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Abstract
This paper reports a series of novel photodetectors based on one-dimensional array of metal-oxide-semiconductor field-effect transistors (MOSFETs), which were fabricated using the standard 0.8-µm complementary metal oxide semiconductor (CMOS) process. Normally, the metal fingers of MOSFET must be manufactured above active region in standard CMOS process, causing MOSFET insensitive to light. The proposed photodetectors use the metal fingers of MOSFETs in a one-dimensional array to form periodical slit structures, which make the transmittance of incident light higher, due to the surface plasmons (SPs) resonance effect. The number of parallel MOSFETs in one-dimensional array is 3, 5, 7, 9 and 11. The experimental results show that all responsivities (Rv) are greater than 103 A/W within visible and near-infrared spectra under room temperature and a maximum value of 1.40 × 105 A/W is achieved, which is at least one order of magnitude larger than those of published photodetectors. Furthermore, a minimum noise equivalent power (NEP) of 5.86 fW/Hz0.5 at 30 Hz and a maximum detectivity (D*) of 2.21 × 1013 Jones are obtained. The photodetectors still have good signal-to-noise ratio when the bandwidth is 1 GHz. At the same time, the optical scanning imaging was completed by utilizing the photodetectors. This combination of high Rv, excellent NEP, high speed and broad spectrum range photodetectors will be widely used in imaging systems.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
In recent years, the photodetectors, which are the essential component of optical receivers, imagers and other systems, have been researched extensively with the development in optical field [1,2]. In order to achieve high-performance photodetectors, some applications have already implemented photodetectors using dedicated semiconductor technologies like GaAs, InP, and InGaAs [3–6] or new materials such as photoelectric crystals [7–9] and two-dimensional (2D) materials [10–12]. However, there is still a tremendous need for a low cost solution for industrial photodetectors, imagers and receivers [13–16]. The complementary metal oxide semiconductor (CMOS) technology provides the opportunity of monolithic integration of the photodetectors with other processing circuitry, including transimpedance amplifiers, low noise amplifiers, mixers, clock and analog to digital conversion circuit, etc. The cost of silicon (Si) materials is very low when they are fabricated, and the standard Si process has an extremely mature design method. Therefore, the Si-based CMOS technology dominates in industrial applications owing to its lower cost and power consumption, smaller size and easier integration.
At present, Si-based CMOS photodetectors are based on photodiode or metal-oxide-semiconductor field-effect transistor (MOSFET), which absorb photons through PN junctions and separate electron hole pairs [17]. Photoexcited electron-hole pairs are separated by a built-in electric field at the PN junctions, the carriers diffuse in the surface layer and are collected by two terminal electrodes. The current/voltage difference detected by the two electrodes is then used to determine the power of light [16]. Generally, the CMOS integrated circuits are based on MOSFETs, hence integration with photodiodes will increase the circuit area and design complexity. However, due to the limitation of standard CMOS process, the metal fingers on top of MOSFET active regions cannot be removed, resulting in a large amount of incident light reflected.
In this work, high-performance photodetectors based on one-dimensional MOSFET arrays were proposed and fabricated by employing Central Semiconductor Manufacturing Corporation (CSMC) 0.8-µm standard CMOS process for the first time. The number of parallel MOSFETs m is 3, 5, 7, 9 and 11. The proposed photodetectors are uniform arrays of MOSFETs with identical size. The metal fingers of MOSFETs in the arrays, which cannot be removed, can form a periodical slit structure. The period of the slits in the arrays is comparable to the wavelength of the incident light, allowing the light to be transmitted effectively, due to the surface plasmons (SPs) resonance effect. The phenomenon of extraordinary optical transmission (EOT) occurs in the periodic slit structure, which makes the transmittance of incident light greater than the percentage area of slits in the whole structure. This shows that even the incident light arrived at the metal fingers can be transmitted. This greatly improves the performance of the photodetectors, and provides responsivities (Rv) that are one order of magnitude larger than those for existing photodetectors.
2. Device structure and principles
The schematic diagram for the one-dimensional MOSFET array photodetector is shown in Fig. 1(a) (taking 7 MOSFETs as an example). The channels of MOSFETs are formed on silicon by doping. There is gate oxide layers with a thickness of 20 nm between the polysilicon gates (thickness of 320 nm) and the channels. The channels on both sides of gates are connected with upper metals (thickness of 430 nm) through vias with a thickness of h = 1.3 µm, forming the source and drain electrodes. Finally, multiple MOSFETs are connected in parallel to the top pad. The dielectric layer with thicknesses of about 8 µm and 9.5 µm were fabricated on top of the source/drain electrodes and gate electrodes, respectively (not shown in the structure). It is noticed that this dielectric layer was essential during the fabrication process by CSMC, which would reduce the transmissivity of incident light. Therefore, the dielectric layers can be removed in the post processing to further enhance the performance of photodetectors. As shown in Fig. 1(a), the single finger length l and width w are 2 µm and 24 µm, and the interval between any two fingers in the MOSFET arrays is 500 nm. Figure 1(b) shows the optical micrograph of the photodetectors when the number of parallel MOSFETs m is 3, 5, 9 and 11 (the 7 parallel MOSFETs structure is not shown here), whose length of the parallel MOSFETs is less than 100 µm.
