Semiconductor symbols are used to represent various electronic devices and components as pictograms. A typical electrical or electronic symbol comprises of an outline together with one or more identification symbols. Letter symbols, numbers and subscripts based on the Roman and Greek alphabets are commonly used with a semiconductor symbol to identify their different input or output terminals with regards to conventional current flow.
The shape and outline of a semiconductor symbol is used to represent exactly their operation and current flow direction for connection to the correct electrical polarities allowing them to be connected together. The physical layout and positioning of semiconductor devices on a circuit board is generally quite different from that of a circuit diagram.
Graphical symbols used on electrical circuit diagrams are wide and varied and many of these symbols vary from country to country. These days semiconductor symbols are being standardised internationally with the symbols for diodes, transistors, and power electronic devises being of a common design. The symbols listed below conform to the recommendations of the International Electrotechnical Commission (IEC) and the British Standards Institution (BSI).
Schematic Symbols for Semiconductor Diodes
There are many different types of semiconductor devices which can be classed as a diode, and for most of these types they have a distinct circuit symbol. Some of the major diode types include here are the pn-junction, Schottky, photo diode, and light emitting diode. The schematic symbols for each of these diode types uses the same basic diode format, but is modified to indicate their different functions.
Diodes are two-terminal devices which consist of two semiconductor materials fused together to produce a pn-junction where the “n”-type material is the cathode and the “p”-type material is the anode. Generally a diodes cathode lead is identified by a coloured band.
The basic schematic symbol for a diode looks like an arrow head that points in the direction of conventional current flow from its Anode (A) terminal to its Cathode (K) terminal. The schematic symbol of a diode also shows that if forward-biased, current will flow through the direction of the arrow. However, if the polarity of the voltage is reversed, no current flows.
The schematic symbol for a zener diode and a Schottky diode are similar to that of the basic pn-junction diode except that the line representing the Cathode (K) terminal of the symbol is bent at both ends. The schematic symbol for a light emitting diode (LED) shows light energy dipicted by smaller arrows radiating from it when forward-biased.
| Schematic Symbol |
Symbol Identification | Description of Symbol |
|---|---|---|
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Semiconductor Diode | Semiconductor pn-junction diode which passes curent when forward-biased, and blocks current flow when reverse-biased. Commonly used in small-signal, rectification or high current applications |
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Zener Diode | Zener diode used in its reverse voltage breakdown region for voltage limiting, transient suppression and regulation applications. Available in a range of reverse breakdown voltage values |
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Schottky Diode | Schottky diode consisting of an n-type semiconductor and metal electrode junction producing a very low forward voltage drop and power dissipation and faster switching speed compared to a pn-junction diode |
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Light Emitting Diode (LED) | A semiconductor diode which emits a range of visible and non-visible coloured light from its pn-junction depending on the materials and doping used when forward biased |
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Photodiode | A semiconductor photosensor which allows current to flow through itself in the reverse direction when exposed to incident light energy |
Schematic Symbols for Bipolar Junction Transistors
The schematic symbols used for bipolar junction transistors, or BJT’s indicate the two main types, NPN (Negative-Positive-Negative) transistors and PNP (Positive-Negative-Positive) transistors. Bipolar transistor symbols with a circle around them indicate a descrete device while those without a circle indicate its use within an internal circuitry. For example, logic gates and digital IC’s.
Bipolar transistors are 3-terminal devices with the schematic symbol of a bipolar transistor marked with the letters of “C”, “B” and “E” corresponding to the terminals of Collector, Base and Emitter respectively. Conventional current flows through a bipolar transistor from the Emitter terminal to the Collector terminal while the Base terminal controls the amount of current flow. Normally, these identification letters are not used in circuit diagrams but are included here for clarity.
