Digital CMOS Abbreviation S Alphabet Dictionary
Saturation Region (BJT)
In the context of Bipolar Junction Transistors (BJTs), the saturation region is an operational state where both the emitter-base and collector-base junctions are forward-biased. This state is characterized by a significant flow of charge carriers (electrons or holes) from the collector to the emitter. It creates a low-impedance path between the collector and emitter terminals, resulting in a low resistance state. Saturation is one of the two steady states in logic gates.
Saturation Region (Pinch Off)
In Metal-Oxide-Semiconductor Field-Effect Transistors (MOSFETs), the saturation region is a mode of operation where the drain current becomes relatively constant as a function of drain-source voltage (Vds). In this region, the drain voltage no longer controls the current, especially in long-channel devices. However, in short-channel devices, saturation can be affected by velocity saturation or velocity overshoot.
Saturation velocity is the velocity of charge carriers (e.g., electrons or holes) in a semiconductor material when their speed becomes relatively independent of the applied electric field. It’s the point at which their kinetic energy resonates with the crystal lattice structure.
The saturation voltage of a MOSFET refers to the drain-to-source voltage (Vds) at which the MOSFET’s drain current (Ids) reaches saturation. It’s the point at which further increases in Vds do not significantly affect Ids.
A scan flip-flop is a modified flip-flop designed to function within a scan path. It includes additional input ports, such as Test Data In (TDI) and Test, along with a 2-to-1 multiplexer. In normal mode, it operates like a regular flip-flop, but in test mode, it allows data to be shifted in and out, enabling comprehensive testing of combinational logic blocks.
A scan path is a design technique used in digital circuits. All registers in a circuit are replaced with scan registers, creating a complete scan path throughout the die. In normal mode, data flows through the pipeline in a parallel manner. In test mode, the scan path is activated, turning all registers into a shift register. This enables full control and observability of inputs and outputs for testing purposes.
A scan register consists of flip-flops replaced with scan flip-flops. In scan registers, the output of each flip-flop is connected to the Test Data In (TDI) of the next flip-flop. Test pins are used to enable the scan mode, allowing data to be shifted in and out of all the registers in the circuit during testing.
A select mask is a photomask used in semiconductor fabrication to define areas where specific types of doping (n+ or p+) should be implanted. Select masks don’t need to be perfectly precise; they only need to include the areas requiring implantation.
A self-aligned process is a semiconductor fabrication technique in which MOSFET gates are fabricated before the sources and drains are implanted. This approach makes the fabrication process more robust against misalignment issues. However, it often requires the use of polysilicon gates because metallic gates may melt during subsequent annealing.
Self-loading refers to the load seen at the output of a logic gate when it is not externally loaded. In CMOS circuits, this self-loading is mainly due to drain capacitance.
Self-timed memory design is a philosophy where the sequence of operations required for a read operation is autonomously managed without relying on an external clock or prompt. It typically involves precharge, word-line activation, cell discharge, and sense amplifier triggering. Self-timing is often initiated upon detecting a change in address.
A semiconductor is a type of material characterized by a moderate bandgap and the ability to conduct electricity. Its electrical properties can be significantly altered by adding dopants. Semiconductors are essential for designing electronic devices and integrated circuits.
Semiconductor Grade Silicon
Semiconductor-grade silicon is a silicon material that has undergone rigorous mechanical and chemical purification processes. It is of exceptionally high quality and is suitable for the fabrication of silicon wafers used in semiconductor manufacturing.
A sense amplifier is used in memory systems, particularly for each bit line. It helps resolve the trade-off between small memory cell sizes (which increase memory density) and slow access times (due to the high capacitance of bit lines). The sense amplifier detects the direction in which a memory cell is trying to drive the bit line and then takes over, rapidly amplifying the signal to accelerate the voltage swing. Sense amplifiers are typically designed differentially and also perform differential-to-single-ended signal conversion.
Sensitivity List (VHDL)
In VHDL (VHSIC Hardware Description Language), the sensitivity list is a section associated with a process. It specifies which signals can trigger or sensitize the process. When any signal in the sensitivity list experiences an event (e.g., a change in value), it causes the process to be executed again. Processes with wait statements do not require a sensitivity list, as they are not event-driven.
