4.2 IHP’s SG13G2 MOSFETs

Table of contents

MOSFET Types and Symbols

The SG13G2 PDK provides parameterizable MOSFET symbols in Xschem:

  • Low-Voltage MOSFETs (1.5 V)
    • sg13_lv_nmos: \(V_{th}\) ~ 0.5 V, \(L_{min}\) = 0.13 µm
    • sg13_lv_pmos: \(V_{th}\) ~ -0.47 V, \(L_{min}\) = 0.13 µm
  • High-Voltage MOSFETs (3.3 V)
    • sg13_hv_nmos: \(V_{th}\) ~ 0.7 V, \(L_{min}\) = 0.45 µm
    • sg13_hv_pmos: \(V_{th}\) ~ -0.65 V, \(L_{min}\) = 0.45 µm

Symbols support the configuration of width (W), length (L), and multiplier (m). Isolated variants of NMOS are also available.

What is an Isolated NMOS?

In standard CMOS, all NMOS devices share a common P-substrate, meaning they share the body terminal.

An isolated NMOS:

  • Is fabricated inside a deep N-well embedded in a P-type epitaxial layer.
  • This allows the body terminal to be independently biased.
  • Provides true 4-terminal operation (G, D, S, B).

Benefits of Isolated NMOS

  • Independent body control
  • Reduced parasitic coupling
  • Mitigates latch-up
  • Improves analog and RF performance

CMOS Transistor Terminals (4-pin mode)

Terminal Symbol Function
Gate G Controls the channel (via voltage)
Drain D Where current exits the transistor
Source S Where current enters the transistor
Body B Substrate; influences threshold voltage and leakage

The Role of the Body Terminal

  • The body terminal in an NMOS is typically connected to the lowest voltage (GND).
  • This prevents the body effect, which increases the threshold voltage \(V_{th}\) if the source and body are not at the same potential.
  • The body terminal also helps prevent leakage and latch-up issues.
Xschem with Skywater 130 PDK

Figure : CMOS using normal process(not isolated)

Xschem with IHP SG13G2 PDK

Figure : Isolated NMOS

MOSFET analysis for analog design

A Metal-Oxide-Semiconductor Field-Effect Transistor (MOSFET) is a voltage-controlled semiconductor device widely used in analog and digital circuits. In analog design, MOSFETs are crucial for building amplifiers, filters, current mirrors, bias circuits, and operational transconductance amplifiers (OTAs).

  • Four-terminal structure: Gate (G), Drain (D), Source (S), Bulk/Substrate (B)

Types of MOSFETs

  • NMOS: Conducts when \(V_{GS}\) > \(V_{th}\) , uses electrons as carriers (higher mobility).
  • PMOS: Conducts when \(V_{SG}\) > \(\|V_{th}\|\) , uses holes as carriers (lower mobility).

NMOS vs. PMOS

NMOS is preferred for speed; PMOS complements NMOS in CMOS logic for power efficiency.

MOSFET as a Voltage-Controlled Current Source

The MOSFET allows current to flow from drain to source, controlled by the gate voltage.
In saturation, the drain current is mainly controlled by VGS:

Why Use MOSFETs in Analog Design?

  • Low input current due to insulated gate (nearly zero gate current).
  • Scalability and high integration in CMOS processes.
  • Design flexibility: Can act as resistors, current sources, amplifiers, and switches.
  • Matching and noise performance for precision analog blocks.

MOSFET Symbols and Conventions

Symbol Type Description
Enhancement-mode Most common in CMOS (normally OFF)
Depletion-mode Less common (normally ON)
4-Terminal Gate, drain, source, and bulk
3-Terminal Bulk tied to source (simplified view)
Xschem with Skywater 130 PDK

Figure : MOSFET symbols for each

DC Characteristics (Ideal Behavior)

Region Condition Behavior
Cutoff \(V_{GS} < V_{th}\) MOSFET is OFF, \(I_D = 0\)
Triode \(V_{GS} > V_{th}\), \(V_{DS} < V_{GS} - V_{th}\) MOSFET behaves like a resistor
Saturation \(V_{GS} > V_{th}\), \(V_{DS} \geq V_{GS} - V_{th}\) Current saturates, good for amplifiers
Xschem with Skywater 130 PDK

Figure : Switching characteristics of both the N-channel and P-channel type MOSFET.

Now, in the next section we will continue to analyse the characteristics of a MOSFET.