A high-RF-bandwidth, high-IF-bandwidth monolithic terahertz heterodyne receiver based on AlGaN/GaN nonlinear transmission lines as a local oscillator and mixer

We report a monolithic heterodyne receiver that uses the AlGaN/GaN nonlinear transmission line as a local oscillator and a mixer simultaneously. This heterodyne receiver has a high RF bandwidth from 80 to 360 GHz and a high intermediate frequency bandwidth of 18 GHz. These results indicate that this nonlinear transmission line receiver has promising potential in broadband spectrum analysis.


D
ue to the attractive advantages of higher sensitivity and phase detection compared with direct detection, [1][2][3] terahertz heterodyne detection is widely used in radio-astronomy receivers, 4) wireless communication, 5) imaging 6) and gas detection.7) A heterodyne receiver requires an independent local oscillator (LO) with a frequency close to that of the signal to be measured, which brings an additional cost above direct detection.The use of harmonic mixers reduces the frequency dependence of the LO source, but second-and third-harmonic terahertz heterodyne receivers still require a relatively high-frequency LO source.5][16][17][18][19] The application of a nonlinear transmission line (NLTL) to a spectrometer demonstrates that a NLTL can be used as a broadband local frequency comb generator for heterodyne receivers.[20][21][22] Another important indicator for a terahertz heterodyne receiver is the intermediate frequency (IF) bandwidth. Af][24][25][26][27] By using matching networks, integrating a load resistance, optimizing the structure to reduce the internal resistance, and so on, the IF bandwidth of a FET detector is effectively increased to several gigahertz.[28][29][30][31] However, these solutions increase the area of the circuit.Therefore, the proposal of a heterodyne receiver with an impedance of 50 Ω is critical for broadband heterodyne receivers.
In this paper, a high-RF-bandwidth, high-IF-bandwidth monolithic terahertz heterodyne receiver based on AlGaN/ GaN NLTL is presented.The noise-equivalent power (NEP) and IF bandwidth of the NLTL as a heterodyne receiver are characterized.By simultaneously utilizing the heterodyne receiver as a local comb generator, the performance of the higher-order harmonics receiver based on the same NLTL is characterized.
The device is fabricated based on an AlGaN/GaN wafer customized by Enkris Semiconductor, the concentration (n s ) range of the two-dimensional electron gas (2DEG) is (0.8−1.2) × 10 13 cm −2 and the electron mobility (μ) of the 2DEG is 1700 cm 2 V -1 s -1 . 32)An optical microscope image of the AlGaN/GaN NLTL heterodyne receiver is shown in Fig. 1(a); it consists of a NLTL and a broadband bow-tie antenna.Twenty-nine pairs of AlGaN/GaN Schottky barrier diodes (SBDs) are periodically loaded into the coplanar waveguide (CPW) and form the NLTL.To obtain a broadband frequency comb as the LO source, the NLTL frequency comb generator is designed with three sections, 33) and the distances (d) between AlGaN/GaN SBDs from section#1 to section#3 are 600 μm, 300 μm and 150 μm, respectively.The characteristic impedance (Z p ) of the CPW at the input port of the NLTL is 50 Ω.Then the CPW is connected to the NLTL through a taper structure.The output port of the NLTL is integrated with a broadband bow-tie antenna to receive the RF signal.
Figure 1(b) displays the structure of the NLTL unit.The CPW is fabricated on a sapphire substrate with a thickness (H) of 0.4 mm and a relative dielectric constant (ε s ) of 9.6.The width (W) of the center conductor and the distance (G) between the center conductor and the ground plane are 40 μm and 60 μm, respectively.Such a structure determines the characteristic impedance of the CPW is Z 0 = 71 Ω, and the effective dielectric constant of the CPW is ε e = 5.3.During the propagation of an electromagnetic wave in the CPW, the phase velocity can be expressed as v c 1.3 10 m s ´-, and the delay per unit length of the CPW is τ = 1/v = 7.7 ps mm -1 .According to transmission line theory, a coplanar waveguide can be equivalent to a LC circuit.The equivalent inductance and equivalent capacitance of the CPW can be expressed as L a = τZ 0 = 547 pH mm -1 and C a = τ/Z 0 = 108 fF mm -1 , respectively. 18)The anode area (A) of the AlGaN/GaN SBD is A ≈ 1μm × 20 μm = 20 μm 2 , and the thickness of the AlGaN barrier layer is h = 25 nm, as schematically shown in Fig. 1(d).Thus, the zero-bias capacitance of a pair of SBDs is .The minimum Bragg cut-off frequency of the NLTL is f Bragg = 66 GHz in section#1 at V B = 0 V, which corresponds to the theoretical IF bandwidth.The structure of the bow-tie antenna is shown in the Fig. 1(c).The input port of this antenna completely matches the CPW of the NLTL.After optimizing the antenna structure by electromagnetic simulation, the return loss of the antenna shown in Fig. 1(c) is less than −3 dB in the frequency range of 40-400 GHz.
