Tunable AC/DC converter using graphene-germanium barristor based half-wave rectifier

As advanced CMOS technologies have been driven toward lower operating voltage and more power efficiency, on-chip power supply circuits are getting more attention to supply the multiple low DC voltage (V_DD), such as the DC-DC converter and rectifier for AC-DC conversion.^1–11 A well-known representative simple circuits are the diode based full/half-wave rectifier circuits and Zener diode based voltage regulator circuits. Their performances are dominantly determined by the characteristics of the rectifying device and other components.

To meet the diverse requirements for many different applications, various DC supply circuits have been proposed using novel devices and materials.^1–8 Steudel et al. demonstrated the 50-MHz half-wave rectifier to provide V_DD to the logic circuit using a pentacene diode.¹ Heljo et al. proposed poly(triarylamine) (PTAA) semiconductor diodes based half-wave and full-wave rectifiers that are successfully used in passive radio frequency identification (RFID) tags.² An integrated a-IGZO Schottky diode with a high cutoff frequency of 3 GHz was proposed for a double half-wave rectifier in passive RFID Tags which yields a DC voltage.³ While these two terminal device-based circuits are simple and suitable for integrated circuits, they can only generate a single value output voltage. Thus, additional elements should be used to generate multiple low V_DD values, such as step-down DC-DC conversion systems connected to the rectifiers, which will affect the design efficiency and chip area.^9–11 In addition, for monolithic integration architecture, it is desirable to move less performance sensitive circuits such as power supply module to upper level of circuits embedded in the back end of line structures. In this case, devices with a low temperature integration capability has a great advantage. Thus, there is a strong need for a low temperature fabricated-tunable AC/DC converter to reduce the chip area and power consumption. Again, several devices have already been investigated to achieve this goal. Wang et al. demonstrated that graphene field effect transistors (GFETs) could be used as frequency-doubling devices.¹² Using the ambipolar transport properties of GFETs that resemble the I–V characteristics of an ideal full-wave rectifier, a frequency multiplier was developed without additional components.

Graphene barristor is an attractive candidate for these types of BEOL device applications because of the very low device fabrication temperature, flexibility, high on/off ratio, and very low V_DD.^13–18 By modulating the Schottky barrier height between a graphene barristor and a semiconductor, it can function like a Schottky diode with an adjustable rectification ratio.

In this work, we developed a tunable DC voltage supply circuit using a half-wave rectifier based on a graphene–germanium (Ge) barristor. This device exhibited a very low turn-on voltage of ∼ 0.17 V as well as high V_DC/V_AC peak ratio compared to other devices based rectifier fabricated with low temperature integration below 300 °C. Moreover, the output voltages could be modulated from 1 to 3.5 V for input signal frequencies between 60 and 1 MHz.

Fig. 1(a) shows the optical image of the fabricated graphene/Ge barristor. The 45-nm AgSb drain electrode (Ge region) and 100-nm Au source electrodes (graphene region) were deposited to form Ohmic contacts to Ge substrate. Then, a 30-nm Al₂O₃ gate dielectricr and a 80nm Au metal gate were deposited on the active channel having the graphene/Ge Schottky junction. The highest process temperature was 300 °C annealing used to enhance the adhesion of graphene to Ge substrate. As shown in Fig. 1(b), the quality of monolayer graphene grown via chemical vapor deposition appears to be maintained quite well during the device fabrication process. Further details about the device fabrication can be founded in Ref. 19.

The electrical characteristics of base device, graphene/Ge barristor, were measured using a semiconductor parameter analyzer (Keithley 4200). Fig. 2(a) shows the typical behaviors of the Schottky diode at V_G = 0 V. A rectification ratio (I_F/I_R) of up to 10³ was achieved at |V_SD| = 4 V, which is high enough for AC signal rectification.^5,6 A turn-on voltage (V_F) of 0.17 V and breakdown voltage (V_BR) of 27.2 V were observed at V_G = 0 V, as shown in Fig. 2(b). Fig. 2(c) shows that the I_D in a reverse bias region increases from 0.3 to 100 A/cm² as the Schottky barrier height formed in graphene and Ge is modulated from 0.4 to 0.17 eV by the gate bias.^18–21 The Schottky barrier height was extracted from the I-V characteristics and Table I presents the device parameters used for this calculation.^19,21 The rectification ratio of the graphene–Ge barristor was modulated from 3380 to 21 for the V_G ranging from 0 to 25 V, as shown in Fig. 2(d). The range of V_G used in this work is relatively high owing to a relatively thick gate dielectric, but it can be scaled down as necessary.¹⁶ 

The performance of the half-wave rectifier using the graphene–Ge barristor was measured using an oscilloscope (Agilent DSO7104A), a function generator (Agilent 81150A), parameter analyzer (Keithley 4200), capacitor C_L, and 1-MΩ load resistor R_L, using the set-up shown in Fig. 3(a). The parameter analyzer was used to apply the gate voltage to the barristor.

Fig. 3(b) shows the response characteristics of half- wave rectifier with different C_Ls. Although a slight voltage drop was observed owing to the high resistance of graphene–metal contacts and other series resistance components, the basic function of the barristor-based rectifier circuit was successfully demonstrated. The ripple voltage generated because of the charge and discharge of the capacitor decreased as the load capacitance increased. Fig. 3(c) shows the output waveforms as a function of the input AC frequency. As the input signal frequency increased from 1 kHz to 10 MHz, the transition from the output voltage peak to peak was smoothed to closely resemble the constant DC voltage with 100 pF capacitor. The peak rectified voltage decreased from 3.4 to 2.8 V when the input signal was modulated from 1 k and 14 MHz at100 pF, as shown in Fig. 4. At 14 MHz, the output voltage was reduced by approximately 30% compared to the input peak voltage.

Table II summarizes the performance of this emerging diode based rectifier that can be fabricated at low temperatures below 300 °C. The turn-on voltage of the barristor is relatively lower than the other devices owing to the Schottky junction. At input signal frequencies in the range of a few MHz, the barristor-based half-wave rectifier has better V_DC/V_AC peak ratio. Moreover, the frequency dependence of the output voltage in 1 kHz to 14 MHz range is competitive to the other examples.

Fig. 5(a) shows the final waveforms of rectified DC output obtained for 60 Hz and 1 MHz input signals with different gate bias conditions. A 100 μF smoothing capacitor (C_L) was used for the 60 Hz signal, and a 100 pF capacitor was used for the 1 MHz signal. As the gate bias decreased from 15 V to 0 V, the rectified DC output voltage could be modulated from 1.1 to 3.4 V at 60 Hz. At 1 MHz, the range of the output voltage decreased from 0.85 to 3.1V. Fig. 5(b) summarizes the range of DC output modulation range as a function of gate bias. The Schottky barrier height was changed by 0.4eV (∼62%) for a 70% DC output modulation level.

We demonstrated a microscale tunable DC power supply using a half-wave rectifier based on a graphene–Ge barristor. The DC output voltage could be controlled up to 70% of the initial output voltage. This device can be useful in tunable DC power supply circuits, especially for monolithically integrated circuits or flexible circuit applications.

This work was partially supported by Global Frontier Program through the Global Frontier Hybrid Interface Materials (GFHIM) (2013M3A6B1078873), Creative Materials Discovery Program on Creative Multilevel Research Center (2015M3D1A1068062) and Nano Materials Technology Development Program (2016M3A7B4909942) through the National Research Foundation (NRF) of Korea, funded by the Ministry of Science and ICT, Korea.