Electronics/Diodes

Electronics | Foreword | Basic Electronics | Complex Electronics | Electricity | Machines | History of Electronics | Appendix | edit

---

Diode

Theoretically, a diode allows current flow in only one direction. An ideal diode acts as a perfect insulator for currents flowing in one direction and as a perfect conductor for currents flowing through it in the other direction. The direction in which the diode allows current to flow is called the forward bias direction and that in which current is resisted is called reverse bias direction. Diode has a symbol as shown

Construction

The modern semiconductor diode consists of two regions of semiconductor each having impurities of different types such that one side has excess holes (p-region) and the other has excess electrons (n-region). Such a junction of p and n regions is called a pn junction diode. The p-region has about twice as much area as the n-region to compensate for the lesser mobility of holes compared to electrons.

+ o-- [P | N]--o - .Theoretically, a diode allows current flow in only one direction the most common type today is a crystalline piece of material with a p junction connected to two electrical terminals.

Operation

I V Curve

As seen in the graph above the diode actually works in both the forward region and the backward region. In the forward region the value of I and V are positive and in the backward region I and V are negative.

Forward Region

Current and Voltage are positive

When V < V_d . I = 0 . Diode does not conduct

Khi V = V_d . I = 1mA . Diode starts to conduct . V_d = 0.3v|Ge , 0.6v|Si

V_d is called Forward Break Over Voltage

Khi V > V_d . Diode conducts current . Current is calculated by $I=I_{o}[e^{({\frac {V_{d}}{nV_{T}}})t}-1]$

Backward Region

Current and Voltage are Negative

When the value of voltage is more negative than the Peak Inverse Voltage (PIV) Voltage the Diode will be destroyed

Ideal Diode

The real diode approaches the ideal diode in the sense that the reverse current is extremely small (less than 1fA) at least for a significant part of the characteristic, and the forward current is very high (on the order of 1mA). Although a real diode does not have the characteristics as the ideal diode, in theory it is possible to make an ideal diode if the concentrations of dopants in both the regions are infinite. However, there is no way of actually doing this and experiments do not agree.

The Shockley equation

The diode reverse (saturation) current is governed by the doping concentration. The current flowing through the device varies as the voltage applied across it changes as given by the Shockley diode equation (not to be confused with Schottky):

$I_{D}=I_{S}[exp(V_{D}/nV_{T})-1]$

In the equation above $V_{T}$ is defined as $V_{T}=kT/q$, where $k$ is Boltzmann's constant, $T$ is the temperature in Kelvin, and $q$ is the magnitude of the charge on an electron.

In the forward bias direction, current flows with low voltage. If one draws a characteristic for this equation, a sharp increase in current can be seen at a particular voltage called the cut-in voltage or the on-voltage.

In the reverse bias mode, the diode current is approximately $-I_{S}$. This is called the reverse saturation current because it looks like the diode is saturated with charge and cannot allow more current in the reverse bias direction than this.

It initially appears that as the temperature $T$ increases, the total diode current would decrease. However, the saturation current $I_{S}$ increases with temperature faster than the $V_{T}$ term decreases the current. This leads to a negative temperature coefficient for the entire device: as the diode heats up, it will pass more current.

Break-down

However, a break from the above equation takes place at a point called break-down voltage. One could think of it as the point where the Shockley equation breaks down and is no longer valid. There are two reasons for breakdown to occur.

Avalanche Breakdown

This occurs as a result of excess minority carriers in a region. Minority carriers are those carriers that are in the wrong region. For example, electrons will be minority carriers in the p-region.

Zener Breakdown

This is basically due to a size difference or dopant concentration difference. One of the regions has a greater region of depletion (Reverse bias voltage induces a depletion region, which is sparse in a densely doped region and dense in a sparsely doped region. )

See also Zener diodes

Summary

So basically, there are three modes in which a diode operates:

Forward

No current flows until a small forward voltage is reached called cut-in voltage.

Reverse

The diode prevents current from flowing in the opposite direction. Current is small, and voltage can be large (but not exceeding the Zener voltage.)

Breakdown

Once the diode voltage is more negative than the Zener voltage, the diode allows current to flow in the reverse direction.

When there is no voltage applied, the excess electrons of the N type semiconductor flow into the holes of the P type semiconductor. This creates a depletion region that acts as a voltage.

Diode Variations

Bridge Rectifier:

A diode circuit that 'rectifies' alternating current (AC) into direct current (DC). The bridge rectifier is a full-wave rectifier, meaning that both the positive and negative portions of the wave become positive. (In a half-wave rectifier, positive stays positive, and negative becomes zero.) The bridge rectifier has advantages over other full-wave rectifier designs, because it reduces peak-inverse voltage (PIV), the largest negative voltage across a single junction diode. By reducing the PIV, it becomes possible to use diodes with lower breakdown (Zener) voltages. This allows the use of cheaper diodes to perform the same function.

A bridge rectifier shown as 4 discrete diodes.

LED (Light Emitting Diode)

A diode that emits light!

Schottky:

A diode made from a metal-semiconductor junction, rather than an p-type/n-type silicon junction. These diodes typically have a much lower forward voltage drop than standard diodes (around 0.2V versus 0.6V).

Zener:

A diode that is meant to be operated in the breakdown region. These diodes have lower Zener (breakdown) voltages, so that they can achieve the breakdown mode without melting. Unlike other diodes, these have very specific breakdown voltages, typically between 2 and 200 volts. See also Zener diodes

Varicap:

A diode that is build to use the capacity in dependance of the cross voltage. Although they rectify, they will be used in tuning circuits as a replacement of manual operated variable capacitors. They permit the use of electronic equipment to be controlled, this permits a phase-locked loop and other circuits which needs stability and electronic control. Semiconductor diodes begin conducting electricity under certain conditions.