The capacitance between the signal line and the ground plane of the NLTL heterodyne receiver is characterized onchip using a Keithley 4200 semiconductor characterization system.The measured capacitance at 10 kHz comprises two parts and can be expressed as C n = C TL + C SBD , as depicted in Fig. 1(d), where the voltage-independent capacitance from the CPW is C TL ≈ 2.62 pF and the nonlinear capacitance of the 29 pairs of SBDs is C SBD ≈ mC s ≈ 3.73 pF.
Benefiting from the nonlinear capacitance of the AlGaN/ GaN SBDs, the AlGaN/GaN NLTL can be used as a mixer.In order to characterize the performance of the NLTL mixer, a quasi-optical testing setup, as shown in Fig. 2(a), is proposed to receive the LO signal and RF signal simultaneously.The LO signal and RF signal are both collimated by the off-axis parabolic mirror (OAP).The two beams are combined through a high-resistance silicon beam splitter and converge through OAP#3 to the NLTL mixer.A DC bias is applied through a bias-T to the NLTL mixer.The IF signal  generated by the NLTL is output through the AC terminal of the bias-T to a low-noise amplifier (LNA) with a gain of 30 dB and a bandwidth of 3 GHz, then the amplified IF signal is measured by the spectrum analyzer.When the RF source is turned off, the noise of the heterodyne system can be obtained.
For LO radiation with a frequency of 100 GHz and power of −5.3 dBm, the power of the IF signal, as well as the noise power with a resolution bandwidth (RBW) of 1 Hz, changes as a function of the bias voltage as shown in Fig. 2(b) for f RF = 101 GHz.In the bias range from V B = 0 V to −2.5 V, the power of the IF signal varies slightly with bias voltage due to the weak nonlinear capacitance.At a bias voltage around V B = -3 V, the lower 2DEG concentration under the anode causes an increase in the series resistance of the AlGaN/GaN SBD and a decrease in the IF power.In the bias range from V B = −3 V to −4 V, the nonlinearity of the capacitance increases rapidly and the power of the IF signal increases faster than the loss.At V B = -4.07V, the power of the IF signal reaches its maximum value of −41.89 dBm.In the bias range from V B = -4 V to −5 V, the relatively weak nonlinearity of the Schottky capacitance leads to a comparatively low power of the IF signal.Since the NLTL is integrated with the bow-tie antenna, which is equivalent to an open circuit structure, the shot noise can be disregarded.The 1/f noise of the NLTL heterodyne detector is also negligible as the frequency of the IF signal reaches the gigahertz level.The measured noise in the heterodyne system shown in Fig. 2(b) is mainly generated in the LNA.With a Golay detector, the RF power converged to the AlGaN/GaN NLTL detector is confirmed to be P RF = -31 dBm.The NEP is defined as NEP=P noise P RF /P IF , and the minimum NEP is −132 dBm Hz -1 at V B = -4.07V and f LO = 100 GHz.
To characterize the IF bandwidth of the NLTL heterodyne detector, another LNA with a bandwidth of 18 GHz is used to amplify the IF signal.Here, the power and frequency of the LO signal are P LO = -5.4dBm and f LO = 100 GHz, respectively.The power of the RF signal (P RF ), the power of the IF signal (P IF ) and their ratio (P RF /P IF ) are depicted in Fig. 3 as the frequency of the RF signal increases from f RF = 82 GHz to f RF = 118 GHz.It can be seen that the ratio fluctuates considerably.Still, there is no decreasing trend as the IF frequency increases, indicating that the IF bandwidth of the NLTL heterodyne detector exceeds 18 GHz.It is assumed that the fluctuations in the power ratio between the RF and IF signals may be related to the receiving capability of the antenna, deviations in the packaging, etc.
The heterodyne receiver shown in Fig. 1(a) can also be used as a frequency comb generator.By using this receiver as a frequency comb generator and a mixer simultaneously, a harmonic mixer can be obtained and HF signals can be detected for relatively low-frequency signal inputs.To characterize the performance of the NLTL harmonic mixer, a quasi-optical system for heterodyne detection was established, as shown in Fig. 4(a).A microwave signal with a frequency f 0 = 20 GHz and a power of 10 dBm is amplified by a power amplifier with a gain of 20 dB.After traveling through the high-pass filter (HPF) with a frequency of 8 GHz and a three-port SubMiniature Version A (SMA) connector, the microwave signal with a power of 22 dBm combined with a DC bias through the bias-T is input to the NLTL.The RF signal ( f RF ) from the terahertz source is collimated by OAP#1 and then converged to the bow-tie antenna by OAP#2.The received RF signal and the local frequency comb are mixed in the NLTL, then the IF signal with f nf f The IF signal is eventually detected by the spectrometer after it has traveled through the LNA (30 dB), the bias-T, the three-port SMA connector, and the low-pass filter (LPF) with a frequency of 6 GHz.The RF signal and IF signal are separated by the LPF and HPF.
For RF radiation with a frequency f RF = 99 GHz and power P RF = 4.6 dBm, the RF signal mixes with the 100 GHz

