BACKGROUND OF THE INVENTION 
     a. Field of the Invention 
     The present invention relates to semiconductor devices and, more particularly, to a method and circuit for increased device reliability through protection against the effects of hot carriers. 
     b. Description of the Background 
     Advances in the semiconductor industry continue to provide smaller device geometries. As geometries have been reduced, some failure mechanisms have become more pronounced. One of the most significant contributors to device failure in sub-micron range devices is hot carrier injection (HCI), also referred to as hot carrier effect. Hot carrier injection is an effect where high-energy charges are injected into the gate dielectric of a MOSFET device and may become lodged in the dielectric. Trapped charges tend to accumulate over time and affect the turn-on voltage, and drain current of transistors and may eventually lead to the failure of the device. Carrier injection is a function of field strength between the source and drain of a transistor. Field strength is a function of the physical distance and voltage difference between source and drain channels. The reduction in geometries of semiconductor devices has been accompanied by a reduction in the operating voltage of the device. Many logic devices that operated at 5 volts a number of years ago now operate at 3.3 volts or less. 
     Many devices have input/output buffers that meet TTL specifications. However, many of these devices generate an output voltage level that is close to the positive supply voltage when generating a logic one voltage. When a device having a 5 volt supply is interfaced with a device having a 3.3 volt power, a voltage near 5 volts may be presented to the 3.3 volt device as a logic one level. The input structure of the 3.3 volt device may employ diodes to accommodate the 5 volt level. Output drivers for a 3.3 volt device, and more specifically n-channel devices employed to drive a logic zero value, however, may experience a voltage difference near 5 volts. This higher voltage results in higher field strength for n-channel devices and may result in higher HCI rates and reduced device reliability. 
     One approach to reducing HCI effects is to employ longer channel transistors. A longer channel device must be wider in order to provide the same current capabilities as a shorter channel device, resulting in greater area being employed for the drive transistors and higher cost for the device. A new method is needed that provides the reduced HCI susceptibility of longer channel devices but without the area and cost penalties of longer devices. 
     SUMMARY OF THE INVENTION 
     The present invention overcomes the disadvantages and limitations of the prior art by first employing a long channel transistor to discharge a signal node to a voltage level at which shorter channel devices may be enabled. This provides lower field strength across the short channel device, reducing charges deposited in the dielectric layer. The long channel transistor is less susceptible to HCI damage such that the long and short channel transistor combination provides higher device reliability. 
     The invention therefore may comprise a high reliability output driver comprising: a first transistor having a first gate channel length, the first transistor being operatively connected between an output node and a first voltage terminal, a second transistor having a second gate channel length wherein the second gate channel length is greater than the first gate channel length, the second transistor being operatively connected between the output node and the first voltage terminal, a drive signal line connected to the gate of the first transistor, and a delay element having an input connected to the drive signal line and an output connected to the gate of the second transistor. 
     The present invention employs a long channel device that is designed to operate across a first voltage range, such as zero volts to 5.25 volts, for example, and employs a short channel transistor that is designed to operate at a second voltage range, such as zero volts to 3.5 volts, for example. The two-transistor architecture of the present invention may be employed to charge or discharge a signal node with lower surge current than may be exhibited by a single transistor. The method of the present invention is applicable to external driver circuitry and to internal circuitry where different voltage levels may be encountered. 
     The invention may further comprise a method for increased reliability in a semiconductor driver comprising: connecting a first transistor between a voltage terminal and an output node, connecting a second transistor between the voltage terminal and the output node wherein the second transistor has a gate channel length that is less than the gate channel length of the first transistor, applying a first gating signal to the gate of the first transistor, applying the first gating signal to the input of a delay element to generate a delayed output, and applying the delayed output to the gate of the second transistor. 
     Advantageously, the present invention provides higher reliability and desirable drive characteristics without incurring the space and cost penalties of large long channel devices. 
    
    
     DESCRIPTION OF THE FIGURES 
     In the figures, 
     FIG. 1 depicts an n-channel transistor. 
     FIG. 2A depicts an n-channel transistor with HCI charge accumulation. 
     FIG. 2B illustrates that the trapped charge area does not scale with transistor channel length. 
     FIG. 3A depicts interconnection between a 5 volt device and a 3.3 volt device. 
