Depletion Mode Fet



The P-N junction diode appeared in the year 1950. It is the most essential and the basic building block of the electronic device. The PN junction diode is a two-terminal device, which is formed when one side of the PN junction diode is made with p-type and doped with the N-type material. The PN-junction is the root for semiconductor diodes. The various electronic components like BJTs, JFETs, MOSFETs (metal-oxide–semiconductor FET), LEDs and analog or digital ICs all supports semiconductor technology. The main function of the semiconductor diode is, it facilitates the electrons to flow totally in one direction across it. Finally, it acts as a rectifier. This article gives brief information about the PN junction diode, PN junction diode in forwarding bias and reverses bias and the VI characteristics of PN junction diode

Construction (a) and symbol (b) of n-channel depletion-mode IGFET/MOSFET. It has a normally-open n-type channel between drain and source, but the channel width is controlled by the electrostatic field of the gate bias. The channel can be closed by applying suitable negative bias, or can be increased by applying positive bias. The depletion MOSFET with adjusted drain current powers up the IC. If the voltage delivered by the auxiliary winding is high enough the NPN transistor is triggered, pulling down the gate of the depletion MOSFET below its threshold voltage and switching it completely off. Example: The depletion MOSFET BSS126 (600 V, 700 Ω, SOT-23) is a good choice.

What is a PN Junction Diode?

There are three possible biasing conditions and two operating regions for the typical PN-Junction Diode, they are zero bias, forward bias and reverse bias.


When no voltage is applied across the PN junction diode then the electrons will diffuse to P-side and holes will diffuse to N-side through the junction and they combine. Therefore, the acceptor atom close to the P-type and donor atom near to the N-side is left unutilized. An electronic field is generated by these charge carriers. This opposes further diffusion of charge carriers. Thus, no movement of the region is known as the depletion region or space charge.

If we apply forward bias to the PN-junction diode, that means the negative terminal is connected to the N-type material and the positive terminal is connected to the P-type material across the diode which has the effect of decreasing the width of the PN junction diode.

If we apply a reverse bias to the PN-junction diode, that means the positive terminal is connected to the N-type material and the negative terminal is connected to the P-type material across the diode which has the effect of increasing the width of the PN junction diode and no charge can flow across the junction

Mode

Zero Biased PN Junction Diode

In the zero bias junction, potentially provides higher potential energy to the holes on the P and N side terminals. When the terminals of the junction diode are shorted, few majority charge carriers in the P-side with plenty of energy to overcome the potential barrier to travel across the depletion region. Therefore, with the help of majority charge carriers, the current starts to flow in the diode and it is denoted to as forwarding current. In the same way, minority charge carriers in the N-side move across the depletion region in reverse direction and it is referred to as reverse current.


Potential barrier opposes the movement of electrons & holes across the junction and permits the minority charge carriers to drift across the PN junction. However, the potential barrier helps minority charge carriers in P-type and N-type to drift across the PN-junction, then equilibrium will be established when the majority charge carriers are equal and both moving in reverse directions so that the net result is zero current flowing in the circuit. This junction is said to be in a state of dynamic equilibrium.

When the temperature of the semiconductor is increased, minority charge carriers have been endlessly generated and thus leakage current starts to rise. But, electric current cannot flow since no external source has been connected to the PN-junction.

PN Junction Diode in forwarding Bias

When a PN-junction diode is connected in a forward bias by giving a positive voltage to the P-type material and a negative voltage to the N-type terminal. If the external voltage becomes more than the value of the potential barrier (estimate 0.7 V for Si and 0.3V for Ge, the opposition of the potential barriers will be overcome and the flow of current will start.Because the negative voltage repels electrons near to the junction by giving them the energy to combine and cross over with the holes being pushed in the opposite direction to the junction by the positive voltage.

The result of this in a characteristic curve of zero current flowing up to built-in potential is called as “knee current” on the static curves & then a high current flow through the diode with a slight increase in the external voltage as shown below.

Depletion Mode Fet

VI Characteristics of PN Junction Diode in forwarding Bias

Enhancement Mode Vs Depletion Mode Mosfet

The VI characteristics of PN junction diode in forwarding bias are nonlinear, that is, not a straight line. This nonlinear characteristic illustrates that during the operation of the N junction, the resistance is not constant. The slope of the PN junction diode in forwarding bias shows the resistance is very low. When a forward bias is applied to the diode then it causes a low impedance path and permits to conduct a large amount of current which is known as infinite current. This current starts to flow above the knee point with a small amount of external potential.

The potential difference across the PN junction is maintained constant by the depletion layer action. The max amount of current to be conducted is kept incomplete by the load resistor because when the PN junction diode conducts more current than the normal specifications of the diode, the extra current results in the heat dissipation and also leads to serving damage to the device.

PN Junction Diode in Reverse Bias

When a PN junction diode is connected in a Reverse Bias condition, a positive (+ Ve) voltage is connected to the N-type material & a negative (-Ve) voltage is connected to the P-type material.

When the +Ve voltage is applied to the N-type material, then it attracts the electrons near the positive electrode and goes away from the junction, whereas the holes in the P-type end are also attracted away from the junction near the negative electrode.

In this type of biasing, current flow through the PN junction diode is zero. Though, the current leakage due to minority charge carriers flows in the diode that can be measured in a UA (microamperes). As the potential of the reverse bias to the PN junction diode ultimately increases and leads to PN junction reverse voltage breakdown and the current of the PN junction diode is controlled by an external circuit. Reverse breakdown depends on the doping levels of the P & N regions. Further, with the increase in reverse bias, the diode will become short-circuited due to overheating in the circuit and max circuit current flows in the PN junction diode.

Depletion Mode Fet Spice Model

VI Characteristics of PN Junction Diode in Reverse Bias

P Channel Depletion Mosfet

In this type of bias, the characteristic curve of the diode is shown in the fourth quadrant of the below figure. The current in this biasing is low till breakdown is reached and hence the diode looks like an open circuit. When the input voltage of the reverse bias has reached the breakdown voltage, reverse current increases enormously.

Therefore, this is all about PN junction diode in zero bias, forward bias and reverse bias conditions and VI characteristics of PN junction diode. We hope that you have got a better understanding of this concept. Furthermore, any doubts regarding this article, or electronics projects please give your feedback by commenting in the comment section below. Here is a question for you, which diode is used in the phototransistor?

Depletion Mode Fet

Photo Credits:

Depletion Mode Fet Symbol

  • VI characteristics of PN junction diode by tutorvista
  • Zero Biased PN Junction Diode by expertsmind




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