Unit 1 Semiconductor Devices

1.1 Silicon controlled Rectifier (SCR) or Thyristor

SCR is four layers, three terminal semiconductor device

Fig 1.1 construction and symbol of Silicon controlled Rectifier (SCR)

It is mainly used in the devices for the control of high power. Silicon controlled rectifier is also sometimes referred to as SCR diode, 4-layer diode, 4-layer device, or Thyristor. It is made up of a silicon material which controls high power and converts high AC current into DC current (rectification). Hence, it is named as silicon controlled rectifier.

Silicon controlled rectifier is a unidirectional current controlling device. Just like a normal p-n junction diode, it allows electric current in only one direction and blocks electric current in another direction. A normal p-n junction diode is made of two semiconductor layers namely P-type and N-type. However, a SCR diode is made of 4 semiconductor layers of alternating P and N type materials.

1.2 Characteristics of SCR

Fig 1.2 Characteristics of SCR

1. Forward Breakover voltage - It is voltage at which scr is swithed on from its off position to on position

2. Holding current -It is value of anode current below which scr switches from its on position to OFF position under the given condition

3. Gate trigger current -It is value of anode current necessary to switch from its off position to on position under specified condition

4. Average forward current – It is maximum continuous value of anode current which scr can handle in its ON position under specified condition

1.3 SCR works in 3 modes

1. Forward blocking mode2. Reverse blocking mode3. Forward conduction mode

1. Forward blocking mod e [VAK = +ve & VG = 0]

When a positive voltage is applied to anode with respect to cathode, the junctions J1 and J3 are forward biased, junction J2 is reverse biased.

The SCR is in Forward Blocking state. At this time the Gate signal is not applied.

A depletion layer is formed in junction J2 and no current flows from anode to cathode.

As shown in VI Characteristic, a small amount of current called forward leakage current flows through the device.

2.Forward conduction mode [V AK = +ve & VG  =   +ve]

When the small amount of positive voltage is applied to gate terminal, while positive voltage is applied to anode with respect to cathode, the junction J3 becomes forward biased.

So the SCR acts as a closed switch and conducts a large value of forward current with small voltage drop.

With the application of gate signal the SCR changed from forward blocking state to forward conducting state. It is called as latching.

Without gate signal SCR changed from forward blocking state to forward conducting state at forward breakdown voltage (Vfbd).

When the gate signal value is increased, the latching happens for low Vak

voltages as mentioned in the figure. In the presence of forward current (i.e. after the thyristor is turned on by a

suitable gate voltage) it will not turn off even after the gate voltage has been

removed.  The thyristor will only turn off when the forward current drops below holding current.

The holding current is defined as the minimum current required to hold the SCR in the forward conduction state.

3.Reverse conduction mod e

As the reverse voltage is further increased, at the reverse breakdown voltage(VBR) Avalanche breakdown occurs at junction J1 and J3.

SCR acts as a closed switch in reverse direction A large current gives more losses in SCR, dissipating in the form of heat,

thereby damaging the SCRSCR triggering

1. Gate triggering In this method scr is operated with anode voltage slightly less than the rated forward breakover voltage and scr is triggered into conduction by low power gate pulse.

2. Forward breakover voltage If the voltage across scr exceeds the rated forward breakover voltage,scr will start conducting due to avalanche breakdown.

3. dv/dt triggering -

In this method scr is turn on by rapidly increasing the anode to cathode voltage.The rapidly increasing voltage produces charging current which triggers the scr to conduction

4. Light triggering

In this method scr is triggered by irradiating with the light.When light falls on middle junction J2 of SCR ,SCR turns on.

1.4 Applications of SCR

SCRs are used for controlled rectifiers.

AC regulators, lighting and heating applications. DC motor drives large power supplies and electronic circuit breakers

1.5 DIAC (Diode as a switch)

The diac is two terminal device which can pass current in either direction when breakover voltage is reached in either polarity across two terminals. The basic construction of DIAC is similar to triac,but without gate terminal. As shown in figure

Fig1.3 Symbol of DIAC Fig1.4 Construction of diac

DIAC consist of four layer diodes connected in parallel but i opposite direction. Four layers of dioded are P1N1P2N2 and P2N1P1N3 as shown by broken lines in figure.

