.By calculating the signal-to-noise ratios (SNRs) of the devices,the optimal value of Ae/Aa can be obtained to be 0.16.The device with the area ratio of 0.16 has the maximum SNR.This result can be used for future design principles for high performance HPTs.