     FIG. 3B provides an example signal sequence for a signal line interfacing a 5-volt device and a 3.3-volt device. 
     FIG. 4 depicts a circuit embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Hot carrier degradation is a key reliability issue prominent in deep submicrometer MOSFET&#39;s. The degradation results from a buildup of charge near the drain junction of a transistor that results in threshold voltage shift, transconductance degradation, and drain current reduction, and may eventually lead to device failure. 
     FIG. 1 depicts an n-channel transistor. Substrate  100  is lightly positively doped. Disposed in substrate  100  are source channel  102  and drain channel  104  which comprise a strongly negatively doped material. A thin oxide region  106  is formed near the surface of the substrate between source channel  102  and drain channel  104  and serves as a gate dielectric. A gate material  108  is co-planarly disposed on top of thin oxide region  106 . If a positive voltage is applied to source channel  102  and drain channel  104  is connected to ground, a positive voltage may be applied to gate material  108  such that a depletion region forms under thin oxide region  106  and current may flow. 
     FIG. 2A depicts an n-channel transistor with HCI charge accumulation. Substrate  200  contains source channel  202  and drain channel  204 . Gate material  208  is co-planarly disposed above thin oxide region  206 . Within thin oxide region  206 , and in proximity to drain channel  204 , trapped charge area  210  illustrates the manner in which charges may accumulate in the thin oxide region. FIG. 2B illustrates that the trapped charge area  212  does not scale with transistor channel length. In other words, the trapped charge area is approximately the same size for a value of field strength. In a shorter channel length transistor, the trapped charge area occupies a larger portion of the channel between source and drain, resulting in a higher probability of device degradation and device failure. Transistors depicted in FIGS. 1,  2 A, and  2 B are intended to illustrate the nature of hot carrier injection and may not reflect the scale or physical positioning of elements for any particular semiconductor process. 
     As device geometries have grown smaller, the operating voltage of devices has been reduced. The industry has seen voltage drop from 5 volts, to 3.3 volts, and shall likely see further reductions in voltage. A potential problem may occur when devices operating at different voltages are interfaced. FIG. 3A depicts interconnection between a 5-volt device and a 3.3-volt device. Signal line  300  connects 5-volt device  302  with 3.3-volt device  304 . Signal line  300  may be bi-directional and may be driven at some periods of time by 5-volt device  302  and maybe driven at other periods of time by 3.3-volt device  304 . In a bi-directional interface, there are typically periods of time between when 5-volt device  302  or 3.3-volt device  304  drive signal line  300  such that signal line  300  is  30  allowed to ‘float’ wherein drive signals from both 5-volt device  302  and 3.3-volt device  304  are in a high impedance state. Many devices conform to TTL input and output voltage level specifications wherein a logical 1 may be represented by a voltage level of not less than 2.7 volts. However, it is often the practice to produce a logic one level that is equal to or near to the positive supply voltage of the device. As such, this may result in a logic one level being driven as 3.3 volts or 5 volts depending on the operating voltage of the device. FIG. 3B provides an example signal sequence for a signal line interfacing a 5-volt device and a 3.3-volt device. At time A (ref  306 ), 5-volt device  302  drives signal line  300  to five volts. At time B (ref  308 ), the output driver of 5-volt device  302  is placed in a high impedance state and signal line  300  is allowed to float. Signal line  300  may remain at approximately 5 volts between time B (ref  308 ) and time C (ref  310 ) due to line capacitance and high impedance of input/output buffers. At time C (ref  310 ), 3.3-volt device  304  drives signal line  300  to a logic zero. When 3.3-volt device  304  first begins to drive signal line  300 , signal line  300  is at approximately five volts. This results in the output buffers of 3.3-volt device  304  being subjected to high field strength for a period of time until signal line  300  is discharged to a lower voltage. The high field strength during this period of time results in a higher rate of charge accumulation in the gate dielectric of the output driver of 3.3-volt device  304 . This results in a higher rate of degradation and earlier potential failure of the device. In CMOS devices, n-channel transistors that are employed to discharge signal nodes may exhibit the greatest degradation from HCI. In order to address HCI reliability issues, a designer may elect to employ longer channel devices for output driver circuitry. The longer channel devices employed for driver circuitry would also require a wider channel to provide the same drive as shorter channel devices. While the longer channel devices offer increased reliability, they may also result in circuit cost due to larger circuit area. 