Fig. 1. One-dimensional MOSFET array photodetectors. (a) Schematic diagrams; (b) Optical micrograph; (c) Equivalent circuit diagram.
Figure 1(c) shows the equivalent circuit diagram of the photodetectors. When MOSFETs are radiated by incident light, the electron-hole pairs in the depletion layer will be separated, leading to an increase in the electron concentration in the channel. This phenomenon is electrically equivalent to adding an additional voltage Ua to the gate. As shown in Fig. 1(c), each MOSFET is uncoupled electronically since the MOSFETs are isolated from each other. Therefore, the photocurrent I generated by each of the MOSFETs is independent in the arrays. During the test, the incident light was uniform and the spot size was much larger than the photodetectors area. Meanwhile, each MOSFET in the arrays has identical gate bias and common source and drain, ensuring each MOSFET in the arrays receives identical radiation and generates identical photocurrent I. The sum of the photocurrents generated by each individual MOSFET is represented by the total output photocurrent Itotal. By connecting more MOSFETs in parallel, a higher photocurrent Itotal is obtained. However, the responsivity (Rv) cannot be increased, due to the increase in photodetectors area.
The fingers of MOSFETs in one-dimensional arrays can form a periodic array of slits with comparable size to the wavelength of the incident light, allowing the light to be transmitted effectively. The EOT occurs of the light is realized by SPs resonance and cavity resonance mechanisms [18]. The periodic slit structure could excite the SPs resonance and enhanced the fields related to the evanescent waves. Since SPs on the upper and lower surfaces can interact through slits, tunneling will occur through the resonance when the finger thickness is thin enough. In addition, The propagating guided waves in the slits can also couple the SPs on the upper and lower surfaces. Cavity resonance is the Fabry Perot resonance, which is the fundamental TM guided wave in the slits and can exist in any slit [19,20]. Then, the phenomenon of EOT occurs in the periodic slit structure [21,22]. In other words, the actual transmissivity can be much higher than anticipation by the classical diffraction theory, and greater than the percentage of slit area in the whole structure [23]. Figure 2 shows the simulated transmissivity for the periodic array of slits under different incident wavelengths, simulated by the commercial Comsol Multiphysics software. It is clearly seen that the transmissivity at most wavelengths is much higher than the percentage of slits in the area of the whole structure, which is 14.29%. This shows that the structure is effective for EOT. The great difference in transmissivity for different wavelengths in the figure is caused by the stack of the dielectric layers in the 0.8-µm standard CMOS process.
Fig. 2. The simulated transmissivity of periodic array of slits formed by MOSFET fingers at different incident wavelengths.
3. Measurement results
3.1 Electrical characteristics
Figure 3 (a) shows the electrical output characteristic curves for various gate voltages with m = 5. With the constant increase of source-drain voltage (Vds), the photodetector firstly enters the triode region, then gets to the saturation region, and finally gets into the breakdown region. Moreover, the source-drain current (Ids) increases as the gate voltage (Vg) increases. The results imply that the parallel MOSFET photodetector still has good electrical characteristics and can effectively realize the amplification function. Figure 3 (b) shows the electrical output characteristic curves of the photodetectors for various numbers of MOSFETs at Vg = 1.5 V. All photodetectors in the figure have good electrical characteristics. At the same time, Ids increases linearly with the increase in the number of parallel MOSFETs. This verifies that each MOSFET in the parallel is independent of each other.
Fig. 3. (a) Electrical output characteristic curves of the photodetectors as a function of the source-drain voltage for various gate voltages with the number of MOSFETs m = 5; (b) Electrical output characteristic curves of the photodetectors as a function of the source-drain voltage for various number of MOSFETs at Vg = 1.5 V
3.2 Optical characteristics
A continuous optical radiation was generated by a single LED automatic light source, which was controlled by the laser controller (CEL-LED35, Ceaulight). The device has various wavelengths and adjustable power intensity. The diameter of the parallel radiated light is 8 mm, which is much larger than the size of the photodetectors. A sourcemeter (Agilent B2911A) is utilized to supply Vds and also detect the Ids for the photodetectors packaged on the printed circuit board (PCB). Finally, the computer records the response signal. In addition, a power supply provides a stable Vg, and a laser power meter (LP10, Sanwa) measured the power of the source, which are not shown in the Figure 4.
