PIN Diode: Characteristics & Applications

Instructor: Gerald Lemay

Gerald has taught engineering, math and science and has a doctorate in electrical engineering.

In this lesson, we explore the characteristics of the diode called a PIN diode. In addition to explaining how this diode differs from the more common PN diode, several application examples are presented.

Ever paint a wall, and after the first layer, realize the wall would look better with a second coat of paint or a few artistic dabs of color? Material scientists do this all the time by adding layers to an existing device to try to create something new or better. Sometimes their efforts don't work out, but other times, the results are a new device like the PIN diode.

What Is a PIN Diode?

A pure crystal of silicon or germanium is called intrinsic and does not pass electrical current very well. We will use the letter ''I'' for intrinsic. Scientists change the properties of an intrinsic material by adding atoms of other elements like boron and phosphorous. N-type materials have an excess of negative charge carriers (the ''N'' stands for negative). P-type have an excess of positive charge carriers (''P'' for positive). The intrinsic material by itself is not a good conductor of electricity. An element like copper is naturally a good conductor. Somewhere in between is the semiconductor, which is intrinsic material with intentionally added impurities. Organizing a P-type material with an N-type material creates a PN junction. If wires are attached, we have a PN diode.

But what if the scientist tries layering?

Enter the PIN diode. This diode has a layer of intrinsic material between a P-type semiconductor and an N-type semiconductor. Intrinsic is the ''I'' in PIN. Hence, the name ''PIN''.

What Can the PIN Diode Be Used For?

At low frequencies (usually, less than 1Mhz), the PIN diode behaves like a common PN diode. Typically, a PN diode conducts current when the junction voltage is at least 0.7V. This is when a charge gathers on each side of the junction separating the P and N portions. At lower voltages, the charges reset, and the PN diode is like an open circuit, passing nearly zero current. If we reverse the voltage far enough, the diode enters its breakdown region. All of this is true for the PIN diode, but at low frequencies.

At high frequencies (above 1MHz), there isn't time for the charge carriers to reset through the relatively large intrinsic region. At these frequencies, the relationship between voltage and current is nearly linear, and we effectively have a resistor where the diode used to be. This resistance between the P and N sides depends on the current flowing through the diode. In an actual circuit, this current is controlled by a dc bias voltage.

Thus, we have a voltage-controllable resistor. The resistance of the PIN diode can range from 0.1Ω to 10kΩ. Another word for a device that resists current flow like a resistor is an attenuator.

We can also use the PIN diode as a switch. A switch usually has two states:

• On: there is a short circuit between two points in the switch and current flows
• Off: the switch is an open circuit and no current flows

The components labelled ''C'' are capacitors. In this circuit, they are blocking any direct current (DC) from entering or leaving the circuit. So, the direct current flowing through the PIN diode is due entirely to the DC bias voltage.

The components labelled ''L'' are inductors. The inductor is a short circuit for direct current. Thus, the inductor has no impact on the direct current flowing through the diode. At high frequencies, the inductor is an open circuit. Thus, the high frequency IN signal is blocked by the inductors. This allows the IN signal to flow through the diode and appear at OUT.

And this is exactly what happens when the DC bias voltage produces a relatively high direct current like 100mA. The resistance of the PIN diode is then very low (0.1Ω) and the diode acts like a short circuit.

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