How many junction in a transistor

What is the symbol for a transistor? The arrow in the symbol indicates the direction of flow of conventional current in the emitter with forward biasing applied to the emitter-base junction. What is Ebers Moll model? Ebers Moll model is a simple and elegant way of representing the transistor as a circuit model. The Ebers Moll model of transistor holds for all regions of operation of transistor. This model is based on assumption that base spreading resistance can be neglected.

What is depletion region in transistor? In semiconductor physics, the depletion region, also called depletion layer, depletion zone, junction region, space charge region or space charge layer, is an insulating region within a conductive, doped semiconductor material where the mobile charge carriers have been diffused away, or have been forced away by an. Can transistor be replaced by two back to back connected diodes?

Once a metal introduced in this path which is what two back-to-back diodes represent , no BJT functionality is possible. Biasing is controlling the operation of the circuit by providing power supply. The function of both the PN junctions is controlled by providing bias to the circuit through some dc supply. The figure below shows how a transistor is biased.

The N-type material is provided negative supply and P-type material is given positive supply to make the circuit Forward bias. The N-type material is provided positive supply and P-type material is given negative supply to make the circuit Reverse bias. By applying the power, the emitter base junction is always forward biased as the emitter resistance is very small.

The collector base junction is reverse biased and its resistance is a bit higher. A small forward bias is sufficient at the emitter junction whereas a high reverse bias has to be applied at the collector junction.

The direction of current indicated in the circuits above, also called as the Conventional Current , is the movement of hole current which is opposite to the electron current.

The operation of a PNP transistor can be explained by having a look at the following figure, in which emitter-base junction is forward biased and collector-base junction is reverse biased. The voltage V EE provides a positive potential at the emitter which repels the holes in the P-type material and these holes cross the emitter-base junction, to reach the base region.

There a very low percent of holes re-combine with free electrons of N-region. The Bipolar Junction Transistors are only turned ON by the input current, which is given to the base terminal. BJTs can operate in three regions. They are:. The NPN transistor consists of two n-type semiconductor materials and they are separated by a thin layer of p-type semiconductor. Here, the majority charge carriers are electrons while holes are the minority charge carriers.

The flow of electrons from emitter to collector is controlled by the current flow in the base terminal. A small amount of current at base terminal causes a large amount current to flow from emitter to collector. Nowadays, the more commonly used bipolar transistor is NPN transistor, because the mobility of electrons is greater than mobility of holes.

The standard equation for the currents flowing in the transistor is. The PNP transistors contain two p-type semiconductor materials and are separated by a thin layer of n-type semiconductor.

The majority charge carriers in the PNP transistors are holes while electrons are minority charge carriers. The arrow in the emitter terminal of transistor indicates the flow of conventional current. The symbol and structure for PNP transistor is shown below. For the connections in the circuit, we also consider a fourth terminal called Base or Substrate. The FETs have control on the size and shape of a channel between Source and Drain, which is created by voltage applied at Gate.

The Field Effect Transistors are uni-polar devices, as they require only the majority charge carriers to operate unlike BJT, which are bipolar transistors. The JFETs are used as switches, amplifiers and resistors. This transistor is a voltage-controlled device. The voltage applied between gate and source controls the flow of electric current between source and drain of the transistor.

When voltage is applied between gate and source, a channel is formed between source and drain for current flow. This channel is called N—Channel. In this type of JFET, the current flow is because of holes. The channel between source and drain is called P—Channel.

Here, the arrow marks indicate the direction of current flow. It is used in switching and power circuits and it is a main component on Integrated Circuit designing technologies.

The amount of current flowing through the transistor is proportional to the magnitude of the biasing voltage applied to the base terminal. This allows the transistor to act like a current-controlled switch. Depending on whether the bipolar transistor is PNP or NPN, the controlled current will flow from the collector to the emitter or from the emitter to the collector while the smaller controlling current will flow from base to emitter or from emitter to base respectively.

The transistor contains a maximum allowed current that is able to restrict the amount of current as it passes from terminal to terminal.

Depending on the order of the terminals in the transistor, the transistor will act as either a conductor or an insulator when in the presence of a controlled current. This ability to change between these two states, insulator or conductor, enables the transistor to act like a switch or as an amplifier of small amplitude signals applied to the base depending on the structure and order of the three semiconductor regions. Bipolar Junction Transistors contain three doped extrinsic semiconductor regions each connected to a circuit.

The transistor is not symmetrical due to the different doping ratios of the emitter, collector and base regions. The base region consists of a lightly doped materials that exhibits high resistivity. The base is located between the heavily doped emitter region and the lightly doped collector region. The collector engulfs the emitter region which eliminates the ability for electrons injected into the base region to escape the base region without being collected. The emitter region is heavily doped to increase the current gain of the transistor.

For high current gain, a high ratio of carriers injected by the emitter to those injected by the base is needed. Increasing the emitter injection efficiency results in the majority of the carriers injected into the emitter-base junction coming from the emitter region. The high doping ratio of the emitter and collector regions, also means the collector-base junction is reverse biased. The collector-base junction can therefore have a high magnitude reverse bias voltage applied before the junction breaks down.

For the transistor as a whole, the fundamental difference between the NPN Transistor and the PNP Transistor is current directions and voltage polarities of the transistor junctions. Making sure these two are always opposite each other ensures the transistors are properly biased. NPN bipolar transistors are the highest used bipolar transistors due to the ease of electron mobility over electron hole mobility. For this type of transistor, large magnitude collector and emitter currents get produced through the amplification of a small current which enters through the base.

This small current only gets amplified when the transistor becomes active. In this active state, a positive potential difference is found between both the base region to the collector region and the emitter region to the base region which results in current that gets carried by electrons, between the collector and emitter regions.

For a bipolar NPN transistor to conduct the Collector is always more positive with respect to both the Base and the Emitter. The Base terminal is always positive with respect to the Emitter.

The current flowing out of the transistor must be equal to the currents flowing into the transistor as the emitter current is given as. Since the physical construction of the transistor determines the electrical relationship between these three currents, Ib , Ic and Ie , any small change in the base current Ib , will result in a much larger change in the collector current Ic.