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© 2024 The Author(s).Published on behalf of The Japan Society of Applied Physics by IOP Publishing Ltd harmonic (n = 5) and the power of the IF signal ( f IF = 1 GHz) changes with bias voltage, as depicted in Fig. 4(b).At V B = −4 V, the power of the IF signal reaches P IF = −32.9dBm, which is close to the maximum power P IF = −32 dBm at V B = −4.16V.The noise power is obtained when the RF source is turned off, and is maintained at a fixed value around P noise = −138 dBm.The NEP = P noise P RF /P IF = −100.5dBm Hz -1 at V B = −4 V.
At an input frequency of f 0 = 20 GHz and a bias voltage of V B = −4 V, the power of the IF signal, the power of the RF signal, the noise power and the NEP vary with the frequency of the RF signal, as shown in Fig. 5 for f IF = nf 0 − f RF = 1 GHz.When the RF signal with higher frequency and the higher-order harmonics with lower power are mixed, the power of the produced IF signal is relatively weak.As the RF signal frequency increases from 79 to 359 GHz, there is a corresponding increase in the NEP from −104 dBm Hz -1 to −50 dBm Hz -1 , alongside an escalation in the conversion loss (CL) from 64 dB to 115 dB.
Limited by the power of the frequency comb, the NLTL heterodyne receiver is more suitable for high-power, broadband terahertz wave detection.Thus, future work should focus on improving the sensitivity of NLTL heterodyne receivers, which can be realized through the following means.Firstly, increasing the input power of the microwave signal to enhance the power of the frequency combs, as referred to in our previous work, 33) will improve the sensitivity of the NLTL heterodyne receiver.Secondly, optimizing the structure of the AlGaN/GaN SBD to decrease the series resistance can reduce the loss of the frequency comb, increase the power of the LO frequency comb, reduce the loss of the IF signal, etc, and finally improve the sensitivity of the NLTL heterodyne receiver.
Table I compares the performance of microwave and THz receivers at similar frequencies.Compared with heterodyne receivers based on fundamental and subharmonic mixers, combbased receivers require a relatively low-frequency LO source and have a higher bandwidth.However, limited by the power of the frequency comb, the NEP of a comb-based receiver is relatively poor.The comb-based receivers reported in Refs.15 and 16 contain both a frequency comb generator and a mixer, while the comb-based receiver in this work simultaneously uses the NLTL as a frequency comb generator and mixer, which effectively increases the integration.The 1 μm AlGaN/GaN SBD technology utilized in this work is not sufficiently advanced compared with the other technologies listed in Table I and requires further optimization to reduce the NEP.
In conclusion, a broadband heterodyne receiver based on an AlGaN/GaN NLTL is designed, fabricated and characterized.This NLTL heterodyne receiver operates with an IF bandwidth of 18 GHz.At a LO input with a frequency of 20 GHz and power of 22 dBm, the NLTL heterodyne receiver achieves heterodyne detection in the frequency range from 79 to 359 GHz.These results indicate that the requirement for a heterodyne receiver for the LO source frequency is effectively reduced.Future research will concentrate on improving the sensitivity of NLTL heterodyne receivers because their lower sensitivity restricts applications that require low-power terahertz detection.
272 fF, where the relative dielectric constant of the AlGaN barrier layer is ε r = 9.6 and the vacuum dielectric constant is ε 0 = 8.85 × 10 −12 F m -1 .Affected by the nonlinear capacitance of the AlGaN/GaN SBDs, the characteristic impedance of a NLTL can be expressed as voltage V B = 0 V, the characteristic impedance of the NLTL from section#1 to section#3 is 40 Ω, 31 Ω, and 23 Ω, respectively.At the reverse bias at which the 2DEG is completely depleted, the Schottky nonlinear capacitance can be ignored and the parasitic capacitance of the SBD is about 3 fF; the characteristic impedance of the NLTL from section#1 to section#3 increases to 68 Ω, 65 Ω and 61 Ω, respectively.This indicates that the characteristic impedance of the NLTL varies around 50 Ω, ensuring a low reflection transmission of the IF signal.The periodic structure of the NLTL forms a low-pass filter, and the Bragg cut-off frequency can be expressed as f d

