Characteristic of Junction Field Effect TransistorPublished on 19/11/2018 & updated on 20/11/2018
There are two types of junction field effect transistor. N channel JFET P channel JFET Let us discuss the characteristics of n channel and p channel transistor separately one by one for better understanding.
Characteristic of N Channel JFETWe know that there is a channel of n-type semiconductor material in n channel JFET. The gate region of the n channel JFET is a highly doped p-type region. A voltage is applied across the channel i.e. between drain and source terminal. First, let us plot the values of the drain to source current for different applied drain to source voltages. At first case, we will short circuit the gate terminal with the source terminal and ground them commonly. Now we will slowly increase the drain circuit voltage VDD from zero. The drain to source current or simply saying drain current increases linearly as the channel will have a resistance. But this resistance is not perfectly constant at that region of the characteristic. As the voltage of the n channel is positive in respect of zero potential gate region the gate to channel pn junction will be in reverse biased condition. As a result, there will be reverse biased depletion layer along the junction. The typicality of this depletion layer is that it has more width towards the drain terminal. Due to voltage distribution along the channel the portion of the junction nearer to drain gets more potential stress. With increasing drain voltage the depletion layer towards drain terminal gets thicker more rapidly than that towards source terminal. The resistance of channel increases as the width of the depletion layer increases and hence channel opening decreases. The rise of resistance is more prominent at higher drain potential and hence the characteristic curve drawn against drain current and drain to source voltage gradually gets aligned along horizontal axis means along the drain to source voltage axis. After a certain drain voltage the depletion layer towards drain terminal touches each other. At that point the curve gets very nearly horizontal. At that drain voltage the depletion layers do not perfectly touch to block the channel rather there will be a narrow opening between the layers through which the drain current continues to flow. If we further increase the drain voltage the depletion layer will try to increase its thickness further but they in no way can touch each other rather the depletion layer more towards source terminal get closure and increases the effective length of the narrow channel opening. This phenomenon is known as channel modulation. Due to this phenomenon, the effective resistance of the channel gets increased and that increment is almost proportional to increment of drain voltage. As a result, the drain current becomes almost constant. The drain current to drain to source voltage curve gets its horizontal portion. The voltage after which the drain current becomes almost constant is known as pinch-off voltage. The drain current at pinch-off voltage when get terminal is in ground potential, is denoted as IDSS and known as Shorted Gate Drain Current. Now if we go on increasing the drain voltage after a certain value of drain to source voltage, the depletion layers get broken down and drain current gets suddenly rises. This region of the characteristic is called breakdown region. The portion of the curve when drain current increases with increasing drain to source voltage is known as linear region or ohmic region and the portion of the curve when drain current remains almost constant is known as constant current or active region. Now we will open the gate terminal from grounded source terminal and apply certain negative voltage at gate terminal. In this situation the junction between gate region and channel gets more quick reverse biasing and hence the drain current for same drain to source voltage becomes lower. The entire curve against drain current and drain to source voltage for applied negative gate voltage, is shifted below the zero gate voltage curve. If we apply more negative voltage at gate terminal the curve will shift more downwards as shown in the figure of the characteristic of an n channel JFET, below
Transfer Characteristic of N Channel JFETThe transfer characteristic is drawn between gate voltage and drain current by keeping drain to source voltage at pinch-off voltage. When the gate is in zero potential the maximum drain current flowing through the transistor is shorted gate drain current(IDSS). Now as the negative potential of the gate increases the corresponding drain current get decreased. After a certain negative gate voltage, the drain current becomes zero. This negative gate terminal voltage at which drain current becomes zero for the applied drain to source voltage same as pinch-off voltage is called gate to source cut off voltage VGS(off).
Characteristic of P Channel JFETIn p channel JFET we apply negative potential at drain terminal. If we make grounded both source and gate terminal and increase the negative potential of the drain from zero we will get the same curve as in the case of n channel JFET. Here at the beginning the drain current flowing from source to drain due to drift of holes in the same direction, linearly gets increased with increasing negative drain voltage. As the negative potential of the channel is more towards drain terminal hence the reverse biasing of the junction nearer to drain is more. This causes thicker depletion layer towards drain terminal. So just like the previous case the pinch-off occurs after certain negative drain voltage and the curve becomes horizontal. If we go on increasing negative drain voltage, after a certain negative drain voltage the depletion layers go through avalanche breakdown and the channel gets free from any further obstruction and drain current suddenly rises to a higher value. Hence, the curve will have a linear region at the beginning, an active region in the middle and breakdown region at the end. Now if we apply positive voltages at the gate terminal, the reverse biasing of the junction becomes more rapid and as a result, the characteristic curve gets shifted downward as shown below.
Transfer Characteristic of P Channel JFETThis is drawn between positive gate voltage and drain current. The pattern will be the same as in the case of n channel JFET but the polarity of the applied voltage and direction of the drain current differ.
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