Low Voltage SC Circuit Design Using Short-Channel MOSFET Switches

Analog switches, which are known to be the bottle-neck for reducing the supply voltage, are analyzed. MOSFET transistors with channel lengths shorter than the minimum feature size (L 0 ) in a given technology are proposed for use as switches operated at a low supply voltage. These MOSFETs have lower threshold voltage and higher punchthrough currents compared to a transistor of L 0 length (due to short channel effects). Measurements and Medici simulations show that a narrow window of optimum channel length exists where punchthrough current is acceptable for low voltage short channel switches. 1.0 Introduction Low voltage circuit design in existing 5V BiCMOS (CMOS) processes is desired in applications requiring low power consumption, battery operation, line fed telecommunication capability, etc. Reducing the supply voltage degrades the performance of digital circuits. However, speed loss is not significant (down to supply voltage of ) and is tolerated in many applications. If higher speed is required, parallel architectures can be employed. Analog switched-capacitor (SC) circuit design, on the other hand, becomes more challenging. Lowvoltage/high-speed operation (>10MHz) of SC circuits is limited by the gate voltage of a switch that must be more than for full swing signal handling (to keep MOSFETs in strong inversion). In a typical BiCMOS (CMOS) process with threshold voltages , such as Northern Telecom’s 0.8μm BiCMOS process (BATMOS) [1], temperature, process variation, and back bias (of 1.5V) together bring to about 2.7V. Thus, high speed SC circuit design with a supply voltage less than 3V will be problematic in this technology. Increasing the width “ ” of a MOSFET helps to reduce switch on-resistance at the expense of increased clock feedthrough and channel charge injection. For extremely small , a better solution might be to apply clock voltage multiplication to increase , although this technique requires extra circuitry that is noisy and may need off-chip capacitors. In this paper, a simple third solution is presented. MOSFET transistors with channel lengths shorter than the minimum feature size of a given technology (called “short-channel devices” hereafter), have lower threshold voltage due to short channel effects (SCE). These “short-channel devices,” can be used as efficient switches due to higher (as a result of lower ), higher aspect ratio( ) for the same width, and lower charge injection (based on channel length reduction). However, undesirable SCEs, such as increased subthreshold swing and punchthrough current, set a limit to the minimum channel length acceptable for the “short channel devices.” Since these devices will be used solely as analog switches, a small number of them (typically less than hundred) are needed on a chip. Thus device yield may not be degraded significantly. Furthermore, low voltage operation (2.5V) allows the use of shortchannel devices as switches with acceptable punchthrough current. Medici, a 2-D device simulator, was used to find the minimum achievable channel length for a useful short-channel device. Measurements were also carried out on different sizes of “short-channel devices” to verify the simulation results. This technique was applied to an existing second order ∑∆ modulator designed in the BATMOS process for 3.3V operation. Modification was minor and involved changing all the switches. The new design which is expected to be capable of operating at has been resubmitted for fabrication. 2.0 Switches An ideal switch has the following properties: (a) zero on-resistance; (b) infinite off-resistance, i.e., no leakage current, and; (c) full signal swing capability. In a SC circuit, analog switches are implemented with MOSFET transistors which have non-idealities and limitations as described below. • On-resistance: MOSFET transistors used as a switch operate in triode mode with on-resistance of approximately VDD 3Vt 2.4V ≈ = Vc Vtn Vtp + = Vtn Vtp 0.8V = = Vc W Von Vgs Vt – = vGS