JP3256965B2 - Method for deriving drain voltage or load line of field effect transistor - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a method of deriving a drain voltage of a field effect transistor (hereinafter referred to as an FET) operating at RF, calculating a drain current value from the voltage value, and connecting a load line of the FET from both values. How to get.

[0002]

2. Description of the Related Art Active load line measurement is an effective means for designing high power FETs. As a method to find this active load line, for example, J.
G. Leckey et.al, "A Vector Corrected Waveforms and
Load Line Measurement System for large Signal Trans
istor Characterization ", IEEE MTT-S Digest, p1243,
The 1995 literature is known. In this method, a voltage oscilloscope is used to directly observe the voltage waveform outside the tuner, and the internal load line is calculated from the S para of the tuner. In the case of this method, there is a problem that a signal component passing through the tuner is reduced, for example, when a harmonic is reflected on the FET by harmonic processing or the like, and a load line cannot be obtained with high accuracy.

[0003]

[0009] In addition to the above, FETs
There is substantially no method capable of measuring the load line described above. Therefore, a method of directly measuring the voltage waveform of the electrode of the FET and obtaining the current has been desired.

[0004]

SUMMARY OF THE INVENTION The present invention derives a relative drain voltage by measuring an electric field generated at a drain electrode of an FET operating in RF by electro-optic sampling, and calculates the relative drain voltage by using the FET. By comparing the direct current (DC) component and the minimum value of the voltage waveform obtained by adjusting the impedance given to the output side and overdriving the FET so that the minimum value of the RF drain voltage becomes 0 V, with the drain bias voltage, A method for deriving a drain voltage of an FET, comprising calculating an absolute drain voltage, and a method for calculating an absolute drain current from the absolute drain voltage, and deriving a load line from the voltage and the current. .

Hereinafter, the present invention will be described in detail. In order to derive the load line of an FET that operates RF, it is necessary to measure the voltage applied to the FET and the current flowing through the FET, but measure the values such as the drain current and drain voltage on a fine FET. It is virtually impossible to do. Therefore, it has not been possible conventionally to derive a load line derived from a current value or a voltage value. However, in order to properly design an integrated circuit or the like, it is essential to understand the characteristics of the FET, and a load line is indispensable for understanding the characteristics. An object of the present invention is to provide a method for deriving an absolute drain voltage that is indispensable for manufacturing the load line, and a method for obtaining the load line from a measured value of the absolute drain value. In the method of the present invention, first, electro-optic sampling (EO)
S, Electro Optic Sampling) to measure the electric field of the FET to obtain a relative voltage curve. This relative voltage curve is only a relative representation of the intensity and not an absolute value. Then, by applying a sufficiently high load impedance to the output side and determining the relative voltage waveform of the overdriven FET, the voltage of the relative voltage curve, that is, the absolute value of the drain voltage can be determined. Further, the value of the drain current can be derived from the absolute value of the drain voltage, and the load line can be obtained from both values.

[0006]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of the present invention will be described with reference to the accompanying drawings, but the embodiments do not limit the present invention. FIG. 1 is a schematic view showing an example of a drain voltage measuring device usable in the method of the present invention, FIG. 2 is a plan view of an FET in the device, and FIG. 3 is a relative view of an FET obtained by using the device of FIG. It is an example of a waveform of a drain voltage. A sample stage 3 is installed in the recess 1 of a base 2 having a recess 1 in the center, and an FET 4 is set on the sample stage 3. An EOS device 5 is provided above the FET 4.
Is fixed to the EOS device 5, and a connection line 6 hangs downward, and an optical crystal 7 such as ZeTe which exhibits a Pockels effect is located at the end of the connection line 6 so that on-wafer measurement of an electric field can be performed. It has become.

The optical crystal 7 is located just above the electrode of the FET 4 to be measured, in this embodiment, the drain (D) electrode, that is, the drain voltage waveform measurement point 8 shown in FIG.
A pair of auto-tuners (load-pull devices) 9 are located on the left and right of the EOS device 5 apart from each other, and the auto-tuners perform RF measurement of the FET 4. The data of the load of the auto-tuner is calibrated including harmonics. Each of the auto tuners 9 is electrically connected to a pair of probe heads 10a and 10b. One probe head 10a is connected to the drain (D) of the FET 4 and the other probe head 10b is connected to the source (S) of the FET 4. Close,
A required amount of impedance is applied between the input and output terminals of the FET 4, so that the FET 4 operates normally and can be overdriven. Normally, since the spatial resolution of the EOS method is determined by the beam diameter of a laser which enters the optical crystal and reads the change in the refractive index, it has a spatial resolution of several microns and measures the electric field intensity waveform on the drain electric field as shown in FIG. Becomes possible.

In this state, the electric field of the FET 4 that performs RF operation is measured by irradiating a pulse laser from the EOS device 5 to the optical crystal 7 at the tip. The change in the electric field strength is converted into a change in the refractive index of the optical crystal 7, and the change in the refractive index is measured by polarization spectroscopy with a laser. The electric field in the optical crystal 7 is integrated as the rotation of the polarization plane of the laser light, and a voltage waveform as shown in FIG. 3 is obtained. FIG. 3 is a measured value of the voltage waveform at the drain voltage waveform measurement point 8 shown in FIG. 2 at 2 GHz. This voltage waveform is a relative value in terms of the performance of the EOS device, and the vertical scale in FIG. 3 is not specified (numbers indicate relative values). This relative drain voltage is converted to an absolute drain voltage as follows.