Figure 5(a) shows the on-off photocurrent characteristics for different numbers of MOSFETs. Here, the photocurrent current ΔI is defined as the difference between the on and off currents. It can be clearly seen from the results that ΔI is insignificant with m = 3, since the number of fingers is too small to form a periodic slit array and the EOT cannot be realized. When the number m increases, ΔI basically increases linearly with m because each MOSFET in the array is independent of each other. It is also verified that the proposed structure formed by fingers of parallel MOSFET array can effectively improve the light transmissivity, thereby improving the performance of the photodetectors. The responsivities (Rv) under the illumination of the incident laser with various power intensity of 0.24, 0.53, 1.77, 2.62, 3.84, 7.58 and 11.44 mW/cm2 are shown in Fig. 5(b). With the increase in incident power intensity, ΔI of the photodetector gradually saturates. This is because the high light power will increase the depth of the depletion layer and the incident light will pass through the thicker silicon material, resulting in more loss and a reduction in photocurrent. Hence, Rv of the photodetector gradually decreases.
Fig. 5. Photodetection performance of the photodetectors under 590 nm laser illumination. (a) Photoresponse behaviors with different numbers of MOSFETs at Vg = 1.5 V and Vds = 700 mV. (b) Rv with different power intensity at Vg = 1.5 V, Vds = 700 mV and m = 5.
Figure 6(a) shows the gate-voltage-dependent photocurrents under 590 nm laser illumination with the incident power intensity of 0.24 mW/cm2. The ΔI increases as Vg increases, since the gate voltage can affect the carrier concentration in the channel. With the increase in transconductance, ΔI also increases. But excessive Vg will reduce the amount of charges in the depletion region, resulting in the decrease in the number of electron-hole pairs separated by the incident light. Meanwhile, large values of Vg will increase the thickness of the inversion layer, as well as increase the penetration depth of the light and the incident light show more loss. It will result in a reduction in ΔI. As shown in the figure, ΔI decreases greatly when Vg is 3 or 4 V. Figure 6(b) shows the variation of ΔI when Vds increases from 0 to 4 V. At the beginning, ΔI increases with the increase in Vds. When Vds is large enough, the MOSFETs enter the saturation region. At this time, Vds increases and ΔI is basically unchanged. In applications, on the premise of ensuring that the MOSFETs work in the saturation region, Vds should be as small as possible, so that the lower power consumption and low noise will be obtained. Figure 6(c) shows the measured Rv of the photodetectors under different incident wavelengths. From results, a maximum Rv of 1.40 × 105 A/W is achieved at 590 nm. Excluding m is equal to 3, the Rv of other photodetectors show little difference, which again verifies that the structure can achieve EOT, and the MOSFETs are independent of each other in the array. The Rv of all photodetectors are shown in Table S2.
Fig. 6. Photodetection performance of the photodetectors under incident power intensity of 0.24 mW/cm2 (a) ΔI vs Vg under Vds = 700 mV and m = 5. (b) ΔI vs Vds under Vg = 1.5 V and m = 5. (c) Rv against different wavelength of the incident ligh at Vg = 1.5 V and Vds = 700 mV.
In addition to the Rv, NEP is also one of the important indicators to evaluate the performance of the photodetectors. The photodetector noise spectral density at Vds = 700 mV and Vg = 1.5 V was measured by a dynamic signal analyzer (Hewlett Packard, 35670A). Figure 7(a) shows the measured and modelled noise density for different numbers of MOSFETs at 30 Hz. The simplified flicker noise model is employed with an expression of i2 = K1 × gm2/f, where K1 is a constant for a particular device with a value of 3.56 × 10−13, f is the frequency of 30 Hz, gm is the transconductance which has been measured by the sourcemeter, when m is 3, 5, 7, 9 and 11, gm is 10.68, 28.09, 37.31, 51.83 and 61.98 mS respectively. It can be seen from the results that the two curves basically coincide, indicating that the measured noise densities are credible. The measured NEP of the photodetectors under different incident wavelengths is shown in Fig. 7(b). A minimum NEP of 5.86 fW/Hz0.5 is achieved for m = 5, since the current noise increases as m increases, but the Rv does not change significantly. Therefore, the smaller the number m is, the lower the noise is. The correlation between the detectivity (D*) and m is shown in Fig. 7(c). The value for the maximum D* is 2.21 × 1013 Jones (1 Jones = 1 cm·Hz0.5·W−1) at 590 nm and m = 7.
