Transistor Diode Equation

The diode equation gives an expression for the current through a diode as a function of voltage. The following equation is called the shockley ideal diode equation when n the ideality factor is set equal to 1.

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Transistor diode equation. The current your equation describes is the collector current. At the same time holes flow from the positive terminal of the voltage source vbe into the base p region. The ideal diode equation will be derived.

The shockley ideal diode equation or the diode law named after the bipolar junction transistor co inventor william bradford shockley gives the iv characteristic of an ideal diode in either forward or reverse bias or no bias. I the net current flowing through the diode. Current flow in an npn transistor.

The equation is called the shockley ideal diode equation when n the ideality factor is set equal to 1. These holes push the holes near the emitter base junction to start diffusing into the emitter. Silicon small signal transistors typically have a b in the range of 100 300.

The ideality factor n typically varies from 1 to 2 though can in some cases be higher depending on the fabrication process and semiconductor material and is set equal to 1 for the case of an ideal diode. There is no diode current in a bjt only the base collector and emitter currents which can be modeled in different ways. The current is equal to is times e to the qv on kt minus one.

So thats the iv characteristic for the diode where this is v diode right there and the corresponding equation for the resistor is i equals v resistor over 330 ohms. Lets look at the construction of a bjt first to provide some insight into the physics of the device then i can show where this diode equation comes into effect. Their bias voltages and currents are in opposite directions.

The current in a bipolar npn transistor is the ratio of these two currents icib called the dc current gain of the device and is given the symbol of hfe or nowadays beta b. Assuming that we have a b100 transistor what value of the base bias resistor is required to yield an emitter current of 1ma. V applied voltage across the terminals.

While current flow is because of the movement of holes in pnp transistors its due to the movement of electrons in npn transistors. So for the diode we write a current law that looks like this. The main characteristic of a pn junction or a diode is that for positive voltages forward bias the diode is conduction whereas for negative voltages reverse bias the current flow is blocked.

The value of b can be large up to 200 for standard transistors and it is this large ratio between ic and ib. Solving the ie base bias equation for rb and substituting b vbb vbe and ie yield 930kw. I0 dark saturation current the diode leakage current density in the absence of light.

The ideal diode law expressed as. A pnp transistor is a dual of an npn transistor and so they are complementary to each other.

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