First, the set current of the device shown in FIG. 1 is set to the class B operation, and the impedance is selected to be sufficiently high by the load-pull device 9 to overdrive the FET 4. As shown in FIG. 4, the operation of the FET at this time is as shown in FIG.
The drain current decreases in the order of load line 3 and load line 4,
When the impedance further increases and the impedance approaches infinity, the load line coincides with the horizontal axis of the graph. At this time, the bias point is the point shown in the figure.

The drain voltage waveform in the case of overdrive of the load line is a rectangular wave clipped at Vds = 0 V as shown in FIG. When this square wave is Fourier-expanded, Vds = VdsDC + Vds1sin (ωt + φ1)
+ Vds2sin (2ωt + φ2) + Vds3 sin (3ωt + φ3) + ・ ・
VdsDC, which is the first term, corresponds to the voltage in which the DC component is biased to the drain, that is, the middle voltage in FIG. 5, and the minimum signal voltage in FIG. 5 corresponds to Vds = 0, and FIG. ΔV = (DC component value of signal) − (minimum signal value) can be calculated.

A load line at a desired output load impedance can be obtained from ΔV. For example, the voltage waveform obtained in FIG. 3 is an EOS signal, but the DC component of the waveform is plotted as 0 for simplicity. Since the DC component of the waveform is the drain bias voltage, for example, if the set bias is 10 V, the EOS
A signal of 0 corresponds to 10V. Further, since the scale of the EOS signal is calculated by ΔV, an absolute value can be given. Thus, the absolute value of the drain voltage as shown in FIG. 6 can be measured.

Next, if the load pull device is calibrated so that the load impedance is known for each of the fundamental wave and the harmonic, and the load impedance of the nth harmonic is Zn, the current waveform is Ids = IdsDC +
Vds1sin (ωt + φ1-ang (Z1)) / | Z1 | + Vds2 sin (2
ωt + φ2−ang (Z2)) / | Z2 | +... At this time, IdsDC is a drain current component of DC. With this operation, a current waveform as shown in FIG. 7 is obtained. This FE
The load line at the time of overdrive of T is shown as a load line 12 in FIG. 8 on the DC drain current-voltage characteristic.

[0013]

According to the present invention, the relative voltage waveform of the drain electrode of a field-effect transistor operating at RF is measured by electro-optic sampling, and the relative drain voltage is adjusted by adjusting the impedance to overdrive the field-effect transistor. A method of calculating an absolute drain voltage by comparing the calculated drain bias voltage corresponding to a DC component of the voltage waveform obtained by the method, and calculating an absolute drain current from the absolute drain voltage; The method is characterized by deriving a line.
The present invention proposes a method of measuring a drain voltage of a FET which has not substantially existed and a method of deriving a load line. A voltage component obtained as a relative drain voltage is converted into a DC waveform of a voltage waveform obtained by overdrive of the FET. The relative drain voltage is displayed as an absolute drain voltage by comparing with a computable drain bias voltage corresponding to the component, and the drain current is also expressed as an absolute drain current value based on the relative drain voltage. Can,
This load line can greatly contribute to understanding the characteristics of the FET.

[Brief description of the drawings]

FIG. 1 is a schematic diagram showing an example of a load line measuring device that can be used in the method of the present invention.

FIG. 2 is a plan view of an FET in the device of FIG.

FIG. 3 is a diagram showing a waveform of a relative drain voltage of an FET obtained by using the device of FIG. 1;

FIG. 4 shows current when an FET is overdriven.
The figure which shows a voltage curve and a load line.

FIG. 5 is a diagram showing a waveform of a drain voltage when the FET of FIG. 4 is overdriven.

FIG. 6 is a view showing a waveform of an absolute drain voltage obtained by converting the relative drain voltage of FIG. 3;

FIG. 7 is a diagram showing a waveform of an absolute drain current obtained from the absolute drain voltage of FIG.

FIG. 8 is a diagram showing load lines of the FET obtained from the voltage waveforms and the current waveforms of FIGS. 6 and 7.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Concave hole 2 Base 3 Probe stage 4 FET 5 EOS device 6 Connection line 7 Optical crystal 8 Drain voltage waveform measurement point 9 Auto tuner (load pull device) 10a, 10b Probe head 11 Current-voltage curve 12 Load line

Claims (2)

(57) [Claims] 1. A relative voltage waveform of a drain electrode of a field effect transistor operating in RF operation is measured by electro-optic sampling, and the relative drain voltage is obtained by adjusting the impedance and overdriving the field effect transistor. A method for deriving a drain voltage of a field-effect transistor, comprising calculating an absolute drain voltage by comparing a drain bias voltage that can be calculated with a DC component of a waveform. 2. A voltage obtained by measuring a relative voltage waveform of a drain electrode of a field effect transistor operating in RF by electro-optic sampling, adjusting the impedance, and overdriving the field effect transistor. Calculating the absolute drain voltage by comparing the calculated drain bias voltage corresponding to the DC component of the waveform, calculating the absolute drain current from the voltage, and deriving the load line from the voltage and the current. A method for deriving a load line of a field effect transistor, which is a feature of the present invention.