| Detectors with low noise level and high signalto-noise ratio?SNR?have been widely used in weak optical signal detection fields,such as telecommunication,remote sensing,astronomical observation,medical imaging,homeland security and non-destructive material evaluation.Today,the mainstream detector technologies for weak optical signal detection are still focused on positive-intrinsic-negative photodiodes?PIN-PDs?and avalanche photodetectors?APDs?.Heterojunction phototransistors?HPTs?are detectors that can provide large internal amplification without avalanche noise and operate in linear mode at low bias voltage,compared to mainstream PIN-PDs and APDs.In addition,HPTs have the same layer structure as heterojunction bipolar transistors.The intrinsic gain of the transistor can amplify the signal without interconnects.Accordingly,HPTs can be widely used in the detection of weak optical signals and have the potential to overtake PIN-PDs and APDs in certain fields.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/Ab?and the performance of HPTs,and to find the optimum value of Ae/Ab for the geometric structure design,we develop an analytical model for the optical gain of HPTs.Moreover,five devices with different Ae/Ab 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/Ab 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 at3 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/Ab 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.Further,in order to reduce the dark current level of the device,we investigated the effect of su-8 as a passivation layer on the photodetector performance.Considering the simple structure of Ga Sb photodiode,and compared with other materials of diode devices,the dark current is larger.Here,we measured the dark currents of devices with different sizes of mesa area and thicknesses of passivation layer,found that the thickness of passivation layer is a key factor for the performance of Ga Sb based devices.And we have found the extrema?minimum?of the curve of dark current density at 3.01?m.The dark current density of the optimum passivated diode is 9.80×10-6 A/cm2?at-0.3 V bias?and the calculated surface resistivity based on dynamic resistance-area product?R0A?of the passivated diode is 854?·cm,which are about twentyfold decrease and sixfold increase than that for unpassivated diodes at 77 K,respectively.The calculated quantum efficiency is 36.3%for the passivated device,whereas the value for the unpassivated device is 19.2%.However,the passivated devices?with thicker or thinner passivation layer?unexpectedly show higher dark current densities than that of the unpassivated devices,which can demonstrate that the SU-8 passivation layer does introduce stresses,and the stresses could aggravate the surface leakage current.Thus,according to the results,the importance of the passivation layer on the performance of Ga Sb based material was confirmed,and the stress introduced by the passivation layer should be carefully treated during the device fabrication process.Finally,we apply the passivation method to HPT based on In P.After passivation,the dark current of the device BC junction is reduced by 61.2%,and the quantum efficiency is increased by 58.1%.For HPT devices with the smallest Ae/Ab,the dark current density decreased by an order of magnitude and the gain increased by 34%.For HPT devices with the highest Ae/Ab,the signal-to-noise ratio of the devices was increased by two times compared with that before passivation.It can be seen that the SU-8 passivation layer can reduce the dark current of the BC junction and improve its quantum efficiency by improving the surface condition of the device.On the other hand,the gain and signal-to-noise ratio of the device can be improved by reducing the surface recombination coefficient.This proves the feasibility of SU-8 as passivation layer again and provides a method for surface treatment and passivation of other photodetectors. |
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