Sequential logic refers to digital logic circuits or elements where the output depends not only on the current inputs but also on the circuit’s previous state. Typically, a clock signal controls sequential circuits, allowing them to store information and execute sequences of operations.
Sequential Statements (VHDL)
In VHDL and other programming languages, sequential statements are instructions or commands that are executed in a specific order. They form the foundation of program flow and are essential for creating complex logic in HDLs (Hardware Description Languages). In HDLs, sequential statements are often used to describe registers, latches, and memory elements.
For a register, setup time represents the minimum time required before the active (rising or falling) edge of the clock signal for the input data to be stable. This setup time allows the data to settle within the master latch before the clock edge arrives.
A setup-time violation occurs when the input data of a register changes too close to the clock edge, leaving insufficient time for the data to stabilize before the clock edge. Such violations can lead to incorrect operation of digital circuits and need to be addressed by adjusting clock timing, pipeline structure, or placement and routing.
Shallow Trench Isolation
Shallow Trench Isolation (STI) is a modern semiconductor isolation technique used in semiconductor fabrication. It involves etching trenches into the silicon substrate and filling them with insulating material (typically oxide). STI is superior to older techniques like LOCOS (Local Oxidation of Silicon) and helps prevent latch-up issues without imposing significant density constraints.
Shared Variable (VHDL)
In VHDL, a shared variable is a variable that retains its value across multiple processes and calls. It can be accessed and modified by various processes simultaneously, making it suitable for applications like multi-port memories.
Sheet resistance is a material and technology-related property that quantifies the resistance of a square-shaped region of a material. It is primarily determined by the type of material and its thickness. Sheet resistance varies between different layers in semiconductor manufacturing.
A shift register is a series of flip-flops or registers connected in a cascade. It allows the input signal to be delayed by a specific number of clock cycles, making it crucial for aligning signals in synchronous pipelines and performing various serial-to-parallel or parallel-to-serial data conversions.
Also known as crowbar current, it refers to the high current that can flow through a circuit during a short-circuit event. This current can potentially cause damage to components or trigger protective measures in a circuit.
In VHDL, a signal represents a wire or node in a digital design. Signals can be either internal or external (input/output) and play a critical role in specifying the connections and interactions between components in a digital design.
Signal Assignment (VHDL)
A signal assignment in VHDL is an operation that assigns a new value to a signal. It can be executed concurrently outside a process or sequentially within a process. Signal assignments inside a process result in transactions that can later lead to events, which are important for simulation and modeling.
A signature analyzer is a circuit used for testing purposes. It compresses the output of a unit under test (UUT) into a shorter signature. This signature serves as an indicator of any faults or anomalies in the UUT. Effective design of signature analyzers is challenging due to the need to accurately detect and diagnose faults.
Silica is a term used to refer to silicon dioxide (SiO2). While the term is often associated with natural occurrences of oxide materials in soil or minerals, high-purity silica can be reduced with charcoal to obtain silicon, which is used in semiconductor manufacturing.
Silicon Controlled Rectifier (SCR)
An SCR is a semiconductor device with an npnp structure. Under normal conditions, it does not conduct current. However, if a startup current is applied to a third terminal (gate), it can be triggered into conduction and will continue to conduct until the current flowing through it is reduced to zero. SCR is a fundamental component in latch-up phenomena.
In a logic gate with multiple inputs, “simple effort” refers to the logical effort contributed by a single input. It’s a measure of how effectively a gate can drive its output considering only one of its inputs.
Skin depth is the depth within a conductor’s cross-section at which current is observed to flow when subjected to a certain frequency of electromagnetic waves. It’s the depth at which the current density drops to 1/e (about 37%) of its surface value. Skin depth is frequency-dependent and is relevant in high-frequency applications. When the skin depth exceeds half the smallest dimension of the conductor, the skin effect is not significant.
Skin effect is a phenomenon observed at high frequencies where current tends to flow predominantly in an outer shell or surface layer of a conductor. As the frequency increases, the depth of this conducting layer decreases. This effect reduces the available cross-sectional area for current flow, leading to an increase in observed resistance.
In digital circuit design, slack represents the time difference between the critical path delay (the longest delay in the circuit) and the worst-case delay of a specific path (target path). The critical path has zero slack. Positive slack indicates that the path completes its operations earlier than required by the clock edge, while negative slack implies that the path fails to meet timing constraints and may experience setup-time violations.
