Abstract:
A method and a circuit for extracting current-voltage characteristics employ two pulse signals with different duty cycles into a device to be measured in order to extracting current-voltage characteristics of the device to be measured. The present invention may reduce the self-heating effect of the device to be measured and increase the measurable range of the device to be measured.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of Invention 
   The present invention relates to the measuring method and circuit for electrical characteristics, and in particular to the measuring method and circuit for, current-voltage characteristics. 
   2. Description of Related Art 
   Conventionally, the measurement of the device parameters under the static bias is affected by the factors of self-heating effect, causing a difference between the measured result and the ideal value. The self-heating effect is a phenomenon that the device current decreases due to the decrease of mobility of electrons in the device when the device temperature rises. Particularly, the self-heating effect is more obvious for the high bias or high power devices such as Laterally Diffused Metal Oxide Semiconductor (LDMOS) devices and Silicon On Insulator (SOI) devices. 
   As an example to illustrate the self-heating effect conveniently, all the to-be-measured devices in the following drawings are N-channel Metal Oxide Semiconductor (NMOS) transistor.  FIG. 1  is a characteristic curve for drain current versus drain-source voltage of the transistor with/without self-heating effect. It is evident that the self-heating effect causes the decrease of drain current. 
     FIG. 2A  is a circuit drawing, schematically illustrating a conventional measuring circuit for the current-voltage characteristics of transistor. The measuring process is first, applying a constant voltage V IN  to the gate of the transistor  201 . Next, by changing the voltage of power source V DD  and measuring the voltage V OUT  between the drain and the source, the drain current Id=(VDD−Vout)/R can be calculated out. Thus, the characteristic curve for the drain current versus the drain-source voltage Vout under the applying the gate voltage can be obtained. The shortcoming of this method is that the self-heating effect causes the drain current to decrease for the high power circuit. 
   In order to reduce the self-heating effect, the disclosure in EEE Electron Device Letters Vol. 16 No. 4 pp. 145-147 has provided an equivalent circuit to the circuit of  FIG. 2A  of the measuring circuit for measuring the current-voltage characteristic curve of a transistor except that the clock signal as shown in  FIG. 2B  is instead of a static voltage applied to the gate. The measuring process is first applying a clock signal Vin to the gate of transistor  201 . Next, by changing the voltage of the power source VDD and measuring drain-source voltage Vout, then the drain current Id=(VDD−Vout)/R can be calculated to obtain the characteristics curve of drain current versus drain-source voltage under the gate voltage Vg. In order to obtain the intended precision for the current, the resistance R should be large in selection. In above measuring process, even though the temperature gradually rises due to the transistor being at high bias when the clock signal is at the high voltage level (HI), the temperature gradually decreases due to the transistor being at the off state by the clock signal at the low voltage level. Therefore, it is possible to reduce the self-heating effect by a proper duty cycle, which is the ratio of the quantity of time of the clock signal HI to the quantity of time of the clock period T, since the heat accumulation on the transistor is not as fast as the prompt change of clock signal. A shortcoming of above measuring method is that a breakdown may occur on the transistor. For example, in order to let the Vout be the maximum operation voltage od the transistor, VDD=Vout+Id*R. When the clock signal is al low voltage level (LO), the drain electrode has the VDD=Vout+Id*R, then it cause the breakdown on the junction between the drain electrode and the substrate. 
   SUMMARY OF THE INVENTION 
   An aspect of the present invention is to provide a measuring method for the current-voltage characteristics, to measure the current-voltage characteristics of a device to be measured. As a result, the self-heating effect can be reduced and the measurable range of the device to be measured can increase. 
   Another aspect of the present invention is to provide a measuring circuit for the current-voltage characteristics, used to measure the current-voltage characteristics of a device to be measured. By means of such a circuit, the self-heating effect is reduced and the measurable range of the device to be measured increases. 
   The present invention provides a measuring circuit for the current-voltage characteristics to measure the current-voltage characteristics of a device to be measured comprises following steps. First, a first voltage and a first clock signal are applied to the device to be measured, wherein the first clock signal has a first duty cycle, wherein a first current value is obtained for the device to be measured. Then, the first voltage and a second clock signal are applied to the device to be measured, wherein the second clock signal has second duty cycle, different from the first duty cycle, and a second current is obtained on the measured device. Then, the first current, the second current, the first duty cycle, the second duty cycle, and the period of the clock signal are used for calculation, so as to obtain the expected current on the measured device when the first voltage is applied. 
   In accordance with the preferred embodiment of the present invention about the measuring method for the current-voltage characteristics, when I 1  represents the first current, I 2  represents the second current, W 1 /T represents the first duty cycle, and W 2 /T represents the second duty cycle, the expected current is Io=(I 2 −I 1 )/(W 2 −W 1 ), in which W 1 , W 2 , and T are real number. In addition, the foregoing device to be measured can be a transistor device. 
