Broad bandwidth photodiodes (PDs) with high-responsivity and high RF output power are key components for microwave/millimeter-wave photonic transmission and processing systems^[1,2], in which both wide bandwidth and high link gain with large spurious free dynamic range (SFDR) are necessary. Recently, the increasing demand of millimeter-wave and sub-THz applications has driven the need for PDs with bandwidths over 100 GHz^[3–6]. Compared with traditional PIN photodiodes, uni-traveling carrier PDs (UTC-PDs) have demonstrated ultra-broad bandwidth performance due to the elimination of slow hole transportation^[7–9]. Waveguide integrated UTC-PDs have been reported to realize ultra-broad bandwidth and high responsivity^[10–12], but the saturation power performance is limited. Inserting a thin n-doped layer into the depletion region can compensate the space charge screening effect, resulting in improved saturation characteristics together with a bandwidth over 110 GHz^[13–15]. However, as the PD employs a thin absorption region to ensure broad bandwidth performance, its responsivity is relatively low ($<0.2\mathrm{A}/\mathrm{W}$), which is not suitable to attain high link gain and large SFDR.

In this work, a high-responsivity and ultra-broadband backside-illuminated double-cliff-layer (DCL) uni-traveling-carrier photodiode (UTC-PD) is designed and fabricated. An 850-nm-thick InGaAs absorption layer is adopted for high responsivity. The self-induced electric field in the graded p-doped absorption layer under high photocurrents facilitates electron drift out. Meanwhile, the depletion region with the double-cliff layer is incorporated to tune the electric field distribution, thus mitigating the high-density electron-induced space charge screening effect and maintaining the overshoot electron velocity. An inductive coplanar waveguide (CPW) with optimized length is employed to enhance the frequency response of the device. The fabricated 6-µm-diameter DCL-UTC-PD exhibits a high responsivity of 0.51 A/W and a large 3-dB bandwidth of 102 GHz at a photocurrent of 10 mA. The saturation photocurrent at 100 GHz is measured to be 16 mA, corresponding to an RF power of 2.7 dBm.

The epitaxial structure of the proposed MUTC-PD is shown in Fig. 1(a), which is grown by metal organic chemical vapor deposition (MOCVD) on a semi-insulating InP substrate. To ensure a high-responsivity, the PD adopts a thick absorption region consisting of a 650-nm-thick graded p-doped InGaAs absorption layer and a 200-nm-thick n-doped InGaAs absorptive depletion layer. The responsivity ($R$) of the detector can be expressed as $R=\frac{q}{hv}\times (1-e^{-\alpha L})$, where $\alpha \approx 0.7{\mathrm{\mu m}}^{-1}$ is the absorption coefficient of InGaAs material around 1550 nm, $L$ is the effective thickness of the absorption layer, $q$ is the electron charge, and $hv$ is photon energy. Assuming a back-incidence structure with the top metal acting as a mirror, the absorption thickness is $L=1.7\mathrm{\mu m}$ since the incident light passes through the absorption region twice. As a result, the 850-nm-thick absorption region leads to a response of 0.87 A/W.

A thick absorption region helps increase the responsivity of the PD, but it tends to lengthen the transit time of photocarriers, thus degrading the bandwidth performance. To realize high-speed performance, the transit time in both the p-doped absorption region and the depletion region should be minimized. A self-induced electric field will be formed in the 650-nm-thick graded p-doped InGaAs absorption layer due to the variation of the Fermi level. Under high photocurrents, the hole drift current would enhance the self-induced electric field, thus leading to drift dominated electron transport and reduced transit time^[16,17]. Figure 1(b) shows the calculated electric field within the absorption region. It is evident that the self-induced electric field increases with the photocurrent and can reach up to several kV/cm, thus speeding up electron transport through the absorption region.

To alleviate the space charge screening effect, a double-cliff-layer depletion region is proposed. As shown in Fig. 1(c), the electric field in the depletion region can be effectively elevated by the two n-doped cliff layers, thus improving the saturation performance of the PD, and it would only slightly increase the capacitance of the PD. The electric field profile in the depletion region can be adjusted by judiciously positioning the two cliff layers. The variation of the electric field within the depletion region under different bias voltages is illustrated in Fig. 1(d), where the cliff layers are positioned 100 nm and 200 nm away from the InGaAs and InP interface. A flattened electric field within the depletion region helps improve the saturation performance of the PD. Meanwhile, an electric field in the range 5–30 kV/cm is helpful for maintaining overshoot velocity^[18].

