CN101917161A - A Frequency Doubler Based on Carbon Nanotube Field Effect Transistor - Google Patents

Abstract

The invention discloses a frequency multiplier based on a carbon nanotube field effect transistor. The core component of the frequency multiplier is a field effect transistor with a small bandgap carbon nanotube as the conductive channel, with the gate as the input terminal, the source terminal as the ground, and the drain terminal as the output terminal; the AC signal input at the input terminal is biased To the maximum resistance point of the DC transfer characteristic of the field effect transistor, a bias DC source is used at the output end to provide working power for the field effect transistor, so that frequency doubling can be realized. The frequency multiplier of the present invention has high conversion efficiency, high frequency response, large signal gain, low price, simple manufacturing process and no complicated post-processing circuit.

Description

A Frequency Doubler Based on Carbon Nanotube Field Effect Transistor

field of invention

The invention belongs to the technical field of nanoelectronics, relates to a carbon nanotube field effect transistor, and in particular to a frequency doubler based on a carbon nanotube field effect transistor.

Background technique

Nanoelectronics based on carbon materials, especially carbon nanotube (Carbon Nanotube)-based nanoelectronics, is considered to have great application prospects, especially in the field of radio frequency (RF) applications (Rutherglen, C.et al. Nature Nanotechnology, 2009, 4, 811-819.). Since carbon nanotubes were successfully prepared in 1991 (Iijima, S. Nature, 1991, 354, 56-58.), carbon-based electronics has made great progress. Electronics based on carbon nanotubes have attracted more and more attention due to their small size, high speed, low power consumption, and simple process. Generally speaking, carbon nanotubes can be divided into three categories based on their bandgap width. One is carbon nanotubes with large bandgap width (referring to semiconductor-type single-walled carbon nanotubes that can achieve a field-effect switching current ratio of more than 100 times. ), the second is carbon nanotubes without a bandgap (including metallic single-walled carbon nanotubes or multi-walled carbon nanotubes), and the third is carbon nanotubes with a small bandgap (which can be single-walled carbon nanotubes or have The multi-walled carbon nanotubes in the semiconductor shell can realize 1-100 times the field effect switching current ratio). The performance of field-effect transistors based on semiconducting carbon nanotubes has been pushed to the extreme, and finally a perfectly symmetrical complementary metal-oxide-semiconductor (CMOS) inverter (Zhang, Z.Y. et al.ACS Nano, 2009, 3, 3781-3187.). Metallic carbon nanotubes have also been proven to be very suitable for interconnection between field effect transistors, and have extremely high frequency response (Close, G.F. et al. Nano Letters, 2008, 8, 706-709.) . However, carbon nanotubes with small band gaps have not been widely used.

Frequency Multiplier (Frequency Multiplier) is a circuit that makes the output signal frequency equal to an integer multiple of the input signal frequency. Existing frequency multipliers include transistor frequency multipliers, varactor diode frequency multipliers, step recovery diode frequency multipliers, and the like. The existing frequency multiplier has complex circuits and requires many subsequent signal processing circuits, such as rectification circuits, filter circuits, etc., and the conversion efficiency is not very high, and the price of frequency multipliers working at extremely high frequencies (above 10GHz) Also very expensive.

Contents of the invention

The purpose of the present invention is to expand the application range of carbon nanotubes with small bandgap width, and realize a high-performance frequency doubler with a simple structure by using a field effect transistor based on carbon nanotubes with small bandgap width.

Carbon nanotubes with a small band gap can provide good bipolar properties, which can conduct both electrons and holes. The typical structure of a field effect transistor prepared with a carbon nanotube with a small bandgap width as a conductive channel includes a source terminal (ie, source electrode) and a drain terminal (ie, drain electrode) located at both ends of the conductive channel, and a gate located between the source and drain. (i.e. the gate electrode), between the gate and the conduction channel is an insulating layer, the transfer characteristic curve I _ds -V _gs (the situation where the drain terminal adopts a voltage bias) and V _ds -V _gs (the drain terminal) of the field effect transistor The case where the terminal adopts current bias) is shown in Fig. 1 and Fig. 2 respectively. It can be seen that the field effect transistor can exhibit good symmetrical bipolarity, and such performance is very suitable for use as a frequency doubler.

