Some Semiconductor Devices

Diode: Zener: LED:

Photodiode: Schottky: Tunnel:

Varicap: Opto-isolator: Shockley:

Shockley Diode:

Varistor:

A varistor is an electronic component with a

"diode-like" nonlinear current–voltage

characteristic. The name is a portmanteau of

variable resistor. Varistors are often used to

protect circuits against excessive transient

voltages by incorporating them into the circuit

in such a way that, when triggered, they will

shunt the current created by the high voltage

away from sensitive components. A varistor is also known as

Voltage Dependent Resistor or VDR. A varistor’s function is to

conduct significantly increased current when voltage is

excessive. Available in both positive and negative temperature

coefficients, they may be used for overvoltage protection

(MOV) or as a self resetting fuse (polyfuse).

Unijunction transistor UJT: A unijunction transistor (UJT) is an electronic semiconductor device that has

only one junction. The UJT has three terminals: an emitter (E) and two bases (B1 and B2). The base is

formed by lightly doped n-type bar of silicon. Two ohmic contacts B1 and B2 are attached at its ends.

The emitter is of p-type and it is heavily doped. The resistance between B1 and B2 when the emitter is

open-circuit is called interbase resistance.

Graph of UJT characteristic curve, emitter-base1 voltage as a function of emitter current, showing

current controlled negative resistance (downward-sloping region)

There are three types of unijunction transistors:

The original unijunction transistor, or UJT, is a simple device that is essentially a bar of N type

semiconductor material into which P type material has been diffused somewhere along its length,

defining the device parameter \eta. The 2N2646 is the most commonly used version of the UJT.

The complementary unijunction transistor, or CUJT, that is a bar of P type semiconductor material into

which N type material has been diffused somewhere along its length, defining the device parameter

\eta. The 2N6114 is one version of the CUJT.

The programmable unijunction transistor, or PUT, is a close cousin to the thyristor. Like the thyristor it

consists of four P-N layers and has an anode and a cathode connected to the first and the last layer, and

a gate connected to one of the inner layers. They are not directly interchangeable with conventional

UJTs but perform a similar function. In a proper circuit configuration with two "programming" resistors

for setting the parameter \eta, they behave like a conventional UJT. The 2N6027 is an example of such a

device.

BJT (Bipolar Junction Transistor):

Two P-N junctions arranged so that 0.7V at the gate (with respect to the emitter) causes current to flow

in the collector. Up to Beta times the base current can flow in the collector. Can be used for switching or

amplification.

JFET (Junction Field Effect Transistor):

Similar to BJT but a different arrangement of junctions such that NO CURRENT flows in the gate. Each

JFET is characterized by two values: IDSS andf VGSoff.

MOSFET (Metal Oxide Semiconductor FET):

Lots of different MOSFETs… Most often in industry, Enhancement mode N-channel MOSFETs are used. A

positive signal on the gate (wrt source) allows current to flow. “On” resistance can be as low as 0.5m

so it’s best for low voltage/high current applications.

DIAC: AC Diode. Negative dynamic resistance, leading to a decrease in the voltage drop across the diode

and, usually, a sharp increase in current through the diode. The diode remains "in conduction" until the

current through it drops below a value characteristic for the device, called the holding current, IH. Below

this value, the diode switches back to its high-resistance (non-conducting) state. This behavior is

bidirectional, meaning typically the same for both directions of current.

Thyristor: A thyristor is a solid-state semiconductor device with four layers of alternating N and P-type

material. They act as bistable switches, conducting when their gate receives a current trigger, and

continue to conduct while they are forward biased (that is, while the voltage across the device is not

reversed). Thyristor is also known as an SCR (Silicon Controlled Rectifier). Note the similarity between

the SCR and the Shockley Diode above. To turn off, Current must drop below the holding current.

SCR or Thyristors have three states:

1. Reverse blocking mode — Voltage is applied in the direction that would be blocked by a

diode

2. Forward blocking mode — Voltage is applied in the direction that would cause a diode to

conduct, but the thyristor has not been triggered into conduction

3. Forward conducting mode — The thyristor has been triggered into conduction and will

remain conducting until the forward current drops below a threshold value known as the

"holding current"

Thyristor voltage/current characteristic:

Note: Thyristor/SCR is unidirectional.

TRIAC:

TRIAC, from Triode for Alternating Current, is a genericized tradename for an electronic component that

can conduct current in either direction when it is triggered (turned on), and is formally called a

bidirectional triode thyristor or bilateral triode thyristor. To turn off, current must drop below the

holding current. The TRIAC and DIAC are bidirectional and symmetric in operation – that is to say, they

work in an AC circuit, equally well in either direction.

Four conduction modes (quadrants) for TRIAC:

DIAC-TRIAC Dimmer circuit:

When the cap charges up to

about 30V, the DIAC conducts

current into the gate of the

TRIAC, turning it on. Voltage

across the TRIAQC drops to

almost zero and the light comes

on. When the AC voltage drops

to zero, current also drops to

zero and the DIAC and TRIAC

reset. The same thing happens in

the negative AC pulse. Varying

R changes the timing such that

we have an adjustable dimmer

switch like in your dining room.

IGBT:

Insulated Gate Bipolar Transistor. Like a BJT but with no control current due to FET on input.

Unlike SCR’s, the IGBT does not need current to be zero to turn off. Forward voltage drop is

about 2V so it’s mostly used in high voltage/low current applications.

GTO (Gate Turn Off) Thyristor:

Similar to a normal Thyristor but can be turned off by a negative gate pulse.

GCS (Gate Controlled Switch):

Any of a number of devices used to convert signals in digital systems. In this case, positive

pulses turn on the switch and negative pulses turn off the switch.