The normalized frequency response of a PD can be expressed as $H(\omega )=H_{\tau }(\omega )·H_{\mathrm{RC}}(\omega )$, where $H_{\tau }(\omega )$ represents the transit time limited frequency response, while $H_{\mathrm{RC}}(\omega )$ is the RC constant limited frequency response. An inductance can provide improved impedance matching for the capacitive PD, thus enhancing the frequency response. However, implementing inductance at high frequencies is challenging^[19]. High-impedance CPW has been proposed as an alternative way to enhance the bandwidth performance of PDs^[20,21], which takes advantage of the inductive characteristics of the high-impedance CPW.

In our device, a short section of a 115-Ω-impedance CPW electrode is included to optimize the frequency response of the PD. The schematic and equivalent circuits of the PD with high-impedance CPW structure are shown in Figs. 2(a) and 2(b). The signal electrode width is 6 µm, while the gap between the signal and the ground electrodes is 97 µm. The transit time limited transfer function is simplified to be $H_{\tau }(\omega )=\frac{1}{1+j\frac{\omega }{2\pi f_{\mathrm{tr}}}}$, where $f_{\mathrm{tr}}=87\mathrm{GHz}$. The RC constant is referred from our previous 6-µm-diameter PDs with 106 GHz bandwidth, as the thickness of the depletion layer and PD area are similar here^[22]. The junction capacitance is $C_j=4\mathrm{fF}$, the parasitic capacitance is $C_p=16\mathrm{fF}$, the junction resistance is set to $R_j=20\mathrm{k\Omega }$, and the electrode contact resistance is $R_1=5\mathrm{\Omega }$. The RC constant limited bandwidth of the PD without the CPW is estimated to be 150 GHz. The introduction of inductive CPW helps achieve better impedance matching by compensating the capacitance of the PD, thus enhancing the frequency response, especially at low frequencies. Figures 2(c) and 2(d) show the RLC limited frequency responses and the total frequency responses of the PD with different lengths of CPW. As the length of the high-impedance CPW increases from 0 to 75 µm, the RC constant limited bandwidth increases from 150 GHz to 170 GHz, and the total bandwidth is enhanced to 107 GHz. Increasing the CPW length to 100 µm results in a slightly decreased RC constant limited bandwidth. Nevertheless, the entire device exhibits an improved bandwidth, as the transfer function $H_{\mathrm{RC}}(\omega )$ below the LC resonance peak is enhanced at the price of reduced response at higher frequencies. Further increasing the CPW length would reduce the overall bandwidth of the PD. Consequently, a 100-µm-long 115-Ω CPW is adopted as the optimized structure for our device.

Backside-illuminated DCL-MUTC-PDs are fabricated with a 3-mesa structure for structural stability after wet-etching reported in our previous work^[23], as illustrated in Fig. 3(a). Both p- and n-mesas are patterned by a combined inductively coupled plasma (ICP) dry-etching and wet-etching process. Ti/Pt/Au and Ni/Au are sputtered as the p- and n-electrodes, respectively. An 800-nm-thick ${\mathrm{SiO}}_2$ layer is deposited for sidewall passivation and parasitic capacitance reduction. Then high-impedance CPW is formed on the ${\mathrm{SiO}}_2$ layer by sputtering and electroplating. Finally, the device is thinned and back polished, and a 215-nm-thick SiN_x layer (refractive index $\sim 1.8$) is deposited on the backside of the device to reduce incident light reflection down to 1%.

The responsivity of the fabricated device is 0.51 A/W for 6-µm-diameter PDs and 0.56 A/W for 8-µm-diameter ones, respectively. The divergence angle of the optical fiber used in our test is 15°, and the diameter of the optical field is expanded to about 17 µm after propagating through the 100-µm-thick InP substrate. As a result, the measured responsivity is lower than the simulation value of 0.84 A/W. In the future, a substrate lens can be incorporated to improve the responsivity^[24].