Fig. 1 .Fig. 2 .
Fig. 1.(a) Integrated chip of the AlGaN/GaN NLTL and bow-tie antenna.(b) Schematic of a NLTL unit in the NLTL.(c) Schematic of the bow-tie antenna.(d) The capacitance of the AlGaN/GaN NLTL as a function of bias voltage.

Fig. 3 .
Fig.3.The power of the RF signal, the power of the IF signal and the power ratio between these two signals as the frequency of the RF signal varies from 82 GHz to 118 GHz for a LO input with a frequency of 100 GHz and power of −4.5 dBm.

Fig. 4 .
Fig. 4. (a) Quasi-optical setup for heterodyne detection.(b) The power of the IF signal, power of noise as RBW = 1 Hz and NEP at f RF = 99 GHz as a function of bias voltage at the LO input with a frequency of 20 GHz and power of 22 dBm.

Fig. 5 . 4 ©
Fig. 5.For a LO input with a frequency of 20 GHz and power of 22 dBm, the bias voltage of the NLTL is V B = −4 V and the frequency of the RF signal is f IF = nf LO − f RF = 1 GHz: (a) the power of the IF signal and RF signal and noise (RBW=1 Hz) ; (b) changes in NEP and CL with RF.

Table I .
Performance comparison of heterodyne receivers at similar frequencies.High Electron Mobility Transistor.b) Complementary Metal Oxide Semiconductor.c) Bipolar Complementary Metal Oxide Semiconductor.