     FIG. 4 depicts a circuit embodiment of the present invention. Positive supply  400  is coupled to p-channel transistor  408 . P drive signal  402  is input to inverter  404 . The output from inverter  404  is input to inverter  406 . The output of inverter  406  is coupled to the gate of p-channel transistor  408 . P-channel transistor  408  is coupled to output node  410  that may be connected to an output pad. Output node  410  is also coupled to first n-channel transistor  422  and second n-channel transistor  424 . The channel length of first n-channel transistor  422  is greater than that of second n-channel transistor  424 . N drive signal  412  is input to inverter  414 . The output of inverter  414  is coupled to the input of inverter  418  and to a first input of two-input NOR  416 . The output of inverter  418  is coupled to the input of inverter  420  and to the gate of first n-channel transistor  422 . The output of inverter  420  is coupled to a second input of two-input NOR  416 . The output of two-input NOR  416  is coupled to the gate of second n-channel transistor  424 . 
     Operationally, P drive signal  402  may be asserted and the signal propagated through inverter  404  and through inverter  406  to the gate of p-channel transistor  408 , enabling current to pass through p-channel transistor  408  to charge output node  410 . To discharge output node  410 , the N drive signal  412  is asserted and the signal propagates through inverter  414  and to a first input of two-input NOR  416  and to the input of inverter  418 . The second input of two-input NOR  416  remains at a logic 1 level until the asserted N drive signal propagates through inverter  418  and inverter  420 . First n-channel  422  is enabled as the asserted N drive signal propagates through inverter  418  and to the gate of first n-channel transistor  422 . After a delay comprising the propagation delay through inverter  420  and two-input NOR  416 , second n-channel transistor  424  is enabled. In this manner, the long channel n-channel transistor is turned on first, allowing discharge of the output node and reduction of voltage prior to the time at which the shorter channel transistor is enabled. This results in higher reliability while occupying less circuit area than a single wider transistor with a long channel. 
     The present invention also may provide less spiking due to current surges when discharging a node. This is a result of sizing the long channel device to reflect desired current flow when the output node voltage is between 5 volts and 3.3 volts, and sizing the short channel transistor to reflect desired current flow when the output node voltage is equal to 3.3 volts. This is in contrast to a single device, either short or long channel, that is sized to produce the desired current flow when the output node voltage equal to 3.3 volts and therefore transfers much more current when the output node voltage is higher than 3.3 volts, since the current through the device varies with the square of the difference between turn-on voltage and gate to source voltage. 
     The present invention provides an efficient and economical solution to HCI effects and provides reduced surge current, reducing EMI radiation and coupled noise. While 5 volts and 3.3 volts have been used for purposes of illustration, the present invention is not constrained to any voltage ranges or specific electronics technologies or fabrication methods. The foregoing description employs inverters and a NOR gate to implement a delay element to delay activation of the short channel transistor. Other delay element implementations, such as inverter chains, RC delays, and other delay techniques known in the semiconductor and circuit arts may be employed within the scope of the present invention. The term channel length has been employed to refer to the length of the gate channel of a transistor. The present invention is well suited to output buffer driver circuitry but also may be employed in internal circuitry where sections operating at different voltages, such as may be found in memory or other devices, are interfaced. 
     The foregoing description of the invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed, and other modifications and variations may be possible in light in the above teachings. The embodiment was chosen and described in order to best explain the principles of the invention and its practical application to thereby enable others skilled in the art to best utilize the invention in various embodiments and various modifications as are suited to the particular use contemplated. It is intended that the appended claims be construed to include other alternative embodiments of the invention except insofar as limited by the prior art.
A long channel transistor and a shorter channel transistor operate in conjunction to drive an output node. The long channel device is first activated by a drive signal and the drive signal is input to a delay element that then activates the shorter channel device. By enabling the long channel device first, hot carrier injection effects are reduced. Employing two transistors that are sized to operate in different voltage ranges reduces surge current. The two-transistor configuration of the present invention occupies less area than a single long channel device with similar drive capabilities.