The frequency response and RF output power are measured with a two-laser heterodyne measurement system^[25]. A set of microwave and millimeter-wave sensor heads is used to cover the frequency range from DC to 40 GHz, 50–75 GHz, and 75–110 GHz. The frequency response at $-2\mathrm{V}$ is shown in Fig. 3(b). The 3-dB bandwidth is only 85 GHz at a photocurrent of 5 mA, while a 3-dB bandwidth as large as 102 GHz is recorded at a photocurrent of 10 mA. The output RF power as a function of photocurrent is plotted in Fig. 3(c). The photocurrent at the 1-dB compression point is 16 mA at $-3\mathrm{V}$ bias voltage, corresponding to an RF output power of 2.7 dBm. To the best of our knowledge, this is the best performance reported for backside-illuminated PDs with such high responsivity and wide bandwidth.

Figure 4(a) shows the equivalent circuit model with variable RC parameters^[14], which takes both the transit time (Region 1) and RC electric circuit parameters (Region 2) into consideration. First, the parameters of Region 2 are tuned to fit the measured ${\mathit{S}}_{22}$, and the transit time of Region 1 can then be extracted by fitting the measured frequency response ${\mathit{S}}_{12}$. $R_u$ and $C_u$ are the resistance and capacitance of the 200 nm InGaAs and 30 nm InGaAsP depletion layer, which are extracted as 20 kΩ and 30 fF, respectively. $R_j$ and $C_j$ are the resistance and capacitance of the 590 nm InP depletion layer. During the fitting process, the bulk material resistance $R_1$ and the contact resistance $R_2$ are extracted as 3 Ω and 8 Ω, respectively. The capacitance $C_j=4\mathrm{fF}$, while the parasitic capacitance of the p-electrode $C_p=17\mathrm{fF}$. Fairly good agreement between the measured and simulated S₂₂ curves is shown in Figs. 4(b)–4(d).

The bandwidths under different bias voltages and photocurrents are shown in Fig. 5(a). The variation of bandwidth with the photocurrent is mainly attributed to the space charge screening effect in the depletion region and the self-induced electric field in the p-absorption region. The self-induced electric field under a high photocurrent helps maintain a high electron drift velocity in the p-doped absorption region, while the double cliff layers inserted into the depletion region can alleviate the space charge screening effect and ensure a suitable electric field for an overshoot drift velocity under high photocurrents. The RC limited bandwidth of Region 2 is estimated to be 145 GHz.

The extracted transit time limited bandwidth $f_{\mathrm{tr}}=1/(2\pi R_t{C}_t)$ is shown in Fig. 5(b). The transit time limited bandwidth under 10 mA photocurrent is larger than that under 5 mA, which is attributed to the enhanced self-induced electric field. Under $-2\mathrm{V}$ bias voltage and 5 mA photocurrent, the transit time limited bandwidth is 74 GHz, corresponding to a transit time of 6.0 ps. When the photocurrent is increased to 10 mA, the transit time limited bandwidth increases to 95 GHz, corresponding to a transit time of 4.7 ps. Assuming an overshoot drift velocity of $3\times {10}^5\mathrm{m}/\mathrm{s}$ for electrons in the depletion region, the electron velocities in the p-doped region under 5 and 10 mA photocurrent are extracted to be $2\times {10}^5$ and $3.3\times {10}^5\mathrm{m}/\mathrm{s}$, respectively.

According to the above analysis, the large photocurrent helps to maintain a high drift velocity of electrons in the p-type absorption region, while the insertion of the double cliff layers into the depletion region can alleviate the space-charge screening effect and ensure electron velocity overshoot under large photocurrents. Figure 6 summarizes the responsivity versus frequency of back-illuminated PDs reported in the literature. The proposed DCL-UTC photodiode shows the highest responsivity with $\sim 100\mathrm{GHz}$ bandwidth^{[11,14,15,17,23,26–28]}.

In conclusion, a broad bandwidth DCL-UTC-PD with high-responsivity and high saturation power is proposed and demonstrated. High responsivity is achieved by adopting an 850-nm-thick InGaAs absorption structure. The graded p-doped absorption layer is designed to ensure an enhanced self-induced electric field under high photocurrent conditions, thus facilitating electron drift out of the region. Meanwhile, double cliff layers are inserted into the depletion region to compensate the high electron density induced space charge screening effect and maintain overshoot electron velocity. In addition, a high-impedance CPW with optimized length is employed to enhance the frequency response of the PD. The fabricated 6-µm-diameter DCL-UTC-PD exhibits a high responsivity of 0.51 A/W and a maximum 3-dB bandwidth of 102 GHz with a standard 50 Ω load. The saturation photocurrent at 100 GHz is 16 mA, corresponding to an RF output power of 2.7 dBm.