Investigation of high sensitivity radio-frequency readout circuit based on AlGaN/GaN high electron mobility transistor
Zhang Xiao-Yu†a), Tan Ren-Bingb), Sun Jian-Donga), Li Xin-Xinga), Zhou Yua), Lü Lia), Qin Hua‡a)
Key Laboratory of Nanodevices and Applications, Suzhou Institute of Nano-tech and Nano-bionics, Chinese Academy of Sciences, Suzhou 215125, China
School of Mathematics and Physics, Chongqing University of Science and Technology, Chongqing 401331, China

Corresponding author. E-mail: xyzhang2010@sinano.ac.cn

Corresponding author. E-mail: hqin2007@sinano.ac.cn

*Project supported by the National Natural Science Foundation of China (Grant No. 61107093), the Suzhou Science and Technology Project, China (Grant No. ZXG2012024), and the Youth Innovation Promotion Association, Chinese Academy of Sciences (Grant No. 2012243).

Abstract

An AlGaN/GaN high electron mobility transistor (HEMT) device is prepared by using a semiconductor nanofabrication process. A reflective radio-frequency (RF) readout circuit is designed and the HEMT device is assembled in an RF circuit through a coplanar waveguide transmission line. A gate capacitor of the HEMT and a surface-mounted inductor on the transmission line are formed to generate LC resonance. By tuning the gate voltage Vg, the variations of gate capacitance and conductance of the HEMT are reflected sensitively from the resonance frequency and the magnitude of the RF reflection signal. The aim of the designed RF readout setup is to develop a highly sensitive HEMT-based detector.

PACS: 52.70.Gw; 85.30.–z; 73.61.Ey
Keyword: radio-frequency circuit; high electron mobility transistor
1. Introduction

The GaN high-electron mobility transistor (HEMT) has great potential applications in high frequency and high power electronic devices.[13] With the development of GaN transistor technology, the GaN HEMT has demonstrated its high power density and high efficiency over the Si- and GaAs-based radio-frequency (RF) and microwave transistors.[4, 5] Thus, for the same output power, the reduction in device size can be realized using GaN-based devices in place of conventional devices. AlGaN/GaN heterostructure has the advantages of the discontinuity of conduction band Δ E and large polarization efficient in nitride, which results in a large piezoelectric polarization in the epitaxial layer of GaN.[68] The high concentration of two-dimensional electron gas (2DEG) is observed at the interface of AlGaN/GaN heterostructure. Therefore, AlGaN/GaN HEMT is an important tunable material for RF and microwave devices by controlling the concentration of 2DEG under gate voltage (Vg). The RF readout circuit, invented by Schoelkopf et al.[9] in 1998, is used to investigate the RF single-electron transistor for the most sensitive and fastest electrometer.[10] The device, such as the single-electron transistor or quantum point contact based on Si or AlGaAs/GaAs heterostructure, is embedded in an RF transmission line of the readout circuit. The changed properties of the device are reflected sensitively from the RF output signal. In China, the use of RF readout to investigate the device has been studied only a very little. In the present paper, we develop a reflective RF readout circuit in which a tunable AlGaN/GaN HEMT device is assembled in the circuit to investigate the performance of the RF readout setup. Under the applied Vg, the changes of impedance including gate capacitance and source– drain conductance are reflected sensitively to the RF output signal. The sensitive changes in resonant frequency and magnitude of output signal indicate that the RF readout setup can be applied to the high-speed detector for the applications in electromagnetic or optoelectronic devices.[11]

2. Experiments

The Al0.3Ga0.7N/GaN heterostructure provided a 2DEG about 25 nm below the surface. The electron mobility μ and density ns at 300  K were μ = 1950  cm2/V· s and ns = 1.4 × 1013  cm− 2, respectively, which were determined by Hall measurement.[12] Ultra-violet exposure, lithography, lift-off, Ohmic electrode preparation, and ion beam etching techniques were used to fabricate the HEMT microstructures. The gate length of HEMT was l = 2  μ m and the channel (mesa) width was w = 8  μ m. A thin film of 10-nm Al2O3 was deposited as the insulating layer by atomic layer deposition. The top electrode of the Ni/Au layer with 200  nm was fabricated by using electron beam evaporation. The microstructure of HEMT was characterized by using scanning electron microscopy (SEM).