Slack borrowing is a mechanism used in latch loops, allowing some stages to have moderate negative slack as long as other stages have sufficient positive slack to ensure that the total running slack remains within acceptable limits. It leverages the transparency of latches to improve efficiency compared to register pipelines.
In a master-slave flip-flop, the slave latch is the second latch. It transfers data through the second phase of the clock signal but masks data from the output when the master latch is active. This dual-latch design helps eliminate glitches and improve stability.
In CMOS semiconductor fabrication, the “Slow-Slow corner” refers to a process corner where both NMOS and PMOS devices are slower than expected due to variability, often caused by lower carrier mobility. Chips manufactured in this corner are typically slower than desired and may suffer from setup-time violations if operated at nominal speeds. They are often sold at a lower price.
SoC (System on a Chip)
SoC stands for “System on a Chip.” It refers to the integration of multiple subsystems or components onto a single semiconductor chip. These subsystems can operate independently but often need to communicate to form a coherent system. SoCs may include processors, memory, peripherals, and communication interfaces.
SOI (Silicon on Insulator)
SOI is a semiconductor technology that replaces the traditional silicon substrate with an insulating material beneath the transistors. This technology helps reduce latch-up susceptibility, increase device density, reduce leakage, and improve performance by isolating transistors from the substrate. It also reduces the Drain-Induced Barrier Lowering (DIBL) effect.
Source (MOSFET Terminal)
In MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor), “source” is one of the four terminals. In NMOS, it is heavily doped n-type, while in PMOS, it is heavily doped p-type. The source is the terminal from which charge carriers (electrons in NMOS and holes in PMOS) flow toward the drain when the transistor is in operation. In most cases, the source and drain are interchangeable depending on the transistor’s operational mode.
Source matching involves matching the impedance of a signal source to the characteristic impedance of a transmission line to which the signal is applied. This matching minimizes signal reflections and reduces delay to a single time-of-flight delay. Failure to match the source impedance can result in signal reflections, causing ringing or undesired waveform distortions.
SRAM (Static Random Access Memory)
SRAM is a type of volatile semiconductor memory used for fast data storage and retrieval. Unlike dynamic RAM (DRAM), SRAM doesn’t require constant refreshing to maintain data. It stores data statically in bistable inverter pairs connected in positive feedback. An SRAM cell typically consists of six transistors, with four forming the bistable inverters and the remaining two serving as access transistors. SRAM is commonly used for processor caches and embedded memories in various electronic devices.
Standard cells are fundamental building blocks used in digital integrated circuit design. Each standard cell has a predefined height, and while its width can vary based on complexity, it must align with the heights of other cells in the library. These cells have specific input and output arrangements in a particular layer, and the placement of wells and supply rails is standardized. They are crucial for creating complex digital logic functions.
Standard Cell Library
A standard cell library comprises a collection of standard cell entries, each representing a specific logic function. These libraries are used in digital circuit design and synthesis. Each library entry includes the cell’s layout, propagation delay characteristics under different conditions (corner cases), logic characterization, and parasitic effects.
A state machine is a design approach used for controlling digital systems. It divides the system’s behavior into a finite number of states, and transitions between these states are determined by the current state and input signals. State machines are often used to implement complex control logic where sequences of events must be managed.
State Transition Diagram
A state transition diagram is a graphical representation of a finite state machine. Each state is depicted as a circle, and transitions between states are represented by lines or arrows. Output signals and conditions causing transitions can be indicated on the diagram, making it a visual tool for summarizing the behavior of a state machine.
State Transition Table
A state transition table is a tabular representation of a finite state machine. It lists all the possible states, the conditions or inputs that trigger transitions between states, and the corresponding output signals. State transition tables provide a detailed view of the state machine’s behavior.
Static current refers to the steady-state DC current that flows in a circuit or device. In CMOS (Complementary Metal-Oxide-Semiconductor) circuits, there is typically zero static current when the transistors are in their non-conductive states. However, in some logic families like ratioed logic, static current flows under certain conditions. This current is also associated with terms like leakage current and crowbar current.
