TRANSISTOR TYPES JFET: Meaning, types, and working principles explained

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JFET is a field effect transistor that controls current flow through voltage variation. The unipolar JFET is used as a resistor, switch, amplifier, and many more. This article explains the JFET symbol, types, operations, advantages, and applications. It also explains the differences between two of the widely used field effect transistors- JFET and MOSFET.

What is a JFET

The concept of field effect transistors was first patented by two electrical engineers in 1925 and 1934. But the two engineers were unable to build a working model of a field effect transistor. However, point contact and bipolar junction transistor (BJT) were the first transistors that were invented and demonstrated. Many unsuccessful attempts were made to invent a field effect transistor but in 1952-1953, the first junction field effect transistor (JFET) was built.

The JFET symbol explained

The direction of the arrow in the JFET symbol explains the direction of the gate current.

• For an n channel JFET transistor, the arrow moves toward the device.

• For a p channel JFET transistor, the arrow moves away from the device.

The arrow may sometimes appear at the center of the device and a circle may or may not encompass JFET.

The two types of JFET

A JFET transistor has three terminals: gate, drain, and source. Similar to BJT, the source is analogous to the emitter and the drain is like the collector. JFET is constructed using metal contacts and doping the transistor with pentavalent and trivalent impurities. The doping profile depends upon the type of JFET used. There are two JFET transistor types: n channel JFET and p channel JFET.

N type JFET

An n channel JFET contains an N-type material between the drain and source terminals. Two P-type materials are embedded along the metal contacts of the gate terminal. The presence of two P-type materials forms two PN junctions inside the JFET transistor.

P type JFET

A p channel JFET contains a P-type material between the drain and source terminals. Two N-type materials are embedded along the metal contacts of the gate terminal. The presence of two N-type materials forms two PN junctions inside the JFET transistor.

When JFET Transistor acts as an amplifier, it has three topologies:

• Common source

• Common gate

• Common drain (source follower)

N-channel Common Source JFET Amplifier is the preferred configuration.

How does a JFET work?

Let us consider the working of an n channel JFET transistor.

Case 1: V_GS = 0, V_DS > 0 - V_DS = V_Drain

At the drain terminal, voltage V_DS is some positive voltage greater than zero. But the source terminal is at ground 0V. It clearly means that voltage drops several times across the length of the n-channel from drain to source. The positive potential of V_DS attracts electrons from the channel towards the source. The conventional current I_D flows from the drain to the source. Since the n-channel provides resistance to the flow of electrons, it can be considered a network of resistances.

An equivalent resistive series network of four voltages explains voltage drops across the length of the JFET n-channel. Suppose V_DS = 4V and a series network of four resistors across the length of the channel. The value of each resistor is 1 ohm.

According to ohm’s law V = IR,

The voltage drop across each point is 3V, 2V, 1V, and eventually 0V at the source ground.

The resistive network explains that the width of the depletion region increases towards the drain terminal. The depletion region towards the source terminal is narrower than the above layers. The gate-to-source terminal is at a lower potential compared to the drain-to-source bias. It implies that the PN junction at the gate terminal is reversed biased.

Pinch off voltage

V_GS = 0, V_DS = V_P

As V_DS increases, the depletion region starts to get bigger near the drain terminal. When drain-to-source voltage V_DS reaches the pinch-off voltage V_P, it appears that both the depletion layers would adjoin. However, the depletion regions never touch due to electrostatic repulsion and allow current flow.

The current I_D increases with increasing V_DSt until the pinch-off point. The current I_D reaches saturation to become constant and does not increase with increasing V_DS. The drain current at the pinch-off point is termed I_DSS. The JFET acts as a constant current source beyond pinch-off voltage V_P.

Case 2: V_GS < 0, V_DS > 0 - V_DS < V_Drain

When V_GS is made more negative and V_DS is positive but lesser than the previous case V_Drain, see figure 8.

The saturation level can be achieved with the much lesser value of V_DS because PN junctions become more reverse biased. The value of pinch-off voltage V_P exponentially drops.

JFET Characteristics

Characteristics of a JFET is a graph of drain current I_D on Y-axis against drain-to-source voltage V_DS at X-axis for different values of gate-to-emitter voltage V_GS.

The characteristic graph explains that drain current I_D obtains a constant value for further increase in drain-to-source voltage V_DS.

When drain-to-source voltage V_DS reaches a maximum level, breakdown starts to occur in a JFET.

JFET advantages

JFET has many advantages compared to other transistors:

• JFET is smaller in size

• More JFETs are fabricated on a single IC because it occupies less space

• JFET has high input impedance

• JFET needs low input current

• JFET consumes less power

• JFET provides very high power gain

• JFET produces less noise at higher frequencies

• JFET is more thermally stable than other junction transistors

• JFET acts as a constant current source

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What are the most common JFET applications?

Applications of JFET are as follows:

• Voltage-controlled resistor (VCR)

• Voltage variable resistor (VVR)

• Signal chopper

• Switch

• Amplifier

• Integrated circuits (IC)

• Constant current source

• analog-to-digital and digital-to-analog converters

• Sample and hold circuits

JFET vs. MOSFET: What is the difference?

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