Subthreshold conduction

In subthreshold leakage (English, German, Unterschwellspannungsleckstrom ') is a so-called leakage current in MOS transistors, which are used in current integrated circuits as processors or microcontrollers.

Meaning progresses miniaturization

A characteristic size of a MOS transistor is the length of the gate. It is considered that in digital circuits, which have been realized to be greater than 0.25 microns in MOS technologies with gate lengths, as well as no subthreshold leakage occurs. If the gate length is less (2008: 45 nm ), increases the subthreshold leakage in MOS transistors but exponentially. This unwanted flow of current results in an increase of the power consumption of the integrated circuits. Studies predict that in current and future processors cause leakage currents up to half of the total energy consumption.

The MOSFET in digital circuits

A normal MOS transistor ( see Figure ) consists essentially of two electrodes ( drain and source ) and an additional insulated control electrode (gate). Lying to a voltage, the transistor is in thermodynamic equilibrium and the charge carriers move mainly by diffusion only; under diffusion understood to be a physical balancing process in which a total fluence of a high to a low concentration, but individual particles can move counter also. Another cause of the particle transport is the thermionic emission. Based on a self-blocking n -channel MOSFET so flows, ideally no current flows between the drain and source as long as the gate-source voltage Vgs is smaller than the threshold voltage Uth. By increasing the gate-source voltage ( Vgs, it is < Uth ) more and more electrons are collected in the interface between the gate insulator and semiconductor ( between drain and source ) to it for inversion and thus leads to the formation of a conductive channel.

In digital circuits there is only the two logic signals '0 'and '1', which via the voltage level GND (= 0 V), and VDD ( power supply ) can be represented. Thus, there are in ideal digital circuits only two states for the MOS transistors:

Weak inversion current in MOS analog circuits

In analog circuits, the gate -source voltage Vgs can take values ​​between VDD and GND. The voltage range in which there is the gate-source voltage between the flat band voltage and the threshold voltage is referred to as a subthreshold region. In this region, the area between the drain and source is in so-called weak inversion (English weak-inversion ) and there is a small number of free charge carriers before. In the subthreshold region, the charge carriers move mainly by diffusion rather than caused by a source -drain voltage drift motion. In analog circuits of the source-drain current is also referred to as a weak inversion current. This shows that it is most desirable, in contrast to digital circuitry. Further, the source-drain current can be approximated as the collector current of a bipolar charged homogeneously doped base in this area. The existing layer structure ( source, substrate, drain ) acts more or less as an npn bipolar transistor. Gives an exponential relationship between the drain current ID and the gate-source voltage ( below the threshold )

The sub-threshold region of the MOSFET is partially important for analog circuits which operate at low voltage and low power of this.

In recent years, this technology is increasingly being used for digital circuits. Objectives here are either very low power consumption (eg wireless nodes for sensor networks ) or high performance due to the sometimes very small achievable delay times. Design challenges are especially the strong influences of manufacturing variations and highly nonlinear dependence of cell - delay time and power consumption of the input signal rise and load capacity.

Problems with small feature sizes

With decreasing feature size of the MOS transistors and the operating voltage is reduced. This is due to the great influence of the operating voltage to the energy consumption of the integrated circuits. However, this reduces the switching speed of the MOS transistors and hence the performance of the integrated circuits. To counteract this trend, also the threshold voltage Uth is reduced. However, this results in that the MOS transistors GND ( NMOS transistor ) and VDD ( PMOS transistor ) can not be completely blocked by means of the digital signals. The area between the drain and the source is located in this case in weak inversion, and by applying an electric field, i.e., the drain-source voltage Vds is greater than 0 V, is connected between the drain and source to a diffusion flow. In addition, the effect of the thermionic emission increases. The current flow resulting from these effects is referred to in digital circuits as subthreshold leakage Isub. The term leakage (Eng. " leakage current " ) indicates that it is an unwanted current flow.

A significant influence on the subthreshold leakage, the threshold voltage Uth. The smaller this is, the greater the number of free charge carriers in the weak inversion layer. This leads to an exponential increase of the diffusion current.

Isub can be determined by:

With

Meaning of symbols:

• Charge carrier mobility ...
• ... Effective gate width
• Effective gate length ...
• Doping in the channel region ...
• Specific ... electron charge ( 1.602 · 10-19 C)
• Dielectric constant of the silicon ...
• ... Surface potential of the substrate
• ... Boltzmann's constant ( 1.381 · 10-23 J / K)
• Temperature ...
• Threshold voltage swing coefficient ...

The effective gate length Leff and the effective gate width Weff is less than the physical dimensions of the transistor. The reduction is based on the so-called short-channel effects which occur in MOS technologies with gate lengths of less than 0.25 microns.