What is Shockley Diode : Working & Its Applications

Shockley diode was manufactured & marketed in late 1950 by Shockley Semiconductor Laboratory. The name of this diode was taken from the inventor namely William Bradford Shockley”. This diode was the first device from the thyristor family. So this diode helps in creating other kinds of thyristors such as TRIAC, DIAC, and SCR. It is a four-layer semiconductor diode and is equivalent to a thyristor including a disconnected gate. So this article discusses an overview of a Shockley diode and its working with applications.

What is a Shockley Diode?

Shockley diode definition is; a semiconductor switching diode which includes four layers like P-N-P-N and two terminals like anode & cathode and this diode is also called the PNPN diode or four-layer diode.

The function of this diode is similar to a normal diode exclusive of any trigger inputs. These diodes include simply two conditions like either ON or OFF so, these diodes are classified as a thyristor. This diode has a negative resistance characteristic property which means on voltage increases across the terminals of the device the electric current throughout it will be decreased. The Shockley diode symbol is shown in the following construction diagram.

Shockley Diode Construction & Its Working

The construction of the Shockley diode can be done by connecting the 4-layers to form a PNPN junction. The two transistors’ equivalent circuit of this diode is illustrated below. In this circuit, the collector terminal of the ‘T1’ transistor is connected to the base terminal of the T2 transistor.

The J1 junction can be formed at the T1 transistor’s emitter-base (EB) junction, J2 junction can be formed at base-collector (BC) junction between T1 & T2. J3 junction can be formed at the BE junction of the T2 transistor.

The four layers will comprise two transistors internally. So, J1 is the EB junction of the T1 transistor and J2 junction is the common connected BC of the T1 & T2 transistor and the BE junction of the T2 transistor is J3.

When this diode is operated in forward bias then both the junctions like J1 & J3 are forward biased & J2 junction is reversed biased. Similarly, once the J1 & J3 are connected in reverse bias & J2 is forward biased then the device will start working in this mode. However, when the diode is operated in reverse biased mode & the voltage goes above the breakdown voltage then the diode will get damaged. So this diode is operated only in forward biased mode.

Schottky Diode Vs Shockley Diode

The difference between the Schottky diode and the Shockley diode is discussed below.

Shockley Diode as a trigger Switch

Schooley diodes are used as switching circuits to switch ON the silicon-controlled rectifier. In the following circuit, SCR can be activated through the Shockley diode. The RC network is given through a DC supply to drive the diode.

When the VDC is provided, then the diode will be forward biased and the capacitor gets charged through the resistance. Once the voltage charging of the capacitor reaches the diode’s break-over voltage, the diode will start conducting & the capacitor will start discharging throughout the diode. This diode’s conduction will make the SCR turn ON state, after that the buzzer will generate an alarm.

Once the SCR is switched ON then it will stay in the latching. So there is no effect for the diode in the circuit to make the SCR turn OFF. But the SCR triggering times can be controlled by choosing proper capacitor & resistors components values.

Shockley Diode Equation Derivation

The Shockley diode equation provides I-V characteristics of the diode in either forward bias or reverses bias. So, the Shockley diode equation can be given as;

I = Is( e^Vd/(nVt)-1

Here,

‘I’ is diode current,

‘Is’ is saturation current in reverse bias,

Vd is the voltage across the diode,

Vt is the thermal voltage,

‘n’ is the ideality factor or the quality factor or the emission coefficient.

This factor ‘n’ ranges from 1 to 2 based on fabrication procedure as well as a semiconductor material.

The thermal voltage like ‘Vt’ is approximately 25.85 mV at a temperature of 300 K, a temperature near to the room temperature usually used within device simulation software. So, at any temperature, it is a called constant that can be given by:

Vt= kT/q

Here,

‘k’ is Boltzmann constant.

‘T’ is the PN junction’s absolute temperature

‘q’ is the charge magnitude of the electron.

Characteristics

The Shockley diode characteristics include the following.

The Shockley diode Voltage-current characteristic is shown in the following figure. This diode includes two operating conditions con­ducting & non-conducting. In a non-conducting state, this diode works on a lower line through negligible current & a voltage is less than break over-voltage or switching voltage.

Once the voltage tries to exceed the break-over voltage, then the device will break down & switches through the dotted line in the direction of the on-state or conducting state. Here, the dotted line is known as an unstable otherwise temporary condition.

On this dotted line,  the device remains in this state for a short time because it switches among the two stable operating conditions. In an on-state or conducting state, the device can operate on the higher line, If the current throughout the device is higher than the holding current (I_H), then the voltage across it is a little higher than knee voltage (VK). Once the current drops under the holding current level, then the device will switches back through the dotted line toward the off-state or non-conducting.

Breakover Voltage & Reverse Breakdown Voltage

Once this diode is forward biased, it provides high resistance to the flow of current. Once the forward voltage increases then reverse bias also increased at the J2 junction. At breakover voltage VBo, reverse breakdown occurs for the J2 junction. This breakdown will reduce the resistance, so the diode will provide less resistance. Thus, it moves into a conduction state and acts as a switch. So current starts flowing throughout it.

Similarly, in reverse bias conditions, if a reverse voltage is enhanced sufficiently, then reverse voltage attains reverse breakdown voltage or VBR. So at this voltage, the two junctions like J1 & j3 will go into reverse voltage breakdown. The reverse flow of current throughout them will increase fast & the heat generated by this flow of current could damage the device. So this diode never operated in a reverse voltage higher than reverse voltage breakdown.

Advantages & Disadvantages

The advantages of the Shockley diode include the following.

• These diodes have a high switching speed.
• It can perform more as compared to a transistor because its construction includes two transistors that are connected through a common terminal.
• It gets latched into the on-state.
• Forward voltage drop is low so power dissipation is also low.
• Reverse voltage can be blocked easily.
• It can be protected by a fuse from over current.

The disadvantages of the Shockley diode include the following.

• Its construction is somewhat difficult because it uses four semiconductor material layers.
• It conducts only in a single direction.
• It can switch ON suddenly because of the rate effect.
• Not simple to turn OFF.
• It cannot control at a fast speed.
• It doesn’t include any controlling terminal.
• There is no control above the forward breakdown voltage.

Applications

The applications of the Shockley diode include the following.

• This diode can be used as a trigger switch within the circuits to switch ON an SCR.
• It can be used in relaxation oscillators.

Thus, this is all about an overview of the Shockley diode which performs as a switch. This diode provides very high resistance & conducts no current practically when the forward voltage is below as compared to the breakover voltage. Similarly, this diode provides very less resistance once the voltage is above the break-over value. Here is a question for you, what is a thyristor?