Fig. 7. (a) Measured and modelled noise density vs m at 30 Hz. (b) NEP against different wavelength of the incident light. (c) D* vs m at Vg = 1.5 V and Vds = 700 mV.
Figure 8 shows the radio frequency characteristics of the photodetectors as determined via impulse-response measurements. Optical pulses of full width at half maximum of 5 ns at a wavelength of 660 nm and repetition rate of 10 MHz were produced by a pulsed diode laser (PDL 800-D, PicoQuant). The impulse photocurrent signals from the photodetectors were monitored by using a 2.5 GHz bandwidth sampling oscilloscope (MSO9254A, Keysight). Because the single cycle time of the voltage pulse signals (100 ns) were exactly consistent with the frequencies of the incident optical pulses (10 MHz), the voltage pulse signals could be used to analyze the photoresponse speed of the photodetectors (Supplement 1). The rise and fall times are defined as the time taken for the signal increasing and decreasing from 10% to 90% and from 90% to 10% of the stable signal, respectively. Figure 8(a) shows the impulse response of the photodetector (m = 3), the rise time (tr) and the fall time (tf) are 24.96 and 21.36 ns, respectively. A Fourier transform of the time domain data provided the spectrum of the photodetector, which is shown in Fig. 8(b). Meanwhile, the noise spectrum of the photodetector is also provided in the figure. The RF bandwidth response is about 15 MHz. When the frequency is 1 GHz, the signal-to-noise ratio is still good, indicating that the photodetector has a very high response speed.
Fig. 8. High-speed photoresponse of the photodetectors. (a) Impulse response of the photodetector (m = 3) and recorded using a 2.5 GHz bandwidth sampling oscilloscope, showing the rise time (tr) and the fall time (tf). The rise and fall times were defined as the signal increasing or decreasing from 10% or 90% to 90% or 10% of the stable signal, respectively. (b) Frequency response obtained via Fourier transform of the time-domain data and spectrum of photodetector noise.
Figure 9(a) shows the schematic diagram of optical imaging scanning system, including a LED light source (660 nm), optical mirrors, chopper, moving stage, lock-in amplifier, PC and power supply. The emitted light beam converged on the photodetector by the optical mirrors. The continuous optical signal was modulated into a pulse signal by a chopper. Place the imaged object on the automatic moving stage (not shown in the figure) for completing the moving scanning. A moving stage (not shown in the figure) is used to complete the moving scanning. The output signals of the photodetector were received by a lock-in amplifier, and the voltage is supplied by a power supply. The scanned image obtained by the photodetector with a step size of 100 µm is shown in Fig. 9(b). It can be obviously seen that the image has relative high resolution, indicating a strong practical application prospects for the photodetector.
Fig. 9. (a) Schematic diagram of optical scanning system. (b) Scanned image using the photodetector (m = 7) with a step size of 100 µm.
In order to show the better performance of the photodetectors, the Table 1 gives a detailed comparison among our photodetectors and others. Our photodetectors achieved high performance and reduced the whole area under earlier technology. The performances of all photodetectors proposed in this paper are shown in Supplement 1. However, a large dark current was generated, which could be resolved by using a proper processing circuit.
Table 1. Comparison of the performance for our one-dimensional MOSFET array photodetector and the other photodetectors
4. Conclusion
In conclusion, the one-dimensional MOSFET array photodetectors proposed and fabricated in this work exhibited high performances. Except m = 3, the Rv of the photodetectors is greater than 2.5 × 103 A/W over a broad near infrared and visible spectral range. When m is small, the array cannot achieve EOT effect, exhibiting a poor performance. When m is large enough, since each MOSFET is independent in the arrays, the increase in m would hardly affect the value of Rv. Meanwhile, the photodetectors have excellent NEP (< 17 fW/Hz0.5), very high D* (> 2.3 × 1012 Jones) and high speed (> 1 GHz). These performances are superior to the existing photodetectors. In addition, the photodetectors have been demonstrated to give good imaging quality. Of course, the performance can be further improved by optimizing the MOSFET size. Thus the photodetectors would provide a new direction for the development in the photoelectric field.
Funding
Fundamental Research Funds for the Central Universities (No. 2020JBM003); National Natural Science Foundation of China (No. 61604009); National Natural Science Foundation of China (Grant No. 61901028).
Disclosures
The authors declare no conflicts of interest.
Data availability
No data were generated or analyzed in the presented research.
Supplemental document
See Supplement 1 for supporting content.
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