Abstract:
A metal oxide semiconductor (MOS) transistor includes a source region having at least one source contact; a drain region having at least one drain contact; and a gate provided between the source region and the drain region, wherein the number of source contacts included in the source region is different from the number of drain contacts included in the source region.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
   The present application claims priority to Korean Application No. 10-2004-0113576, filed on Dec. 28, 2004, which is incorporated by reference. 
   BACKGROUND OF THE INVENTION 
   The present invention relates to a semiconductor device; and, more particularly, to a metal oxide semiconductor (MOS) transistor capable of preventing a saturation current variation. 
   Generally, a saturation current generated between a source and a drain of a metal oxide semiconductor (MOS) transistor is varied due to a temperature variation. That is, characteristics of an integrated circuit, such as a constant voltage generation circuit or a delay circuit, including the MOS transistor are changed due to the temperature variation. If the characteristics of the integrated circuit are excessively changed based on predetermined conditions such as temperature and pressure, the integrated circuit may not be normally operated. 
   Accordingly, during manufacturing steps, integrated circuits are tested at a severe temperature, e.g., −10° C. or 90° C., in order to screen out a defective circuit. Since the defective circuit cannot be used by users, it is desirable to produce a MOS transistor stably operated regardless of the temperature variation. 
     FIG. 1  is a schematic circuit diagram showing a conventional constant voltage generation circuit including conventional MOS transistors. 
   As shown, the conventional constant voltage generation circuit includes a first and a second n-type metal oxide semiconductor (NMOS) transistors M 1  and M 2 ; a first and a second p-type metal oxide semiconductor (PMOS) transistors M 3  and M 4 ; and a resistor R. 
   A source of the second NMOS transistor M 2  is coupled to a ground GND. A gate of the second NMOS transistor M 2  is coupled to a gate of the first NMOS transistor M 1 . The gate of the first NMOS transistor M 1  is also coupled to a drain of the first NMOS transistor M 1 . A source of the first NMOS transistor M 1  is coupled to a first terminal of the resistor R and a second terminal of the resistor R is connected to the ground GND. 
   Each source of the first PMOS transistor M 3  and the second PMOS transistor M 4  is connected to a power supply voltage VCC. Each gate of the first PMOS transistor M 3  and the second PMOS transistor M 4  is coupled to each other. A drain of the first PMOS transistor M 3  and a drain of the second PMOS transistor M 4  are respectively coupled to the drain of the first NMOS transistor M 1  and the drain of the second NMOS transistor M 4 . 
   The first and the second PMOS transistors M 3  and M 4  are served as a current mirror circuit for operating the first and the second NMOS transistors M 1  and M 2 . The MOS transistors M 1  to M 4  shown in  FIG. 1  are served as a Widlar-type current mirror circuit for generating a constant voltage VR 0 . 
   Herein, it is noted that the resistor R is connected between the first NMOS transistor M 1  and the ground GND. A role of the resistor R is described below. 
     FIG. 2  is a lay-out of each MOS transistor, e.g., M 1 , included in the conventional constant voltage generation circuit shown in  FIG. 1 . 
   As shown, the first NMOS transistor M 1  includes a gate line  201 , a source region  202  and a drain region  203 . 
   Each of the source region  202  and the drain region  203  includes a plurality of contacts, e.g., a contact  204  included in the source region  202  and a contact  205  included in the drain region  203 , for supplying a power and connecting internal elements. The gate line  201  also includes contacts, e.g.,  206 . Herein, it is noted that the number of contacts included in the source region  202  is same to the number of contacts included in the drain region  203 . 
   Further, a size of the first NMOS transistor M 1  is determined by a ratio of width to length. The length corresponds to a distance between the source region  202  and the drain region  203 , and the width corresponds to a length of a tangent line between the gate line  201  and the source region  202  or the drain region  203 . 
   Meanwhile, as above-mentioned, a saturation current of a conventional MOS transistor having a structure such as the lay-out shown in  FIG. 2  can be abnormally changed due to the unstable temperature. Accordingly, the conventional constant voltage generation circuit having the conventional MOS transistor can operate abnormally. 
   Therefore, for compensating the change of the performance of the above constant voltage generation circuit, it is necessary that the resistor R is connected between the first NMOS transistor M 1  and the ground GND. That is, for reducing the change of the characteristics caused by the temperature variation, the resistance of the resistor R and the size of the first NMOS transistor M 1  are appropriately adjusted. 
   BRIEF SUMMARY OF THE INVENTION 
   Embodiments of the present invention relate to providing a metal oxide semiconductor (MOS) transistor which is capable of stably operating in response to a temperature variation. 