The diac can be performing for both the direction . DIAC is a diode that conducts after a ‘break-over’ voltage, selected VBO, and is exceeded. When the diode exceeds the break-over voltage, then it goes into the negative dynamic resistance of region. This causes in a reduce in the voltage drop across the diode with rising voltage. So there is a quick increase in the current level that is mannered by the device

The holding current, the DIAC reverts to its non-conducting state. Its behavior is bidirectional and thus its function takes place on both halves of an alternating cycle.

1.6 VI characteristics of DIAC

From the figure above, we can see that a DIAC has two p-type material and three n-type materials. Also, it does not have any gate terminal in it.The DIAC can be turned on for both the polarity of voltages. When A2 is more positive with respect to A1 then the current does not flows through the corresponding N-layer but flows from P2-N2-P1-N1. When A1 is more positive A2 then the current flows through P1-N2-P2-N3.The construction resembles the diode connected in series.

When the applied voltage is small in either polarity, a very small current flows which is known as leakage current because of the drift of electrons and holes in the depletion region. Although a small current flows, it is not sufficient to produce avalanche breakdown, hence the device remains in the non-conducting state.

When the applied voltage in either polarity exceeds the breakdown voltage, DIAC current rises and the device conducts in accordance with its V-I characteristics.

DIAC is an important device in the thyristor family.The main advantage of using this device is-

1. It does not switch sharply to a low voltage condition at a low current level as done by SCR or TRIAC.

2. It has low on state voltage drop until its current falls below the holding current level.

3. Voltage drop decreases with the increase in current.

1.7 TRIAC

TRIAC is three terminal device which can conduct in either direction,when triggered by either positive or negative pulse irrespective of polarity of voltage across its main terminals.

As such the triac can be considered as a pair of parallel but opposite thyristors with the two gates connected together and the anode of one device connected to the cathode of the other, etc.

The triac is a development of the scr . While the scr can only control current over one half of the cycle, the triac controls it over two halves of an AC waveform.

Fig 1.5 construction, Equivalent circuit and symbol of triac resply

1.8 Characteristics of TRIAC

Fig 1.6 Characteristics of TRIAC

1.9 TRIAC operation

1. A positive voltage at MT2 and a positive pulse at the gate2. A positive voltage at MT2 and a negative pulse at the gate3. A negative voltage at MT2 and positive pulse at the gate4. A negative voltage at MT2 and a negative pulse at the gate

1. A positive voltage at MT2 and a positive pulse at the gate

When terminal MT2 is positive with respect to terminal MT1 current flows through path P1-N1-P2-N2. The two junctions P1-N1 and P2-N2 are forward biased whereas junction N1 P2 is blocked. The triac is now said to be positively biased. A positive gate with respect to terminal MT1 forward biases the junction P2-N2 and the breakÂdown occurs as in a normal SCR.

2.A positive voltage at MT2 and a negative pulse at the gate

Though the flow path of current remains the same as in mode 1 but now junction P2-N3 is forward biased and current carriers injected into P2 turn on the triac.

3.A negative voltage at MT2 and positive pulse at the gate

When terminal MT2 is negative with respect to terminal MT1, the current flow path is P2-N1-P1-N4. The two junctions P2-N1 and P1 – N4 are forward biased whereas junction N1-P1 is blocked. The triac is now said to be negatively biased. A negative gate with respect to terminal MT1 injects current carriers by forward biasing juncÂtion P2-N3 and thus initiates the conduction.

4.A negative voltage at MT2 and a negative pulse at the gate

Though the flow path of current remains the same as in mode 3 but now junction P2-N2 is forward biased, current carriers are injected and therefore, the triac is turned on. Generally, trigger mode 4 should be avoided especially in circuits where high di/dt  may occur. The sensitivity of triggering modes 2 and 3 is high and in case of marginal triggerÂing capability negative gate pulses should be used. Though the triggering mode 1 is more sensitive compared to modes 2 and 3, it requires a positive gate trigger. However, for bidirectional control and uniform gate trigger modes 2 and 3 are preferred.