A field-effect transistor based on carbon nanotubes with a small band gap is used as the core of the device, and a high-performance frequency doubler can be simply realized. The frequency multiplier of the present invention includes a field effect transistor, an input terminal bias DC source and an output terminal bias DC source, wherein: the field effect transistor uses carbon nanotubes with a small bandgap width as a conductive channel, and the source terminal and The drain terminals are respectively located at both ends of the conductive channel, the gate is located between the source and drain, and the insulating layer is between the gate and the carbon nanotube as the conductive channel; the gate is the input terminal of the frequency doubler, the source terminal is grounded, and the drain terminal is The output terminal of the frequency multiplier; the input terminal bias DC source biases the AC signal input to the input terminal to the maximum resistance point of the DC transfer characteristic of the field effect transistor; and the output terminal bias DC source is connected to the output terminal, giving the field Effect transistors provide a working power supply.

The input terminal bias DC source is connected to the input signal source, and the input AC signal is biased by the input terminal bias DC source to the maximum resistance point of the DC transfer characteristic of the field effect transistor (that is, the lowest point of DC current, which is the work of the transistor) center point) and then input to the input. Driven by the bias DC source at the output terminal, the output signal can be collected at the output terminal (for carbon nanotube bipolar frequency multipliers, the larger the DC bias at the output terminal, the better, but the device’s The current withstand capability is limited to an appropriate range, otherwise it will cause permanent failure of the device).

The working circuit and basic principle of the bipolar frequency multiplier realized by the present invention can be shown in FIG. 3 . The core part of the circuit shown in Figure 3 is a field effect transistor (FET) 1 based on carbon nanotubes with a small band gap; the gate of the field effect transistor 1 is the input terminal of the frequency multiplier, and the input signal source is connected to a DC bias voltage 4 ( That is, the input terminal is biased to the DC source), the input signal is biased and then input to the input terminal; the source terminal is grounded; the drain terminal is the output terminal of the frequency multiplier, and the output terminal is connected to the DC current source 3 (ie, the output terminal is biased to the DC source) . Both the DC bias voltage 4 and the DC current source 3 are used to provide a DC operating point for the FET, wherein the DC bias voltage 4 is used to bias the V _gs of the input terminal, and the DC current source 3 is used to provide the I of the output terminal. _ds (Note: the DC current source 3 can also be replaced by a DC voltage source connected in series with a resistor, and for voltage bias, it is used to provide the V _ds of the output terminal). The rationale shown is based on the transfer characteristics (V _ds −V _gs ) shown in FIG. 2 . The input signal 2 is a sinusoidal signal with a circular frequency of ω and a peak-to-peak value of V _pp,in , while the output signal is biased by a current source 3 whose magnitude is I _dd . Through the transfer characteristics of the field effect transistor 1 (V _ds -V _gs ), we can get the center point of its work, and then bias the field effect transistor 1 to the working area through the DC bias voltage 4, and then the output can be passed through the oscilloscope 5 Collect the multiplied signal. The DC bias of the output terminal can also use voltage bias, so that a resistor needs to be connected in series with the output terminal, and then connected to the voltage source.

Specifically, when the input signal is at point A, the input voltage is at the working center point, so the resistance of the device is at its maximum value, then the output signal should also be at the maximum voltage point A'. When the input signal changes to point B, the output The voltage at the terminal gradually decreases, so that it reaches the minimum voltage at point B', and then the input signal returns to point C, and the output terminal also reaches the maximum voltage point C' again, and then the input signal reaches points D and E, correspondingly The output signal reaches points D' and E'. In this way, for an input signal of one cycle, there are two cycles of the output signal, so that the frequency multiplication is realized very simply. Moreover, due to the sub-linearity of the transfer characteristic near the working center point of the FET, the output signal can display a good waveform without filtering.

By cascading the frequency multipliers shown in Figure 3, that is, the output signal of the first-stage frequency multiplier is used as the input signal of the second-stage frequency multiplier, that is, the output of the first stage is connected to the input of the second stage In this way, at the output of the second stage, a signal with four times the frequency of the input signal of the first stage can be obtained. By analogy, signals of frequencies such as octave frequency and sixteen frequency frequency can be further obtained. Since the frequency multiplier based on carbon nanotubes has a large gain, it can realize multi-level cascading.

The core of the device is a field-effect transistor with a carbon nanotube as a conductive channel with a small bandgap width. This transistor has a small on-off ratio due to the small bandgap width of its conductive channel material (can achieve 1-100 times switch ratio). At the same time, it also has good bipolar conductivity, that is, the conductivity of electrons and holes is very good, and it has relatively good symmetry. The device is a typical nanometer field effect transistor. The source and drain terminals are used as the two ends of the conduction, and the gate is used as the switch terminal. The regulation and control of the conduction capacity of the conduction channel is realized by adjusting the gate voltage. The gate is not limited to the bottom gate or the top gate (independent gate), and may also be a ring gate and other various structures that can realize gate regulation. There is an insulating layer between the gate and the conductive channel, and regulation is realized through the capacitive effect of the insulating layer. And the structure of the whole device is not limited to the self-aligned structure or the non-self-aligned structure, as long as the structure that can realize the field effect can be used to realize the frequency doubler.