   In one embodiment of the present invention, a metal oxide semiconductor (MOS) transistor includes a source region having at least one source contact; a drain region having at least one drain contact; and a gate, which is connected between the source region and the drain region, wherein the number of source contacts included in the source region is different from the number of drain contacts included in the source region. 
   In another embodiment of the present invention, there is provided a constant voltage generation circuit, including: a first MOS transistor for receiving a power supply voltage; a second MOS transistor for receiving the power supply voltage, a gate of the second MOS transistor being coupled to a gate of the first MOS transistor; a third MOS transistor connected between the first MOS transistor and a ground; and a fourth MOS transistor connected between the second MOS transistor and the ground, a gate of the third MOS transistor being coupled to a gate of the fourth MOS transistor, wherein each MOS transistor includes a source region having at least one source contact, a drain region having at least one drain contact and a gate, the number of source contacts included in the source region being different from the number of drain contacts included in the drain region. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and other objects and features of the present invention will become apparent from the following description of preferred embodiments taken in conjunction with the accompanying drawings, in which: 
       FIG. 1  is a schematic circuit diagram showing a conventional constant voltage generation circuit; 
       FIG. 2  is a lay-out of a MOS transistor included in the conventional constant voltage generation circuit shown in  FIG. 1 ; 
       FIG. 3  is a lay-out showing a MOS transistor in accordance with a preferred embodiment of the present invention; 
       FIG. 4A  is a graph demonstrating a first simulation result of resistance characteristics of an NMOS transistor in accordance with the preferred embodiment of the present invention; 
       FIG. 4B  is a graph demonstrating a second simulation result of resistance characteristics of a PMOS transistor in accordance with the preferred embodiment of the present invention; 
       FIG. 5A  is a graph demonstrating a third simulation result of a source-drain current and a gate-source voltage of the NMOS transistor shown in  FIG. 4A ; 
       FIG. 5B  is a graph demonstrating a fourth simulation result of a source-drain current and a gate-source voltage of the PMOS transistor shown in  FIG. 4B ; 
       FIG. 6A  is a table showing a fifth simulation result of a saturation current variation of the NMOS transistor shown in  FIG. 4A ; 
       FIG. 6B  is a table showing a sixth simulation result of a saturation current variation of the PMOS transistor shown in  FIG. 4B ; 
       FIGS. 7A and 7B  are graphs respectively showing simulated results shown in  FIGS. 6A and 6B ; and 
       FIG. 8  is a schematic circuit diagram showing a constant voltage generation circuit in accordance with the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Hereinafter, a metal oxide semiconductor (MOS) transistor in accordance with the present invention will be described in detail referring to the accompanying drawings. 
     FIG. 3  is a diagram showing a lay-out of a MOS transistor in accordance with a preferred embodiment of the present invention. 
   As shown, the MOS transistor includes a gate line  301 , a source region  302  and a drain region  303 . Each of the source region  302  and the drain region  303  includes at least one contact, e.g.,  304  included in the source region  302  and  305  included in the drain region  303 , for supplying a power and connecting internal elements. 
   A size of the MOS transistor is determined by a ratio of width to length as shown in  FIG. 3 . 
   Herein, in comparison with the conventional MOS transistor, the number of contacts included in the source region  302  is smaller than the number of contacts included in the drain region  303 . Further, the length is smaller than that of the conventional MOS transistor. 
   As the number of contacts of the source region  302  is decreased, the source contact resistance is increased. Since the source contact resistance is increased, the source contact resistance is relatively more sensitive to a temperature variation. That is, a change of characteristics of the MOS transistor caused by the temperature variation is compensated by a change of the source contact resistance. Herein, although a current flow is decreased as the source contact resistance is increased, the current reduction is compensated by decreasing the length. 
     FIG. 4A  is a graph demonstrating a first simulation result of resistance characteristics of an n-type metal oxide semiconductor (NMOS) transistor in accordance with the preferred embodiment of the present invention when the temperature is variously changed. 
   Herein, it is assumed that a ratio of width to length, i.e., W/L, is 10/1. A channel resistance is marked as “1/gmmax_n(W/L=10/1)” and a source contact resistance is marked as “BLC-N+Rc — 1EA”. The source contact resistance 1/gmmax_n(W/L=10/1) shows a resistance of one source contact. A resistance of a source region is marked as “N+Rsh”. 
     FIG. 4B  is a graph demonstrating a second simulation result of resistance characteristics of a p-type metal oxide semiconductor (PMOS) transistor in accordance with the preferred embodiment of the present invention when the temperature is variously changed. 