A static hazard is a logical hazard that occurs when a combinational output remains constant despite changes in input values. This can result from differential delays within the circuit and can manifest as temporary glitches in the output. Static hazards are typically caused by specific circuit structures and can be eliminated by adding redundant terms to the logic.
Static power refers to the power dissipated in a device or circuit due to the flow of static current. This power consumption is associated with constant DC currents and is separate from dynamic power, which arises from switching activities. Static power can include leakage power and short-circuit power in certain logic families.
Steep Retrograde Doping
Steep retrograde doping is a semiconductor doping technique where the surface layer of a device is lightly doped, and the bulk doping concentration increases rapidly with depth. This doping profile is used to improve surface mobility while maintaining the desired bulk properties. However, it can worsen the body effect in MOSFETs.
In semiconductor manufacturing, a stepper is a machine used in photolithography processes. It is responsible for accurately positioning a photomask over a small portion of a semiconductor wafer and exposing that area to light. Steppers are essential for patterning the features on a semiconductor wafer with high precision.
A stick diagram is a simplified, color-coded line diagram used in semiconductor design to represent the approximate position of devices, the layers used for routing, and provide an initial visualization of how the layout might look. It does not specify device sizes or exact positions but serves as a high-level representation of the physical layout.
Storage Capacitor (DRAM)
In dynamic random access memory (DRAM) cells, a storage capacitor is used to store the logic value (either a “0” or “1”). This capacitor can be intentional or parasitic and can be linear or nonlinear. One-transistor DRAM cells use additional linear capacitors to store charge intentionally, reducing the impact of leakage and enabling higher charge-sharing during read operations.
A stress test is a type of testing that subjects a chip or electronic device to challenging and often extreme operating conditions. These conditions simulate tough working environments or extended periods of use. Stress tests are conducted to determine the limits of a circuit’s performance, test its reliability, and identify potential weaknesses. Passing a stress test indicates the device’s ability to function under harsh conditions.
Strong inversion is a sub-mode of operation in a MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor) where the application of additional gate potential mainly affects the oxide layer. It’s characterized by a saturation of the surface potential, leading to a linear relationship between differential gate potential and inversion charge. Strong inversion begins when the surface is as n-type as the deep bulk in NMOS (and vice versa in PMOS) and is significant for transistor behavior.
Stuck-at -0 Fault
A stuck-at-0 fault is a fault model used in digital circuit testing. It assumes that a logic node is “stuck” at logic value “0,” meaning it consistently outputs a logic low signal. This fault model is used to simulate a variety of defects that could cause a node to be held at a logic low level, even though it may not correspond to an actual short circuit to ground.
A stuck-at-1 fault is similar to a stuck-at-0 fault but assumes that a logic node is “stuck” at logic value “1,” consistently outputting a logic high signal. Like the stuck-at-0 fault, this model represents various defects that could lead to a node being held at a logic high level.
A stuck-at fault model is a testing concept where each node in a digital logic circuit can exhibit one of three states: normal functioning, stuck-at-0, or stuck-at-1. This model is a simple yet powerful way to simulate a wide range of defects, as it allows nodes to be “stuck” at logic values, even if there’s no direct short circuit to supply or ground.
A stuck-open fault is a circuit-level fault model where a transistor is assumed to be stuck in an open-circuit state. This typically doesn’t correspond to a complete failure to create the transistor but may represent defects such as issues with vias. Stuck-open faults are detected by identifying high-impedance nodes when low impedance is expected.
A stuck-short fault is a circuit-level fault model where a transistor is assumed to be stuck in a short-circuit state. Similar to stuck-open faults, this may not correspond to a direct transistor failure but could represent defects like via issues. Stuck-short faults are detected when there’s a steady-state current flow in CMOS circuits where it shouldn’t exist.
In a MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor), the substrate, also known as the body, is one of the four terminals. It serves as the semiconductor plate of the MOS capacitor and is where the source and drain regions exist. To prevent current flow from the source or drain into the body, reverse junctions are created between them. The body of NMOS transistors is usually connected to a constant potential (typically ground), while the body of PMOS transistors is connected to another constant potential (typically the supply voltage). The body effect, influenced by the body potential, can significantly impact current flow through the transistor.