The material of the above-mentioned source terminal and drain terminal can be metal or other conductive materials, such as titanium, palladium, gold, etc.; the material of the gate can be metal or other conductive materials, such as titanium, palladium, tungsten nitride, etc.; the material of the insulating layer can be It is an oxide, nitride or oxynitride, such as silicon oxide, hafnium oxide, aluminum oxide, yttrium oxide, silicon nitride, etc.

When the frequency multiplier of the present invention is working, the grid AC input of the entire field effect transistor is biased to the lowest point of the DC current, that is, the maximum point of the channel resistance, which is the center point during operation, and then the source terminal is grounded, And add a DC current source or voltage source to the drain, and use an oscilloscope to detect the AC signal at this point, you can see that a signal with a frequency twice the input frequency can be observed, so it is very simple Frequency doubling is achieved.

Compared with the traditional frequency multiplier, the frequency multiplier based on the small bandgap carbon nanotube field effect transistor of the present invention is mainly reflected in:

1. The frequency multiplier is simple in structure, only needs a field effect transistor, and does not need complex parts of rectification or filtering. Due to the good bipolarity, the output waveform is simple and clean, and most signals are concentrated in the double frequency In terms of frequency, and the reduced process complexity reduces the cost;

2. The frequency response is high. Since the carriers in the carbon nanotubes are hardly scattered by the surface during the transport process, their mobility is very high, so there will be a high cut-off frequency;

3. The signal gain is large, and the ratio of the output signal to the input signal can be increased by increasing the size of the current source, thereby increasing the gain of the device, which is also conducive to the realization of cascading.

Description of drawings

Fig. 1 is a transfer characteristic (I _ds -V _gs ) curve of a field effect transistor based on a carbon nanotube with a small bandgap width, wherein Vds decreases from 1V to 0.1V from top to bottom, and each curve decreases by 0.1V.

Figure 2 is the transfer characteristic (V _ds -V _gs ) curve of a field effect transistor based on carbon nanotubes with small bandgap width, where I _ds is from 15 μA to 5 μA from top to bottom, and each curve decreases by 1 μA.

Fig. 3 shows the working principle and working circuit of the frequency doubler based on the small bandgap carbon nanotube field effect transistor of the present invention. Among them: points A', B', C', D', and E' in the output signal correspond to the same time points as points A, B, C, D, and E in the input signal; 1 is the core of the device—— Field effect transistor (FET); 2 is the input sinusoidal signal, the peak-to-peak value is V _{pp, in} , and the circular frequency is ω; 3 is the current source I _dd at the output terminal; 4 is the DC voltage bias V _{g, DC} of the input signal; 5 is an oscilloscope (OS) used for signal acquisition.

Fig. 4 is a field effect transistor with a bottom gate structure, wherein 11 is a conductive channel carbon nanotube, 12 and 13 are source and drain electrodes respectively, 14 is an insulating layer of a gate, and 15 is a silicon substrate.

5 is a field effect transistor with a top gate structure, wherein 21 is a conductive channel carbon nanotube, 22 and 23 are source and drain electrodes respectively, 24 is an insulating layer of a gate, 25 is a gate electrode, and 26 is an insulating substrate.

Fig. 6 has shown the spectrometer test circuit based on frequency multiplier of the present invention, and wherein 1 is the core of device---field effect transistor (FET); 2 is the sinusoidal signal of input, and peak-to-peak value is V _{pp, in} , and circle frequency is ω ; 3 is the current source I _dd of the output terminal; 4 is the DC voltage bias V _{g of the input signal, DC} ; 6 is the isolator (or bias device, Bias-T) of the DC and AC signals; 7 is the use of collecting AC signals spectrum analyzer (SA).

Fig. 7 is the frequency spectrum curve measured in embodiment 2.

Detailed ways

Below in conjunction with the accompanying drawings, the present invention will be further described in detail through examples, but the present invention is not limited in any way.