The radio frequency (RF) readout setup mainly included a voltage-controlled oscillator (VCO), directional coupler, low noise amplifier (LNA), band-pass filter (BPF), DC block, and RF detector (Agilent 8471D). The output power from VCO (0.4  GHz– 1.3  GHz) was about + 8  dBm. HEMT and high-Q inductors (L = 100  nH) were surface mounted on the 50-Ω coplanar waveguide transmission line, which is fabricated on a high-frequency printed-circuit board (Rogers 3010). The HEMT device was electrically connected to the PCB by using short gold bond wires. The mounted inductor (L) and the capacitance (C) of HEMT formed an LC resonant unit. The LC circuit introduced a discontinuity in the electromagnetic impedance in the transmission line which reflected an incident RF signal. All RF components and transmission lines were connected by soft coaxial cables. The gate voltage Vg was supplied by a Yokogawa DC source meter.

3. Experiments and discussion

Figure  1 shows the schematic diagram of the RF readout circuit. The incoming RF signal from the VCO is coupled to the HEMT device by a directional coupler and the reflected RF signal passes through the directional coupler to LNA, BPF, and DC block to the RF detector. The RF detector is used in RF circuits as a detecting element in the leveling loop for power monitoring. Finally, the reflected RF signal can be monitored by a multimeter. The inset shows the image of the transmission line where the HEMT device and high-Q inductors are surface mounted. The device on the copper holder must be well grounded in order to avoid damaging the HEMT device by static electricity. An SEM photo of the HEMT is also shown in the inset where mesa and Ohmic electrodes are displayed.

Fig.  1. Schematic diagram of the RF readout circuit and equivalent circuit of HEMT. Inset shows the image of the transmission line with the HEMT device and SEM photo of HEMT.

Figure  2 shows the variations of the transfer characteristic curve of AlGaN/GaN HEMT, i.e., drain– source DC current Ids with gate voltage Vg. For IdsVg measurement, a DC voltage Vds is applied to the drain and Vg is applied to the gate. A multimeter through a low-noise current preamplifier with a sensitivity of 10− 4  A/V is used to monitor the Ids at source. At a fixed Vds, Ids decreases with the increase of | Vg| and reaches a saturation state when Vg < − 3.75  V, showing a switch characteristic of depleting the electrons under the control of Vg. The current Ids can be described as follows:

where μ is the electron mobility, Cg is the gate capacitance, and Vth is a threshold voltage where Ids changes abruptly. Generally, equation  (1) is used to describe the Ids under the gate and it is difficult to fit the whole IdsVg curve because the Cg is also a function of Vg. When Vg is zero, Ids increases greatly with the increase of Vds because the electrons in the channel of the HEMT are accelerated by Vds, thus resulting in the enhancement of the drift current.

Fig.  2. Curves of Ids versus Vg at different values of applied Vds. Inset shows the schematic diagram of GaN/AlGaN HEMT.

Figure  3 shows the variations of the RF reflection signal VRF with frequency. Two remarkable characteristics are observed. Firstly, the resonance frequency fr shifts from 805  MHz to 816  MHz with the increase of | Vg| . Secondly, it is found that the magnitude of RF signal VRF also decreases significantly around the resonance frequency with the increase of applied Vg. We consider that the changes of fr and the magnitude of VRF are strongly dependent on the impedance of HEMT under Vg, namely, the changes of gate capacitance and conductance. In an HEMT microstructure, we estimate the geometry capacitance under the gate to be Cgeom = wlɛ 0ɛ AlGaN/dAlGaN = 0.053  pF, where wl = 16  μ m2 is the area under the gate, ɛ 0 is the vacuum permittivity, ɛ AlGaN = 9.4 is the relative permittivity of AlGaN, and dAlGaN = 25  nm is the distance between the gate and 2DEG. With the increase of Vg, the electrons under the gate are gradually depleted, thus resulting in the change of electric charges Δ q in the GaN layer. The reduced electron density in 2DEG can be equivalent to the increase of distance dAlGaN, therefore, the gate capacitance decreases with the applied Vg.[13, 14] According to the resonance of the designed LC circuit, the resonance frequency is expressed as