Subthreshold conduction is a significant source of leakage current in semiconductor devices. It occurs when a transistor operates below its threshold voltage, entering a weak inversion state where some current can flow. The subthreshold current decreases exponentially with gate voltage but remains significant when the gate-to-source voltage is near zero. This phenomenon is exacerbated by drain-induced barrier lowering (DIBL) and can contribute to power dissipation in deep submicron technologies.
Subthreshold swing measures the change in gate voltage required to reduce subthreshold current by a factor of ten (i.e., one decade). A lower subthreshold swing indicates that the gate can more effectively control the channel’s conductivity, resulting in better power efficiency. The ideal subthreshold swing at room temperature is 60 millivolts per decade.
In VHDL (VHSIC Hardware Description Language), a subtype is a subset of a default or user-defined data type. Signals declared under a subtype must adhere to the restrictions imposed by that subtype. Subtypes are used to restrict the range, values, or limits of a particular data type.
In the context of adders, a sum bit is an output bit that is generated in the same bit position during addition. The logic for calculating the sum can be complex, requiring a 2-input XOR gate in half adders and a 3-input XOR gate in full adders to compute the sum bit.
In binary long multiplication, a summand is the result of multiplying one bit from the multiplier with the multiplicand. There are as many summands as there are bits in the multiplier, and each summand has the same width as the multiplicand. Summands are shifted one bit to the left relative to the previous one. Adding all the summands together yields the final product. When the multiplier bit is 0, the summand is all zeros; when the multiplier bit is 1, the summand is a copy of the multiplicand.
Supply bounce is a phenomenon observed in microchips where the power supply voltage temporarily decreases for a short duration. This phenomenon is different from resistive drops and is caused by inductance, particularly at the pad, pin, and PCB (Printed Circuit Board) levels. High-frequency changes in the current supplied by the power source result in significant Ldi/dt drops, leading to a momentary drop in the core voltage. Supply bounce can be problematic, especially during startup, as it can potentially cause latch-up and affect chip reliability.
Surface mount refers to both an integrated circuit (IC) package and a mounting technique for ICs. In surface mount packages, the pins are located on the periphery of the IC and are often positioned on all four sides. These pins typically follow a smooth curve downward and end up flat and horizontal. Surface mount packages allow for thin and densely packed pins, increasing the pin count compared to Dual Inline Package (DIP) chips. However, mounting surface mount chips can be challenging due to their tight packing, and manual soldering is often difficult and requires highly skilled technicians.
Surface potential refers to the amount of band bending observed in the substrate of a Metal-Oxide-Semiconductor (MOS) capacitor. This potential is nonzero at thermal equilibrium and is influenced by the applied gate-to-body potential. In a flat band condition, the surface potential is zero, but it becomes nonzero when a voltage is applied to the gate terminal, affecting the behavior of the MOS device.
A synchronization circuit is a circuit that utilizes synchronizers, flags, and handshaking protocols to enable communication between two clock domains. Synchronizers are used to mitigate metastability issues when transferring signals between different clock domains. These circuits are designed to ensure that signal transitions observed at their output are not metastable with respect to the receiving clock domain. Synchronization circuits are used to synchronize handshaking signals between a transmitter and a receiver. However, they introduce overhead in communication and are not well-suited for burst communication.
A synchronizer is a circuit that typically consists of two flip-flops used to synchronize a signal from one clock domain to another clock domain. Synchronizers are crucial for addressing metastability issues, ensuring that signals transferred between different clock domains are correctly synchronized. While synchronizers introduce a one-cycle uncertainty about the arrival time of a signal, they significantly reduce the risk of metastability-related failures.
A synchronous pipeline is a pipeline architecture where all registers within the pipeline stages share the same clock signal. In digital circuits, most pipelines operate synchronously, meaning that all stages are clocked by the same clock edge. This approach allows for well-defined timing characteristics, making it suitable for many digital applications.
Synthesis is the process of converting a high-level behavioral description of a hardware design, typically described using a Hardware Description Language (HDL) like VHDL or Verilog, into a netlist composed of specific logic cells from a library. The synthesis process maps the behavioral description to the physical hardware components, such as logic gates or standard cells, that will implement the design. In FPGA (Field-Programmable Gate Array) design, synthesis results in configurations of logic cells, while in ASIC (Application-Specific Integrated Circuit) design, it involves mapping to standard cell library entries.