A junction field effect transistor is a type of FET used as a switch that can be controlled electrically. The junction field effect transistor is opposite the bipolar junction transistor. JFETs are voltage-controlled devices. In JFET, the current flow is mostly due to charge carriers. Whereas, in BJT, the current flow is due to both minority and majority charge carriers. Only most of the charge carriers are responsible for the current flow. JFETs are unidirectional. The first working model of the junction field-effect transistor was made in 1953.
Operation of Junction Field Effect Transistor
The working of a JFET can be compared to a garden hose pipe. If there is no blockage then water flows easily through the garden hose pipeline, but if we bend the pipe a bit, the water flow slows down. This is how JFET works. Here the hose is in line with the JFET, and the water flow is equal to a current. We control the current flow by constructing the current carrying channel as per our requirement.
When no voltage is applied to the source and gate, the channel will provide an easy path for electrons to flow. When a polarity that makes the p–n junction reverse biased is applied, the channel becomes narrower by increasing the depletion layer and can put the JFET in the cut-off or pinch-off region.
The types of JFETs are N-channel FET and P-channel FET. In n-channel FET a P-type semiconductor material is added to the N-type semiconductor substrate, whereas in p-channel FET an N-type semiconductor material is added The P-type semiconductor is added to the substrate. JFETs are used as switches, choppers, and buffers, used in oscillatory circuits and cascade amplifiers.
Junction field effect transistors are generally classified into two types based on the source of current flow and they are:
- N-channel Junction Field Effect Transistor.
- P-channel Junction Field Effect Transistor.
N-Channel Junction Field Effect Transistor
The N-channel FET is the most commonly used field effect transistor. For the fabrication of n-channel FETs, a narrow strip of N-type semiconductor material is applied on which the P-type semiconductor material is formed by diffusion on opposite sides. These two sides are joined to form a single connection to the gate terminal.
Two gate depositions of P-type semiconductor material form two P-N diodes. The area between the gates is called the channel. Most carriers pass through this channel.
Ohmic contacts are formed at the two ends of the n-type semiconductor bar, forming a source and a drain. The source and drain terminals can be interchanged.
Operation of N-channel Junction Field Effect Transistor
To turn on the N-channel JFET, a positive voltage of VDD must be applied between the drain terminal of the transistor and the source terminal so that the drain terminal is appropriately more positive than the source terminal. Thus, the current ID is allowed to flow through the drain terminal to the source terminal in the channel. If the voltage at the gate terminal, VGG is 0 V, then the maximum current will be at the drain terminal and here the N-channel JFET is said to be in an ON state.
To turn off the N-channel JFET, the positive bias voltage can be turned off or the negative voltage can be applied to the gate terminal. Thus, the drain current can be reduced by changing the polarity of the gate voltage, and then the N-channel JFET is said to be in off condition.
The power supply at the gate terminal increases the depletion layer and the voltage at the drain terminal allows the drain current to be carried from the source to the drain terminal. Let the source terminal be at point B and the drain terminal points A, then the resistance of the channel is greater than the voltage drop at terminal A, the voltage drop at terminal B. Meaning,
Because of this, the voltage drop is progressive through the length of the channel. The reverse biasing effect is stronger at the drain terminal than at the source terminal. This is because when both VGG and VDD are applied, the reduction layer at point A penetrates more into the channel than at point B.
Operation of Depletion Mode
The width of the depletion layer plays an important role in the operation of the FET. If no potential is applied between the gate and source terminals and a potential VDD is applied between the drain and source, a current id flows from the drain to the source terminal, most of which is greater than the channel width. The voltage applied between the gate and source terminal VGG is reverse-biased. This increases the width of the gap. As the layers increase, the cross-section of the channel decreases, and hence the drain current ID also decreases.
When this drain current is further increased, a state is achieved where both the depletion layers touch each other and stop the current ID flow.
The voltage at which these two depletion layers “touch” each other is called the “pinch-off voltage“. It is denoted as VP. The drain current is almost zero at this point. Hence the drain current is a function of the reverse bias voltage at the gate.
Characteristics of N- Channel Junction Field Effect Transistor
N-channel JFET characteristics or transconductance curve, which is graphed between drain current and gate-source voltage. There are several regions in the transconductance curve and they are ohmic, saturation, cutoff, and breakdown regions.
When the voltage between the gate and source VGS remains zero or is reduced to zero, the source-to-drain current ID is also zero as no voltage VDS is applied. As the voltage Vds between the drain and source increases, so does the current ID flow from source to drain. This increase in current is linear up to a certain point A, known as the Knee voltage.
The gate terminal will remain under reverse biased conditions and the depletion region will shrink as the source of current ID increases. This constriction is uneven in length, causing these regions to move closer to the conduit and away from the conduit, causing a pinch-off voltage. The pinch-off voltage is defined as the minimum drain to source voltage where the drain current reaches a constant value, the saturation value. The point at which this pinch-off voltage occurs is called the pinch-off point, denoted as B.
As the voltage Vds is increased, the channel resistance also increases and the current ID remains practically constant. The region BC is known as the saturation region or amplifier region. All these points along with A, B, and C are plotted in the graph below.
The drain characteristics are plotted for the drain current ID against the drain-source voltage VDS for various values of the gate-source voltage VGS. Below are the overall drain characteristics for different input voltages.
The negative gate voltage controls the drain current, FETs are called voltage-controlled devices. Conduit characteristics are used to obtain the values of drain resistance, permeation, and amplification factor.
- Ohmic Region: The only region in which the transconductance curve shows linear response and the drain current is resisted by the JFET transistor resistance is called the ohmic region.
- Saturation Region: In the saturation region, the N-channel junction field effect transistor is further energized in the on state, as maximum current flows through the gate-source voltage applied.
- Cutoff Region: In this cutoff region, no drain current occurs so the N-channel JFET remains in the OFF position.
- Breakdown Region: If the VDD voltage applied at the drain terminal exceeds the maximum required voltage, the transistor fails to resist current. Because of this, current flows from the drain terminal to the source terminal. Therefore, the transistor enters the breakdown region.
P-Channel Junction Field Effect Transistor
In a P channel JFET, there is a P-type semiconductor material bar with n-type semiconductor material layers on its two sides. The ohmic contacts on both sides form the gate terminal. Like the N channel JFET, the source and drain terminals are connected to the other two sides of the bar. A p-type channel, which has holes as charge carriers, is formed between the source and drain terminals.
A negative voltage applied to the drain and source terminals ensures the flow of current from the source to the drain terminal and the device operates in the ohmic region. The reduction in channel width is ensured by a positive voltage applied across the gate terminal, thus, increasing the channel resistance. If the gate voltage is more positive then the current flowing through the device is less.
Operation of P-channel Junction Field Effect Transistor
To turn on a P-channel JFET, the negative voltage can be applied to the drain terminal of the transistor as the source terminal, such that the drain terminal must be appropriately more negative than the source terminal. Thus, the current is allowed to flow through the drain to the source channel. If the voltage at the gate terminal, VGG is 0 V, then the maximum current will be at the drain terminal and the P-channel JFET is said to be in ON state.
To turn off a P-channel JFET, the negative bias voltage can be turned off or a positive voltage can be applied to the gate terminal. If a positive voltage is given to the gate terminal, the drain current starts decreasing (up to cutoff) and thus the P-channel JFET is said to be in OFF condition.
Characteristics of P- Channel Junction Field Effect Transistor
P-channel JFET characteristics, or transconductance curve, graphed between drain current and gate-source voltage. The transit curve has several regions ohmic, saturation, cutoff, and breakdown regions.
Related Tutorial: Types of Transistors