which is mainly dominated by the capacitance CRF including gate capacitance and parasitic capacitance, when L = 100  nH is a constant in our experiment. Therefore, fr increases with the decrease of CRF under Vg, which is consistent with the observed shift of fr in Fig.  3. On the other hand, the source– drain resistance Rds will also increase with the applied Vg increasing due to the depletion of electrons. It is suggested that the variation of the magnitude of reflection signal VRF originates from the change of source– drain conductance.

Fig.  3. Variations of reflected voltage signal with frequency at different values of applied Vg.

Figure  4(a) shows the calculated capacitance CRF as a function of Vg from Eq.  (2) by using the measured fr where the value of the inductor is L = 100  nH. Clearly, the obtained CRF displays the Vg-controlled characteristics, which indicates that the changed capacitance of the HEMT is reflected sensitively at Vth by the RF readout circuit. We observe that CRF varies from 0.378  pF to 0.366 pF when the value of Vg changes from 0  V to − 4  V. The tunability τ = [CRF(Vg = 0)– CRF(Vg = − 4  V)]/CRF(Vg = 0) is about 3.2% around the frequency of 816  MHz. Actually, the tunability of gate capacitance is remarkable from the measurement of the gate capacitance Cmeas at a low frequency of 1  kHz by using an Agilent B1500A semiconductor device analyzer as shown in Fig.  4(b). The Vg-controlled Cmeas varies from 0.25  pF to 0.042  pF, and the tunability reaches up to 83.2%, showing a good Vg-controlled performance at the low frequency. We consider that the difference in tunability mainly results from the difference in frequency response of gate capacitance.

Fig.  4. Variations of (a) calculated capacitance and (b) measured capacitance with Vg at 1  kHz.

Figure  5 shows the variations of source– drain conductance G(1/Rds) and the valley of RF reflection signal Vm in Fig.  3 with Vg. The conductance of HEMT G = nseμ w/l is mainly dependent on the electron density ns and mobility μ when the micro-structure of HEMT is fixed. With the increase of applied Vg, conductance G decreases abruptly in the vicinity of Vth due to the depletion of electrons. We find that the Vm also exhibits the same variation trend as the GVg relationship. From the RF equivalent circuit in our previous article, [10] the reflection coefficient is Γ = (ZinZ0)/(Zin + Z0), where Zin is the input impedance and Z0 = 50  Ω is a characteristic impedance. The input impedance can be expressed as Zin = j2π fL + Rds/[1 + j2π fRdsC] + Z0. In the experiment, capacitance C mainly includes gate capacitance Cg and the parasitic capacitance of the circuit. The sensitivity of the output signal under the gate voltage is proportional to both d(1 − Γ )/dRds(Vg) and dRds(Vg)/dVg. The first term is determined by the characteristics of the RF transmission line, while the second term is determined by the characteristic of the HEMT. The intrinsic sensitivity | dRds/dVg| of the readout circuit is strongly dependent on the rate of changed drain– source resistance Rds(Vg). Therefore, the design and fabrication of good performance of HEMT with large | dRds/dVg| are the key to improving the sensitivity of the readout setup, namely, the rapid change of source– drain resistance near threshold voltage Vth under Vg. In the present experiment, the sensitivity of the readout setup can increase up to 5.6× 108. When a threshold voltage Vth is applied to the gate of the HEMT, the RF readout setup can be used to detect sensitively the change of impedance of HEMT under the external radiation, such as terahertz radiation or light.

Fig.  5. Variations of the valley of RF reflection signal Vm and conductance with Vg.

4. Conclusions

We designed a reflective RF readout circuit and investigated the performance of the HEMT-based readout setup. The RF setup is used to detect the changed impedance of the HEMT by analyzing the resonance frequency and the magnitude of the RF reflection signal. It is suggested that the designed RF readout setup can be applied to the readout circuit for high-sensitivity optic– electric detection.

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