TRANSISTOR TYPES JFET: Meaning, types, and working principles explained
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.
A junction field effect transistor (JFET) is a three-terminal voltage-controlled device used extensively in digital circuits. The unipolar field effect transistor has two PN junctions inside its structure. JFET works on the principle of depletion region and enables current flow through reverse biasing of gate-to-source terminals. Depending on the dopants, there are two types of JFET transistors- n channel JFET and p channel JFET.
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: VGS = 0, VDS > 0 - VDS = VDrain
At the drain terminal, voltage VDS 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 VDS attracts electrons from the channel towards the source. The conventional current ID 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 VDS = 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
VGS = 0, VDS = VP
As VDS increases, the depletion region starts to get bigger near the drain terminal. When drain-to-source voltage VDS reaches the pinch-off voltage VP, 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 ID increases with increasing VDSt until the pinch-off point. The current ID reaches saturation to become constant and does not increase with increasing VDS. The drain current at the pinch-off point is termed IDSS. The JFET acts as a constant current source beyond pinch-off voltage VP.
Case 2: VGS < 0, VDS > 0 - VDS < VDrain
When VGS is made more negative and VDS is positive but lesser than the previous case VDrain, see figure 8.
The saturation level can be achieved with the much lesser value of VDS because PN junctions become more reverse biased. The value of pinch-off voltage VP exponentially drops.
Characteristics of a JFET is a graph of drain current ID on Y-axis against drain-to-source voltage VDS at X-axis for different values of gate-to-emitter voltage VGS.
The characteristic graph explains that drain current ID obtains a constant value for further increase in drain-to-source voltage VDS.
When drain-to-source voltage VDS reaches a maximum level, breakdown starts to occur in a JFET.
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
What are the most common JFET applications?
Applications of JFET are as follows:
- Voltage-controlled resistor (VCR)
- Voltage variable resistor (VVR)
- Signal chopper
- 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?
JFET is a junction-fet that enables current flow because of the junctions
MOSFET enables current flow by the electric field
Gate terminal is in direct contact with metal contact
Gate terminal is insulated from the device with a silicon dioxide layer
Negative Temperature Coefficient
Positive Temperature Coefficient
Works on the principle of depletion region
Works on the principle of electric field due to parallel plate capacitor
There are only depletion mode JFETs, there is no enhancement mode
There are depletion mode and enhancement mode MOSFETs
Lower than MOSFET
Higher than JFET
High drain resistance
Low drain resistance
Complex but less expensive
Easy but expensive
Extensively used in IC