   Herein, it is assumed that a ratio of width to length, i.e., W/L, is 20/1. A channel resistance is marked as “1/gmmax_p(W/L=20/1)” and a source contact resistance is marked as “BLC-P+Rc — 1EA”. The source contact resistance 1/gmmax_p(W/L=20/1) shows a resistance of one source contact. A resistance of a source region is marked as “P+Rsh”. 
   Referring to  FIGS. 4A and 4B , each source contact resistance of the NMOS transistor and the PMOS transistor is decreased as the temperature is increased. 
     FIG. 5A  is a graph demonstrating a third simulation result of a source-drain current Id and a gate-source voltage Vgs of the NMOS transistor when the temperature is changed. 
   As shown, the source-drain current Id is decreased as the temperature is increased at the gate-source voltage Vgs. 
     FIG. 5B  is a graph demonstrating a fourth simulation result of a source-drain current Id and a gate-source voltage Vgs of the PMOS transistor when the temperature is changed. 
   As shown, the source-drain current Id is decreased as the temperature is increased at the gate-source voltage Vgs. 
   Referring to  FIGS. 4A to 5B , a saturation current variation and a source contact variation caused by the temperature variation can be understood. By appropriately adjusting the number of source contacts based on the data, the saturation current variation caused by the temperature variation can be compensated. 
     FIG. 6A  is a table showing a fifth simulation result of a saturation current variation of the NMOS transistor according to the number of source contacts and the temperature variation. 
   As shown, if the number of the source contacts is  30 , the saturation current variation between a maximum temperature and a minimum temperature is 27 μA. However, if the number of the source contacts is 1, the saturation current variation between the maximum temperature and the minimum temperature is 2.5 μA. 
     FIG. 6B  is a table showing a sixth simulation result of a saturation current variation of the PMOS transistor according to the number of source contacts and the temperature variation. 
   As shown, if the number of the source contacts is 30, the saturation current variation between a maximum temperature and a minimum temperature is 1.0 μA. However, if the number of the source contacts is 1, the saturation current variation between the maximum temperature and the minimum temperature is 0.3 μA. 
     FIGS. 7A and 7B  are graphs respectively showing simulation results shown in  FIGS. 6A and 6B . As shown, each saturation current variation (Idsat) according to the temperature variation is decreased as the number of source contacts is decreased. 
     FIG. 8  is a schematic circuit diagram showing a constant voltage generation circuit in accordance with the present invention. 
   As shown, the constant voltage generation circuit includes a first NMOS transistor M 901 , a second NMOS transistor M 902 , a first PMOS transistor M 903  and a second PMOS transistor M 904 . Herein, the number of source contacts included in each MOS transistor of the constant voltage generation circuit is appropriately determined for compensating a saturation current variation between a source and a drain caused by a temperature variation. Each MOS transistor included in the constant voltage generation circuit has a lay-out shown in  FIG. 3 . 
   In detail, a source of the first PMOS transistor M 903  is coupled to a power supply voltage VCC and a gate of the first PMOS transistor M 903  is coupled to a gate of the second PMOS transistor M 904 . Similarly, a source of the second PMOS transistor M 904  is coupled to the power supply voltage VCC. The gate and the drain of the second PMOS transistor M 904  are coupled each other. 
   A drain of the first NMOS transistor M 901  is coupled to the drain of the first PMOS transistor M 903  and a source of the first NMOS transistor M 901  is coupled to a ground GND. The drain and the gate of the first NMOS transistor M 901  are coupled each other. Similarly, a drain of the second NMOS transistor M 903  is coupled to the drain of the second PMOS transistor M 904  and a source of the second NMOS transistor M 903  is coupled to the ground GND. 
   A constant voltage VR 0  is generated at a node commonly coupled by the drain of the first NMOS transistor M 901  and the drain of the first PMOS transistor M 903 . 
   As a result, by determining the number of source contacts included in each MOS transistor of the constant voltage generation circuit for compensating the saturation current, the constant voltage VR 0  can be stably generated not using a temperature variation compensating resistor, e.g., R shown in  FIG. 1 . 
   The MOS transistor in accordance with the present invention can be used not only for a constant voltage generation circuit but also for other integrated circuits. 
   Accordingly, in accordance with the present invention, it is possible to reduce the number of defective integrated circuits, whereby a cost of production can be reduced. 
   The present application contains subject matter related to Korean patent application No. 2004-113576, filed in the Korean Patent Office on Dec. 28, 2004, the entire contents of which being incorporated herein by reference. 
   While the present invention has been described with respect to the particular embodiments, it will be apparent to those skilled in the art that various changes and modifications may be made without departing from the spirit and scope of the invention as defined in the following claims.