PDF1. NMOS Transistor Basics
Figure 1: NMOS Transistor
NMOS transistors consist of three terminals: gate, drain, source, and body. The source and body are both grounded and the device will operate, or in other words, a drain current (iD) will be induced based on voltages applied at the gate (VGS) and drain (VDS) of the transistor. Every NMOS transistor contains a threshold voltage (Vt) which is constant and unique for each transistor. In order for the transistor to operate, VGS must be greater than Vt. Once this condition has been met, the resulting drain current can be controlled by the voltages supplied at the gate and the drain. The relationship between VGS, VDS, and iD is described by three regions of operation:
1. Cut-off Region: In this region no channel exists (iD = 0) for all values of VD. (VGS < Vt)
2. Ohmic/Triode Region:
The NMOS transistor is active and not “pinched off.” This means the value of VDS affects the value of iD (VGS > Vt and VDS ≤ VGS – Vt). Figure 2 shows the relationship between VGS, VDS, and iD in this region. Notice the linear relationship between iD and VDS. In this region, iD obeys Ohms Law as the NMOS transistor responds as a voltage controlled resistor.
Figure 2: iD vs. VDS (Ohmic/Triode Region)
3. Active/Saturation Region: The channel is “pinched off” because increases in VD have no affect on iD (VGS > Vt and VDS > VGS – Vt). In the saturation region, the amount of drain current is directly related to the values of VGS > Vt.
Figure 3: iD vs. VDS (left) and iD vs. VGS (right)
The two characteristic curves provide important information for engineers implementing NMOS transistors into their system by illustrating whether a particular unit will pass or fail within a certain region of operation. Figure 3 depicts the transition from the triode region into the saturation region given discrete values of VGS and shows the response of iD to changes in VGS while the NMOS transistor is in the saturation region.
2. Hardware Setup for Testing Transistors
Testing the NMOS and acquiring the characteristic curves require a voltage source for both VGS and VD, and a device to measure iD. Often, a source measure unit is the instrument of choice for such applications, as it provides the ability to both source and sweep voltage and measure the resulting current.
Figure 4: The NI PXI-4130 Power SMU
The National Instruments PXI-4130 is a programmable, high-power source measure unit (SMU) in a single-slot, 3U PXI module. The NI PXI-4130 has a single isolated SMU channel that offers a four-quadrant ±20 V output that incorporates remote (4-wire) sense. With five current ranges providing measurement resolution down to 1 nA, this precision source is ideal for design validation and semiconductor test applications that require programmatic sourcing and sweeping as well as high-accuracy measurements. The NI PXI-4130 also includes a utility channel that can source either current or voltage with 16-bit setpoint and measurement resolution. This output, which provides up to 6 V and 1 A, can be used as a complementary power source to the SMU channel, allowing transistor characterization applications to be performed with a single module, thereby reducing complexity, footprint, and cost.
Figure 4: Connection Diagram for Using a single NI PXI-4130 Power SMU for Transistor Characterization.
Although the NI PXI-4130 Power SMU can provide a complete solution for many IV characterization applications, some NMOS transistors can operate at very low saturation levels, which can introduce a need for measurement below 1nA. In this situation, engineers can use a high-resolution DMM, such as the PXI-4071 FlexDMM, which delivers 1 pA (10-12) resolution in the 1μA measurement range, in order to measure very low values of iD. One concern when measuring current at these extremely low levels is environmental noise on the system. Using shielded cables, shielded enclosures, and grounding devices appropriately is crucial for reducing noise to minimal levels.
3. Software Solutions for Acquiring Characteristic Curves
In addition to instrumentation hardware, another key consideration for IV characterization applications is software environment. While the NI PXI-4130 Power SMU can be used with a variety of programming environments including C++ and Visual Basic, NI recommends the LabVIEW Graphical Programming Environment, or the LabVIEW SignalExpress Interactive Measurement Workbench for these applications for both ease-of-use and interactive display capabilities.
NI LabVIEW SignalExpress
LabVIEW SignalExpress is interactive measurement software for acquiring, analyzing, and presenting data with no programming required. Performing IV characterization in LabVIEW SignalExpress is as simple as inserting 2 sweeps around a step to interface with the NI PXI-4130 hardware. Use this environment to get up and running quickly with measurements and display. Shown below is an IV sweep performed on a in the LabVIEW SignalExpress environment.
Figure 5: iD vs. VDS with the LabVIEW SignalExpress Programming Free Environment
National Instruments LabVIEW
For automated test applications, the need can arise to programmatically control hardware and analyze data in a more user-defined fashion. In these scenarios, the LabVIEW Graphical Programming Environment is the recommended option for more customized measurements, analysis and display. To help get started, the NI-DCPower hardware driver for the NI PXI-4130 Power SMU ships with a number prebuilt example programs including a IV characterization example. Here users can enter in their sweep parameters to get up and running quickly, and then customize their application from there.
Figure 6: IV Curve Trace Example in the LabVIEW Graphical Programming Environment
Conclusion
With the ability to measure and characterize these devices using a single PXI instrument , the NI PXI-4130 Power SMU is ideal for IV characterization applications. However, NMOS transistor characterization is one example that demonstrates National Instruments capabilities with low current sourcing and measuring, as precision devices in the PXI form factor can be used for a variety of other applications such as testing and measuring leakage currents or measuring voltage levels and power draw for semiconductor validation. Combining this hardware with the LabVIEW Graphical Programming Environment gives users an open ended tool for acquiring, analyzing, and presenting data. For more information visit:
