I. Field
The present invention relates generally to electronics circuits, and more specifically to current sources and active circuits.
II. Background
Current sources are widely used to provide current for various circuits such as amplifiers, buffers, oscillators, and so on. Current sources may be used as bias circuits to provide bias currents, active loads to provide output currents, and so on. Current sources are often fabricated on integrated circuits (ICs) but may also be implemented with discrete circuit components.
As IC fabrication technology continues to improve, the size of transistors continues to shrink. The smaller transistor size enables more transistors and thus more complicated circuits to be fabricated on an IC die or, alternatively, a smaller die to be used for a given circuit. The smaller transistor size also supports faster operating speed and provides other benefits.
Complementary metal oxide semiconductor (CMOS) technology is widely used for digital circuits and many analog circuits. A major issue with shrinking transistor size in CMOS is leakage current, which is the current passing through a transistor when it is turned off. A smaller transistor geometry results in higher electric field (E-field), which stresses a transistor and causes oxide breakdown. To decrease the E-field, a lower power supply voltage is often used for smaller geometry transistors. However, the lower supply voltage also increases the propagation delay of the transistors, which is undesirable for high-speed circuits. To reduce the delay and improve operating speed, the threshold voltage (Vt) of the transistors is reduced. The threshold voltage determines the voltage at which the transistors turn on. However, the lower threshold voltage and smaller transistor geometry result in higher leakage current.
Leakage current is more problematic as CMOS technology scales smaller. This is because leakage current increases at a high rate with respect to the decrease in transistor size. Leakage current can impact the performance of certain circuits such as phase lock loops (PLLs), oscillators, digital-to-analog converters (DACs), and so on.
Some common techniques for combating leakage current include using high threshold voltage (high-Vt) transistors and/or larger transistor sizes (e.g., longer gate lengths). High-Vt transistors may impact circuit performance (e.g., slower speed) and typically require an additional mask step in the IC fabrication process. Larger-size transistors are marginally effective at combating leakage current since (1) leakage current is a relatively weak function of channel length and (2) there are practical limits on how long the channel length may be extended. Both of these solutions may thus be inadequate for certain circuits.
There is therefore a need in the art for a current source with low leakage current and good performance.