Non-Ideal Transistors Lecture Review

Flashcards covering vocabulary related to non-ideal (real) transistor behavior, including high field effects, threshold voltage effects, leakage mechanisms, and process/environmental variations.

Subthreshold Conduction

Leakage current in weak inversion, occurring when transistors do not turn completely OFF, and the current is exponentially sensitive to Vgs.

Channel Length Modulation

A non-ideal behavior where the effective channel length decreases with increasing drain-source voltage (Vds), causing the saturation current to increase with Vds.

Mobility Degradation

The reduction of carrier mobility at high vertical electric fields (Evert) due to increased scattering caused by collisions with the oxide interface.

Velocity Saturation

At high lateral electric fields, carrier velocity in a transistor ceases to be proportional to the electric field and reaches a maximum saturation velocity due to scattering off atoms in the silicon lattice.

Parasitic Capacitance and Inductance

Unwanted capacitance and inductance inherent in device structures and interconnections that affect signal integrity and overall device performance.

Drain Induced Barrier Lowering (DIBL)

A threshold voltage effect where the electric field from the drain affects the channel, causing the threshold voltage (Vt) to decrease and the drain current (Ids) to increase with high Vds, especially in small transistors.

Body Effect

A threshold voltage effect where the source-to-body voltage (Vsb) alters the amount of charge required to invert the channel, thereby changing the threshold voltage (Vt).

Gate Leakage

Current that flows due to quantum tunneling of electrons through an ultra-thin gate oxide, which is exponentially sensitive to oxide thickness and VDD.

Process Variation

Variability in physical parameters such as channel width (Ws), length (Ls), and threshold voltage (Vt) among adjacent devices or across a wafer, resulting from manufacturing imperfections.

Temperature Dependence

Nonlinear changes in transistor parameters, such as mobility and threshold voltage, with variations in operating temperature, which in turn affects device performance.

Junction Leakage

Leakage current that flows across reverse-biased p-n junctions, consisting of mechanisms like ordinary diode leakage, band-to-band tunneling, and gate-induced drain leakage.

k′ (k prime)

A parameter in ideal transistor models representing the product of the mobility of electrons/holes (μn) and the oxide capacitance per unit area (Cox).

β (beta)

A gain factor in ideal transistor models calculated as k′ multiplied by the transistor's width-to-length ratio (W/L).

Cutoff (Ideal Transistor)

The operating region of an ideal transistor where the drain current (Ids) is zero (or negligible) because the gate-source voltage (Vgs) is less than the threshold voltage (VTH).

Linear (Ideal Transistor)

The operating region of an ideal transistor where the drain current (Ids) is proportional to the drain-source voltage (VDS), occurring when VDS < VGS − VTH.

Saturation (Ideal Transistor)

The operating region of an ideal transistor where the drain current (Ids) is constant for a given gate-source voltage (VGS), occurring when VDS ≥ VGS − VTH.

Ion (ON current)

The drain current (Ids) when the transistor is fully turned ON, typically measured at Vgs = Vds = VDD in the saturation region.

Ioff (OFF current)

The drain current (Ids) when the transistor is in cutoff, typically measured at Vgs = 0 and Vds = VDD, representing the leakage current.

α-power law model

An empirical model used to approximate the velocity-saturated ON current in real transistors, where Ids is proportional to VDD^α, with α being an empirically determined value between 1 and 2.

Ld (depletion region width)

The width of the depletion region formed by a reverse-biased p-n junction, which grows with reverse bias and reduces the effective channel length.

Leff (effective channel length)

The actual active channel length experienced by carriers, which is the physical channel length (L) minus the depletion region width (Ld).

λ (channel length modulation coefficient)

An empirically derived coefficient used to model the increase in saturation current with Vds due to channel length modulation.

φs (surface potential at threshold)

The surface potential required at the silicon-oxide interface for the channel to begin inversion, a critical parameter in understanding the body effect.

γ (body effect coefficient)

A coefficient that quantifies the dependence of the threshold voltage (Vt) on the source-to-body voltage (Vsb), related to doping and intrinsic carrier concentration.

Reverse short channel effect

A phenomenon observed in some processes where the threshold voltage (Vt) unexpectedly decreases with increasing channel length (L), opposite to the typical short channel effect.

Band-to-band tunneling (BTBT)

A type of junction leakage where carriers tunnel across heavily doped p-n junctions, particularly significant at sidewall regions or in devices with very high doping.

Gate-induced drain leakage (GIDL)

A type of junction leakage occurring at the overlap region between the gate and drain, most pronounced when the drain is at VDD and the gate is at a negative voltage.

Reverse Temperature Effect

A phenomenon where for low supply voltages (VDD) and high temperatures, the increase in the (VDD-Vt) term compensates for the drop in mobility (µ), leading to an increase in IDSAT.

Process Corners

Standardized sets of extreme process, voltage, and temperature variations used to describe worst-case scenarios for design verification, ensuring robust circuit operation.

Fast Corner (F)

A process corner characterized by short effective channel length (Leff), low threshold voltage (Vt), thin gate oxide (tox), high VDD, and low temperature, resulting in faster transistor switching speeds.

Slow Corner (S)

A process corner characterized by longer effective channel length (Leff), high threshold voltage (Vt), thick gate oxide (tox), low VDD, and high temperature, resulting in slower transistor switching speeds.

Typical Corner (T)

A process corner representing average or nominal parameters for effective channel length (Leff), threshold voltage (Vt), gate oxide thickness (tox), supply voltage (VDD), and operating temperature.

Vertical electric field (Evert)

The electric field perpendicular to the transistor channel, primarily caused by the gate-source voltage (Vgs) across the gate oxide, which controls carrier accumulation.

Lateral electric field (Elat)

The electric field along the transistor channel, primarily caused by the drain-source voltage (Vds) across the channel length, which accelerates carriers from source to drain.