Patent Publication Number: US-2003234426-A1

Title: Electrostatic discharge protection device

Description:
BACKGROUND OF THE INVENTION  
       [0001] 1. Field of the Invention  
       [0002] The present invention relates generally to an electrostatic discharge (ESD hereinafter) protection device and more particularly to an ESD protection device with high voltage pad. The holding voltage of the ESD protection device is increased by separating the drain and inserting an N well between the separated drains.  
       [0003] 2. Description of the Related Art  
       [0004] In memory products, especially for EPROM and Flash memory, high voltage has to be applied to perform special applications such as program/erase. Possible overshoot may occur when the high voltage is applied. Thus, the high voltage pad should be capable of withstanding such an overshoot. On the other hand, this high-voltage pad should also have enough ESD performance to resist high current, fast transient ESD events.  
       [0005]FIG. 1 shows a block diagram of the input terminal of a memory device. Generally, the ESD protection device  12  is set between a pad  10  and a memory device  14 .  
       [0006]FIG. 2 shows a sectional view of the conventional ESD protection device. The N-type doped regions  23  and  24  are formed on the P-type substrate  22 , wherein the P-type substrate  22  and the N-type doped region  24  are coupled to the ground level, and the N-type doped region  23  is coupled to a pad  20  providing the outside signals.  
       [0007] In addition, the N-type doped region  23 , the P-type substrate  22  and N-type doped region  24  construct a bipolar junction transistor (BJT hereinafter) Q 0 . When the ESD event occurs at the pad  20 , a large ESD current turns on the parasitical BJT Q 0  of the ESD protection device and flows to the ground through the substrate  22  to prevent ESD damage to the memory device.  
       [0008]FIG. 3 shows the drain current of the conventional ESD protection device against the input voltage. Under ESD events, when the voltage level of the pad is higher than the trigger voltage, the ESD protection device is enabled and immediately goes to snapback mode with a low holding voltage to decrease the voltage level of the pad to prevent ESD damage to the memory device. Thus, the low holding voltage is beneficial to ESD resistivity.  
       [0009] However, under normal high-voltage application, when the voltage overshoot larger than the trigger voltage, the ESD device would be also enabled unexpected and goes to snapback mode with low holding voltage. But when high voltage returned to its stable state, the ESD device might be still in its snapback mode because holding voltage was lower than stable high-voltage state. Thus, the drain current of the BJT increases with the increasing voltage and reaches to I HV  as shown in FIG. 3. The large drain current combined with high voltage will damage the ESD protection device due to the high power. Accordingly, the problem occurs when the high level signal of the conventional ESD protection device is higher than the holding voltage. Therefore, the conventional ESD protection device in holding mode outputs large current when receiving high level signals and will be damaged by the large current.  
       SUMMARY OF THE INVENTION  
       [0010] The object of the present invention is to provide an ESD protection device having a holding voltage higher than the high level signal received by the pad to avoid the problem mentioned above. In addition, the ESD protection device of the present invention adds another discharge path to increase discharge effect.  
       [0011] To achieve the above-mentioned object, the present invention provides an ESD protection device set between a memory device, a second voltage level and a pad coupled to a first voltage level. The ESD protection device includes the following elements. A first second type doped region is formed on the first type substrate and coupled to the first voltage level. A second second type doped region is formed on the first type substrate and coupled to the second voltage level. A third second type doped region is formed on the first type substrate. A second type well is formed between the first second type doped region and the third second type doped region. An isolation element is formed between the second second type doped region and the third second type doped region. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0012] The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings, given by way of illustration only and thus not intended to be limitative of the present invention.  
     [0013]FIG. 1 shows a block diagram of an input terminal of a memory device having an ESD protection device.  
     [0014]FIG. 2 shows a sectional view of a conventional ESD protection device.  
     [0015]FIG. 3 shows the drain current of the conventional ESD protection device against the input voltage.  
     [0016]FIG. 4 shows a sectional view of an ESD protection device according to the embodiment of the present invention.  
     [0017]FIG. 5 shows the drain current of the ESD protection device against the input voltage according to the embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
     [0018]FIG. 4 shows a sectional view of an ESD protection device according to the embodiment of the present invention. The ESD protection device according to the embodiment of the present invention is set between the pad  30  coupled to the input signal S IN  and the memory device  34  and the ground level.  
     [0019] The P-type substrate  41  is coupled to the ground level. The first N-type doped region  42  is formed on the P-type substrate  41  and is coupled to the input signal S IN  through the pad  30 . The second N-type doped region  43  is formed on the P-type substrate  41  and is coupled to the ground level. In addition, the third N-type doped region  45  is also formed on the P-type substrate  41 .  
     [0020] The N-type well  46  is formed between the first N-type doped region  42  and the third N-type doped region  45 . Therefore, the N-type well  46  forms the resistance between the first N-type doped region  42  and the third N-type doped region  45 . In addition, an isolation element  48  is formed between the second N-type doped region  43  and the third N-type doped region  45 . The isolation element  48  is a shallow trench or a field oxide formed by LOCOS.  
     [0021] In addition, the second N-type doped region  43 , the P-type substrate  41  and the third N-type doped region  45 , form a parasitical bipolar junction transistor Q 1 , and the first N-type doped region  42 , the P-type substrate  41  and the second N-type doped region  43 , form another parasitical bipolar junction transistor Q 2 .  
     [0022] When an ESD event occurs at the pad  30 , the ESD current flows through the first N-type doped region  42 , the N-type well  46  and the third N-type doped region  45  and causes the junction of the collector and the base of the bipolar junction transistor Q 1  voltage breakdown to turn on the bipolar junction transistor Q 1 . In addition, when the bipolar junction transistor Q 1  is turned on, the large current flowing through the N-type well  46  turns on the bipolar junction transistor Q 2 . According to the ESD protection device of the invention, the N-type well  46  formed between the first N-type doped region  42  and the third N-type doped region  45  increases the resistance between the first N-type doped region  42  and the third N-type doped region  45 . Therefore, the holding voltage is increased. In addition, the depth of the bottom of the N-type well  46  is adjusted to obtain the demanded ESD protection effect.  
     [0023]FIG. 5 shows the drain current of the ESD protection device against the input voltage according to the embodiment of the present invention. When the voltage level of the pad  30  is higher than the trigger voltage, the ESD protection device is enabled and immediately goes to snapback mode to prevent ESD damage to the memory device. In FIG. 5, the holding voltage is higher than the high level signal received by the pad  30  when operating in normal mode. Therefore, the drain is independent of the high level signal received by the pad  30  when operating in normal mode.  
     [0024] Moreover, the ESD protection device according to the embodiment of the present invention comprises two discharge paths, the paths through the bipolar junction transistors Q 1  and Q 2 . Therefore, the ESD protection effect is improved even when the holding voltage is increased. In addition, the large N-type well  46  can withstand a large ESD current, thus the ESD protection effect is further improved.  
     [0025] The foregoing description of the preferred embodiments of this invention has been presented for purposes of illustration and description. Obvious modifications or variations are possible in light of the above teaching. The embodiments were chosen and described to provide the best illustration of the principles of this invention and its practical application to thereby enable those skilled in the art to utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated. All such modifications and variations are within the scope of the present invention as determined by the appended claims when interpreted in accordance with the breadth to which they are fairly, legally, and equitably entitled.