What is Field-Effect Transistor? – FET basics – Unipolar transistor


FET Basics – Unipolar transistor

FET basics

FET basics

Unipolar Transistor (also known as Field-Effect Transistor – FET) –tri-ended (tri-electrode), semiconductor electronic component that has the ability to amplify the electrical signal. They transfer only one type of carriers: majority carriers (hence the name – unipolar), while the control of output current is done with the use of electric field (field effect transistors). Depending on the operating laws, we can distinguish two main types of FET: Junction FET (JFET) and Insulated Gate FET (IGFET). The latter are divided into Metal-Oxide Semiconductor FET (MOSFET) and Thin-Film Transistors (TFT).

The Types of FET:

types of fet

Division of field effect transistors


Construction and operation principles of FETs:

Unipolar Transistor has three electrodes: D – drain, G – gate, and S – source; these are equivalents to the electrodes in bipolar transistors. Two of them: Drain and Source are connected to the properly doped semiconductor crystal. Between these ends, the channel is created through which the current flows. The third end is placed along the channel: Gate, thanks to which we can control the flow of the current. In case of connecting multiple MOS transistors in an integrated circuit, the fourth electrode is often used: B – Body (or Bulk) in order to bias the substrate. But in general this end is connected with/to the source.

Operating modes of FETs:

There are three operating modes of the transistors:

  • Cut-off mode: |UGS| > |UT| at any |UDS|,
  • Active mode (also known as the linear or unsaturated): |UGS| < |UT| and |UDS| <= |UDSsat|,
  • Saturation mode: |UGS| < |UT| and |UDS| => |UDSsat|.

Note: In many countries Voltage unit and symbol is called “V” instead of “U” like in this article. 

FET basics symbols

JFET (normally on)

Jfet symbols

Jfet symbols

construction and working of jfet

Inner structure of JFET with “N” channel type

Construction and operation of JFET

In the JFET construction transistors, the gate is insulated from the channel by the reverse-bias junction (with very high input resistance). There are two variants of this junction:

  • P-N junction (PNFET)
  • M-S junction (Metal-Semiconductor)

The channel through which the current will flow is located between the drain and source. One can control the width of the channel (its resistance) by changing the gate-source voltage (UGS). Increasing UGS voltage (which reverse-bias the junction) causes narrowing of the channel until its completely “closed” – the current won’t flow. To the UGS voltage, the voltage drop between a specific point of the channel and the source (UDS) is added. Increasing the value of UDS voltage will ultimately connect depletion layers and block the channel by saturating the transistor. The value of Drain current ID ceases to grow regardless of further increasing the UDS voltage, and transistor becomes a very good transconductance component.

Types of MOSFET

  • Depletion type – D MOS (normally on):
Mosfet symbols

D MOS symbols

  • Enhancement type – E MOS (normally off):
Mosfet symbols

E Mos symbols

Mosfet structure

Inner structure of E MOSFET with “N” type channel

In this type of transistors, the gate is insulated from the channel with the dielectric layer. The area marked “N+” is heavily doped “N” type semiconductor. In case of E MOS transistors with a voltage of UGS = 0, the channel is blocked (its resistance takes the value of MΩ and the ID current doesn’t flow). By increasing the UGS voltage channel increases its conductivity and after reaching a certain value called UT threshold voltage, the flow of the ID drain current was possible through the channel.

The minimum resistance value of the channel specified by the manufacturer can be found in datasheets as rdson(it’s dependent on the maximum voltage of the UDS transistor. The value of ID current that will flow through the created channel is dependent on the UDS voltage, but it isn’t a linear relation. It’s described by the formula:

βcurrent amplifier ratio
This current affects on the gate bias state by changing it, which results in a narrowing of the channel near the drain. In case of further increasing the gate-source UGS voltage, the cut-off of UGSoff voltage will be exceeded at some point causing the loss of created channel (UGS= UDS)

FET basics parameters:

  • UDSmax – the maximum drain-source voltage,
  • IDmax– the maximum drain current,
  • UGSmax– the maximum gate-source current,
  • Ptotmax– permissible power loss,
  • UP– the threshold voltage at which current begins to flow,
  • IDSS(UGS=0) – the saturation current at determined UDS current,
  • gm [S-Siemens] – transconductance ratio,
  • rds(on)– the minimum resistance value of the channel of the transistor operating in the unsaturation mode,
  • IGmax– the maximum allowable gate current,
  • ID(OFF)– the drain current in cut-off mode – at a voltage | UGS| > | UGS(OFF)|.

Current-voltage FET characteristics:

Transfer characteristics – it describes the relation of ID drain current from the gate-source UGS voltage with determined drain-source UDS voltage.


FET characteristics

D MOS “N”:

Mosfet characteristic

E MOS “N”:

Mosfet characteristics

Drain characteristics (for “N” type unipolar transistor) – it describes the relation of drain Icurrent from the drain-source UDS voltage with determined gate-source UGS voltage. The characteristics area was divided into two parts: the active and saturated scope.

Unipolar characteristics

FET basics practical application of unipolar MOSFET – NMOS transistor

In the practical exercise, the effect of the NMOS transistor in its simplest form is shown as a transistor key. Such use mainly operates in microcontroller applications, it is used to control the signal from the microcontroller to external receivers.

For exercise we need:

The connection schematic looks like this:

Where: V2 – 9V DC power supply, D1 – White LED, R1 – 220 ohm resistor, M1 – BUZ11 transistor, R2 – 1k ohm resistor, V1 – 3V battery (in the schematic sinusoidal source to illustrate transistor operation). Symbols on the schematic are different for the transistor but have similar parameters as BUZ11.

On the breadboard the circuit was connected as follows:

The system after connecting the 9V power did not show any action. After connecting the battery to the circuit, the LED started to glow. This is the simplest way to illustrate the principle of the operation of voltage UT (breakdown voltage) in unipolar transistors. In the BUZ11 transistor, the voltage VGSTh is from 2.1 to 4 V. With the use of 3V batteries we obtain the voltage to open the channel between the drain and the source in the unipolar transistor. The LED starts to glow.

On the waveform below, we see the voltages on the V(n005) battery, which varies between 3 and -3V and are applied to the gate of the transistor, between the current on the LED I(D1). Additionally, on the waveform we can see the VDS voltage, the appearance of the waveform depends on the time of switching on the transistor.

Below you can check a short video of operating unipolar transistor application:

Check Field Effect Transistors: JFET, MOSFET tasks!

2 tasks available


Comments (3)

Write a Message