Example 1

The bottom gate structure field effect transistor as shown in Figure 4, with the carbon nanotube 11 on the insulating substrate as conduction channel; Two titanium electrodes on the carbon nanotube 11 are source electrode 12 and drain electrode 13 respectively; An oxidized silicon wafer, the silicon oxide obtained by thermal oxidation of the substrate surface is used as the gate dielectric 14 (that is, the insulating layer between the gate electrode 15 and the carbon nanotubes 11), and the heavily doped silicon substrate 15 of the base is used as the gate electrode . The specific preparation steps are as follows: form the shape of the source and drain electrodes by photolithography on the carbon nanotubes with small bandgap width distributed on the substrate, vapor-deposit a layer of 50 nanometer thick titanium metal layer as the source and drain electrode layer, and then The sample is put into acetone and peeled off, and the unnecessary metal layer is removed to obtain the source-drain metal electrode.

The prepared field effect transistor obtains its transfer characteristic curve through experimental measurement, and its operating point can be known, and then a 10kHz sinusoidal signal that has been biased to the operating point is input at the gate electrode 15, and the source electrode 12 is grounded. A 10μA DC current source is connected to the pole 13, and the drain electrode 13 is also connected to the oscilloscope at the same time, so that an approximately sinusoidal signal with a frequency of 20kHz can be seen on the oscilloscope, that is, frequency doubling is realized. (The specific circuit diagram is the same as that shown in Figure 3)

Example 2

The field effect transistor of the top gate structure as shown in Figure 5, with the carbon nanotube 21 of small bandgap width being distributed on the insulating substrate 26 as conduction channel, two palladium electrodes on the carbon nanotube 21 are respectively source electrode 22 and leakage current The electrode 23 and the gate dielectric layer 24 are hafnium oxide films obtained by atomic layer deposition, and the gate electrode 25 is metal titanium obtained by evaporation. Concrete preparation steps are as follows:

1. Form the gate shape by photolithography on the semiconductor carbon nanotubes, and grow a layer of hafnium oxide of about 15 nanometers by atomic layer deposition;

2. Immediately evaporate a layer of 20 nm thick metal titanium as the gate electrode layer by electron beam evaporation;

3. Put the sample into acetone and peel it off to prepare the gate electrode;

4. Form the shape of the source and drain electrodes by photolithography, vapor-deposit a layer of palladium metal layer with a thickness of 50 nanometers as the source and drain electrode layer, then put the sample into acetone and peel it off, and remove the unnecessary metal layer to obtain the source and drain metal electrodes .

Connect the device according to the circuit shown in Figure 6, and input a 1kHz sine wave signal 2 biased by a DC voltage 4 at the gate of the field effect transistor (FET) 1, where the bias voltage 4 needs to pass through the measurement field effect transistor The DC transfer characteristics are obtained. Then the drain end is connected to the DC current source 3 through a biaser (Bias-T) 6, and the AC signal is also coupled to the spectrum analyzer (SA) 7 through the biaser 6 at the same time, so that the output signal can be obtained spectrum. The frequency spectrum of the output signal is shown in Figure 7. It can be seen that more than 95% of the signal power is concentrated at the frequency multiplier 2kHz, which can show the high efficiency of this frequency multiplier.

Claims (6)

1. based on the application of field-effect transistor in frequency multiplier of little energy gap carbon nano-tube, wherein this field-effect transistor is a conductive channel with little energy gap carbon nano-tube, source end and drain terminal lay respectively at the two ends of conductive channel, between grid leaks in the source, grid and as being insulating barrier between the carbon nano-tube of conductive channel; Grid is as the input of frequency multiplier, source end ground connection, and drain terminal is as the output of frequency multiplier.

2. application as claimed in claim 1 is characterized in that, the field effect transistor switch current ratio of described field-effect transistor is 1-100.

3. frequency multiplier, comprise a field-effect transistor, an input bias direct current source and an output bias direct current source, wherein: described field-effect transistor is a conductive channel with little energy gap carbon nano-tube, source end and drain terminal lay respectively at the two ends of conductive channel, between grid leaks in the source, grid and as being insulating barrier between the carbon nano-tube of conductive channel; Grid is the input of frequency multiplier, source end ground connection, and drain terminal is the output of frequency multiplier; The AC signal that input bias direct current source will be input to input is biased to the resistance maximum point of the direct current transfer characteristic of described field-effect transistor; And output bias direct current source connects output, provides a working power to field-effect transistor.

4. frequency multiplier as claimed in claim 3 is characterized in that, the field effect transistor switch current ratio of described field-effect transistor is 1-100.

5. frequency multiplier as claimed in claim 3 is characterized in that, described input bias direct current source is a direct voltage source that connects input signal source.

6. frequency multiplier as claimed in claim 3 is characterized in that, described output bias direct current source is a DC current source, or the direct voltage source of the resistance of having connected.

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