Fig. 1. The device structure of the phototransistor with small base size：（a）the schematic diagram of geometric structure；（b）the band diagram.

Fig. 2. The comparison of the experimental and the simulated results：（a）the current gain；（b）the dark current.

Fig. 3. The relative response versus time of the experiment and simulation at 0.8 V（$t_{\mathrm{r}\mathrm{i}\mathrm{s}\mathrm{e}}$ is the time required for the linght response to rise from 10% to 90% of the maximum value of the response）

Fig. 4. The relationship between the gain，the operating bias and the base size：（a）the variation with bias voltage for different base sizes；（b）the variation with base size for different bias voltages.

Fig. 5. The physical field distribution near the base region for different base sizes：（a）the electron density；（b）the electric field（the region of -0.05-0.0 μm is the base，the region larger than 0.0 μm is the collector）

Fig. 6. The current density versus the base size：（a）the dark current；（b）the gain-normalized dark current.

Fig. 7. The SRH generation rate distribution：（a）at the center of device；（b）the whole device.

Fig. 8. The total current density versus the base size for different optical powers at 0.8 V.

Fig. 9. The response time versus the base size for different optical powers at 0.8 V.

Fig. 10. The detectivity versus base size for different optical power at 0.8 V.

Fig. 11. The noise equivalent photon versus base size for different optical power at 0.8 V.