Category:Transistors characterization

Transistors characterization is a fundamental part of understanding how much performance can be expected from a transistor at a given node and process design kit. Having characterized a transistor one can get a rough idea for whether certain specifications for a particular block are possible given a required power budget.

Transistors dimension parameters


 * Length: Device length


 * Width: Device total width
 * Number of fingers (Nf)
 * Parallel multiplier (m)

Some important transistor device parameters are the following:


 * Vgs, Vds, Vbs: gate-to-source, drain-to-source, bulk-to-source DC bias voltages.


 * Vth: Device threshold voltage
 * Transition point from cuttoff (sub-threshold weak inversion) to moderate inversion ("ON").


 * Veff: Effective overdrive voltage
 * A proxy for understanding the device inversion level.
 * More formally Veff = Vgs - Vth
 * Used widely as a design parameter in analog design.


 * Vdsat: Minimum active (saturation) voltage (MOS)
 * Traditionally equal to Veff (no longer the same, but close)
 * Minimum voltage across drain to source that keeps device in active-sat


 * Id: drain current
 * DC bias drain current.


 * Cxx: Device parasitic capacitances
 * Most important here being lumped capacitances to ground Cgg and Cdd (also less importantly Css and Cbb)
 * Also mutual capacitances between terminals.


 * rds or 1/gds: Device output drain-source resistance (or equally inverse conductance)
 * Largely dependent on relative length
 * Critical for enabling large intrinsic device voltage gain

Most important is understanding the performance limits of your devices so as not to engage in designs where it is not possible to meet your goals. Some very important transistor design parameters are the following:


 * gm: transconductance gain
 * Iout/Vin due to nominal "gate" transconductance.


 * gmb: bulk (back-gate) transconductance gain
 * Iout/Vin due to "back-gate" transonductance.
 * Must be considered when source and bulk not at the same potential.


 * gm/Id: transconductor current efficiency
 * How much current do you need to spend to yield a desired transconductance gain.
 * gm/Id (max) scale tends to remain constant over technology nodes (< 30)
 * Can also be used as a proxy for understanding the device inversion level. (but now in terms of other useful design parameters)


 * gm/W: transconductor area efficiency
 * How much IC area do you need to spend to yield a desired transconductance gain.


 * Id/W: Current density
 * How much current can you support for a given device width.
 * This parameter is often times used alternatively to gm/W at larger currents where reliability and electron migration are a concern.


 * ao: Device intrinsic voltage gain
 * Vout/Vin: The maximum intrinsic voltage gain attainable from the device at DC.
 * ao = gm x rds (or equally gm / gds)


 * Ft: Unity-gain frequency
 * Largest frequency where the device current gain is 1.
 * Note this applies also to MOSFET transistors, recall at these frequencies the gate impedance is actually finite.
 * After this point, the device no longer provides amplification for you.
 * In fact it often makes little sense to operate devices near their Ft frequency as there is an exponential decreasing gain returns to power spent. (often times limiting bandwidth to Ft/10 is more reasonable)
 * Often times simulator provides most accurate Ft approximation.
 * However, a good rough approximation for Ft=gm/Cgg


 * Fc: Device -3dB bandwidth corner frequency
 * Assuming a dominant pole approximation
 * A rough approximation being Fc=gm/Cload (Fc=gm/Cdd for self-loaded case)


 * FoM: Device figure-of-merit for given technology
 * FoM = gmid x Ft
 * Typically a good bias operating point for transistors when high-speed power-efficient operation is desired.

As always, the first step in the design process should be understanding the capabilities of the devices you are working with.