   The present invention also provides a measuring circuit for current-voltage characteristics, used to measure the current-voltage characteristic of a device to be measured. This measuring circuit comprises a clock signal generator and a voltage generator. The clock signal generator is coupled to the device to be measured, also called measured device, to generate a first clock signals and a second clock signal, at different time so as to drive the measured device. The first clock signal has a first duty cycle, and the second clock signal has a second duty cycle, and the first duty cycle is different from the second duty cycle. The voltage generator is coupled to the measured device, for setting and applying a voltage to the measured device. In the measuring process, when the measured device is driven by the first clock signal, the first current is generated due to the first voltage and the first clock signal. Also and, when the measured device is driven by the second clock signal the second current is generated due to the first voltage and the second clock signal. The expected current of the measured device can be obtained by means of calculation based on the first current, the second current, the first duty cycle, the second duty cycle, and period of the clock/ signal. Moreover, the aforesaid measured device includes a transistor device. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. The drawings illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention. 
       FIG. 1  is a drawing, schematically illustrating a characteristic curve for drain current versus drain-source voltage of the transistor with/without self-heating effect. 
       FIGS. 2A-2B  are a circuit diagram and a clock signal drawing, schematically illustrating a measuring circuit for the current-voltage characteristics of transistor and the clock signals. 
       FIGS. 3A-3B  are a circuit diagram and a clock signal drawing, schematically illustrating a measuring circuit for measuring the current-voltage characteristics for a transistor, and the needed clock signals with different duty cycles, according to the embodiment of the present invention. 
       FIG. 4  is a drawing, schematically illustrating a flow-process diagram of measuring circuit for the current-voltage characteristics of transistor, according to the embodiment of the present invention. 
       FIG. 5  is a drawing, schematically illustrating a simulated curve of the current-voltage characteristics of transistor, according to on the embodiment of the present invention. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Following preferred embodiment, in which the measured device is an N-channel Metal Oxide Semiconductor (NMOS) transistor as an example, illustrates clearly the measuring method and circuit embodying the present invention. Any person who is familiar with ordinary electronic measurement can similarly apply into other kinds of transistor or electronic device. For example, the following embodiment can be applied to any device to be tested, which has a control terminal and at least one signal terminal. The signal terminal of the device to be tested is controlled according to the voltage level on the control terminal.  FIG. 3A  is the circuit diagram schematically illustrating a measuring circuit for measuring the current-voltage characteristics for a transistor, according to the preferred embodiment of the invention.  FIG. 3   b  is a drawing, schematically illustrating the timing diagram for two the clock signals with different duty cycles, applied to the gate of transistor  301  in  FIG. 3A . Vg is the voltage level of the clock signal at the high voltage level, that is, the gate voltage of the transistor  301  in  FIG. 3A . W 1  and W 2  respectively represent the enabling time for the two clock signals, and T represents the clock period for the two clock signals. 
     FIG. 4  is a drawing, schematically illustrating a flow-process diagram of measuring circuit for the current-voltage characteristics of transistor, according to the embodiment of the present invention. Referring to  FIGS. 3A ,  3 B and  4 , in step S 410 , the voltage generator  310  provides a voltage applied to the drain of the transistor  301 , that is, the signal terminal. The clock generator  320  provides a first clock signal Vin, of which the duty cycle is W 1 /T, to the gate of the transistor  301 , that is, the control terminal. Then, a first current I 1  is obtained by measuring the measured a drain current of the transistor  301  (Step S 411 ). Then, the voltage generator  310  supplies a voltage to the drain of the transistor  301 , the clock generator  320  supplies a second clock signal (of which the duty-cycle is W 2 /T, and W 2 /T≠W 1 /T) to the gate of transistor  301  (Step  420 ). Meanwhile, the transistor  301  is measured to obtain the second current value T 2 , that is, the drain current (Step S 421 ). 
   Since I 1 =(W 1 /T)Io and I 2 =(W 2 /T)/Io, wherein Io denotes the current value when applying a constant voltage Vg on the gate without self-heating effect, the two equations I 1 =W 1 /T)Io and I 2 =(W 2 /T)/Io can be solve in relation, so that the drain current Io=(I 2 −I 1 )T/(W 2 −W 1 ) on the transistor without self-heating effect can be obtained when the transistor is applied with the first voltage at the drain and the constant voltage Vg at the gate (Step S 430 ). 
     FIG. 5  is a simulated result, of the current-voltage characteristics of the transistor, under various combination of different duty cycle for the circuit in  FIG. 3 . The simulated result shows that the higher the drain current is, the more obvious the reduction of the self-heating effect is. Moreover, the influence of self-heating effect can be reduced by mean of clock measurement technique. Particularly, when the enabling time period of the clock signal is shorter, the self-heating effect is more capably reduced. 
   In summary, the influence of the self-heating effect on the measured device can be reduced by using the clock signal and two clock signals with different duty cycles are used to respectively drive the measured device, to respectively obtain current of the measured device. Also and, the two current are used for calculating out the expected current. This method of the invention can increase measuring range of, the measured device. 
   It will be apparent to those skilled in the art that various modifications and variations can be made to the structure of the present invention without departing from the scope or spirit of the invention. In view of the foregoing descriptions, it is intended that the present invention covers modifications and variations of this invention if they fall within the scope of the following claims and their equivalents.