Transistors Basics: MOSFET

2024-09-30 | By Alex Nguyen

A transistor is a type of semiconductor device that can conduct and insulate electric current or voltage. It can act as an amplifier or a switch. There are many different types of transistors, the majority of which can be split into the following diagram:

Transistors Basics: MOSFET Figure 1 Different Types of Transistors.

What is a MOSFET?

MOSFET stands for “metal-oxide-semiconductor field-effect transistor.” The general layout of their structure is that they have a source, drain, gate, and body, also referred to as substrate. MOSFETs are split into two types: n-channel and p-channel.

N-channel MOSFET, also known as NMOS or NFET, creates a current channel between the source and the drain when a positive voltage v_GS is applied between the gate and the source.
P-channel MOSFET, also called PMOS or PFET, creates a current channel between the source and the drain when a negative voltage v_GS is applied between the gate and source.

Transistors Basics: MOSFET

Scheme It

When the voltage at the source is equal to the voltage at the body, then the MOSFET can be represented without the body.

Transistors Basics: MOSFET

Scheme It

What is a MOSFET Made Out of?

The physical structure of an NMOS and PMOS is similar but differs in the semiconductor material used:

NMOS: The body is made of p-type substrate. The source and drain are heavily doped with n-type material.
PMOS: The body is made of n-type substrate. The source and drain are heavily doped with p-type material.

Transistors Basics: MOSFET Figure 2 Physical Structure of NMOS and PMOS.

Focusing on the NMOS structure, “the transistor is built on a p-type substrate, a single-crystal silicon wafer that provides physical support for the device.” As indicated by n⁺, the heavily doped n-type source and drain are created in the substrate. Afterwards, a thin layer of silicon dioxide (SiO₂) which typically varies in thickness t_ox from 1nm to 10nm, is grown on the surface of the substrate. SiO₂ is an amazing insulator, so it covers the area between the source and drain. Metal is then deposited onto the silicon dioxide layer to form the gate of the transistor. “Metal contacts are also made to the source region, the drain region, and the substrate.”

Transistors Basics: MOSFET Figure 3 Physical Structure of NMOS in Perspective View.

How Does MOSFET Operate?

Current Channel

For NMOS, when zero voltage is applied to the gate, it is like having “two back-to-back diodes in series between drain and source.” For PMOS, if a high voltage is applied to the gate, the diode configuration would be the reverse. This situation makes it difficult for the transistor to conduct current between the source and drain. “The path between drain and source has a very high resistance (of the order of 10¹² Ω)”.

Transistors Basics: MOSFET Figure 4 MOSFET Zero Gate Voltage Diode Configuration.

For NMOS, the source and drain are grounded before a positive voltage v_GSis applied to the gate. Since the other terminals are grounded, the positive gate voltage causes holes (positively charged) in the substrate beneath the gate to be repelled, leaving behind a depletion region. This depletion region is left with a net positive charge, making it more susceptible to electrons (negative charge). “The positive gate voltage attracts electrons from the n^{^}+ source and drain regions (where they are in abundance) into the channel region.” After enough electrons gather in the substrate under the gate, an n-channel is created, connecting the source and drain. When a voltage is applied between the source and drain, this induced region allows current to flow. The voltage v_GS required to create a conducting channel under the gate is called the threshold voltage, V_t. Typically, the value of V_t lies between 0.3V to 1V.

Transistors Basics: MOSFET Figure 5 NMOS with Current Channel.

For PMOS, a channel between the source and drain that allows current to flow is created when a negative voltage is applied to the gate v_GS. When the magnitude of the negative v_GS is “beyond the magnitude of the threshold voltage V_t, which by convention is negative, a p-channel is established.”

Transistors Basics: MOSFET Figure 6 PMOS with Current Channel.

Channel Conductance

There are three factors that control NMOS channel conductance: μ_nC_ox, (W/L), and v_OV. “The first factor is determined by the process technology used to fabricate the MOSFET. μ_n represents electron mobility, which is how easily electrons can move on the surface of the channel. C_ox“ is the capacitance of the parallel-plate capacitor per unit gate area. The product μ_nC_ox“ is called process transconductance parameter and given the symbol k_n'.” The second factor is the transistor aspect ratio (W/L). “W is the width of the channel, and L is the length of the channel”. This aspect ratio is dictated by the device designer, “who can select the values of W and L to give the device the i−v characteristics desired”. In 2019, the minimum channel length was 14 nm. The third term is the overdrive voltage v_OV, sometimes referred to as v_GS-V_t. This voltage determines the amount of electron charge in the channel, which directly influences MOSFET’s conductance.

PMOS almost has the same three factors that contribute to the channel conductance. The difference is the process transconductance parameter k_p' is represented by μ_pC_ox. μ_p represents hole mobility, which is how easily holes can move on the surface of the channel. C_ox and (W/L) remain the same from NMOS. To have the correct polarity, overdrive voltage v_OV is sometimes referred to as v_SG-V_t.

Operational Regions

There are two main operation regions for NMOS: triode and saturation. Each region has different characteristics and equations governing the current. The triode region happens when v_GD>V_t or v_DS<v_OV where the current channel is formed. When v_DS is small, the current is proportional to v_OV . “As v_DS increased, the channel becomes more tapered, and its resistance increases correspondingly” causing the increase in current to slow down. The saturation region happens when v_GD≤V_t or v_DS≥v_OV . The “current saturates because the channel is pinched off at the drain end, and v_DS no longer affects the channel”.

Transistors Basics: MOSFET Figure 7 NMOS Drain Current.

PMOS has similar regions to NMOS, which are triode and saturation. There are slight differences in the operation and condition. The triode region is obtained by v_GD>|V_t|or v_SD<|v_OV|. The saturation region is obtained by v_DG≤|V_t| or v_SD≥|v_OV|.

Transistors Basics: MOSFET Figure 8 PMOS Drain Current.