Other types of bipolar transistor circuit symbols include those for Darlington transistors where two bipolar transistors are connected together to form a single descrete device, and phototransistors which use light energy to operate instead of a Base terminal.
| Schematic Symbol |
Symbol Identification | Description of Symbol |
|---|---|---|
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NPN Bipolar Transistor | Characterised as being a lightly doped p-type base region between two n-type emitter and collector regions with the arrow indicating direction of conventional current flow out |
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PNP Bipolar Transistor | Characterised as being a lightly doped n-type base region between two p-type emitter and collector regions. Arrow indicates direction of conventional current flow in |
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Darlington Pair Transistor | Two bipolar transistor npn or pnp connected in a series common collector configuration to increase overall current gain. Available in PNP and Sziklai pair configuration |
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Phototransistor | NPN Phototransistor sealed in a protective case with glass lens or window for detecting external visible and near infrared light sources. Some models have a base (B) lead available to enable biasing and sensitivity control |
Schematic Symbols for Field Effect Transistors
Field Effect Transistors, or FETs are 3-terminal devices which ccome in a large variety of different types, each with its own semiconductor symbol to describe its operation. The schematic symbols used to represent field effect transistors are marked with the letters of “D”, “G” and “S” corresponding to the terminals of Drain, Gate and Source respectively.
The two main types of field effect transistors are: Junction FET’s or JFETs, and Insulated Gate FET’s or IGFETs. Junction FETs have a symbol which uses an arrow to show conventional current flow direction through their diode junction. Insulated Gate FETs, which are commonly termed MOSFETs due to their metal, oxide and silicon form of construction, have a schematic symbol which shows the Gate insulated from the Drain-to-Source channel. Both the JFET and the IGFET (MOSFET) are available as N-channel or P-channel types.
| Schematic Symbol |
Symbol Identification | Description of Symbol |
|---|---|---|
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N-JFET Transistor | N-channel junction field effect transistor having an n-type semiconductive channel between the Source (S) and Drain (D) terminals with the Gate (G) arrow pointing inwards to indicate direction of conventional current flow |
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P-JFET Transistor | P-channel junction field effect transistor having a p-type semiconductive channel between Source (S) and Drain (D) terminals with the Gate (G) arrow pointing outwards to indicate direction of conventional current flow |
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N-channel D-MOSFET Transistor | Depletion N-channel Metal-Oxide Semiconductor FET (nMOSFET) has a Gate terminal insulated from the main conductive channel and which is normally-on and conducting when VG = 0 volts |
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P-channel D-MOSFET Transistor | Depletion P-channel Metal-Oxide Semiconductor FET (pMOSFET) has a Gate terminal insulated from the main conductive channel and which is normally-on and conducting when VG = 0 volts |
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N-channel E-MOSFET Transistor | Enhancement N-channel Metal-Oxide Semiconductor FET (nMOSFET) has a Gate terminal insulated from the main channel and which is normally-off and closed when VG = 0 volts |
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P-channel E-MOSFET Transistor | Enhancement P-channel Metal-Oxide Semiconductor FET (pMOSFET) has a Gate terminal insulated from the main channel and which is normally-off and closed when VG = 0 volts |
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IGBT Transistor | Insulated Gate Bipolar Transistor (IGBT) is a cross between a BJT and IGFET offering high input MOS characteristics and large bipolar output current-carrying capability and low saturation voltage |
Schematic Symbols for Power Electronics Devices
Power semiconductor devices are a range of electronic devices that are used in electrical circuits to convert, control or switch electrical power. The electrical power that these semiconductor devices control is usually much larger than those controlled by bipolar transistors or field effect transistors above.
Power electronics semiconductor devices such as Thyristors and Triacs are voltage-driven components used to switch and control AC supplies for use in controlled rectifiers, power supplies or AC motor drive controllers. Power electronics devices along with the components detailed above are finding new applications in energy, power, industrial and transportation applications, such as renewable energy technologies, battery charging systems, energy storage, solar inverters, electric vehicles, power converters, HVAC, and many more.
Power electronics circuits and applications are different compared to digital or small circuit electronics systems in terms of its efficiency, size, and power handling capabilities with the following components of silicon controlled rectifiers, gate turn-off rectifiers, triacs and diacs being the key components along with their schematic symbols.
| Schematic Symbol |
Symbol Identification | Description of Symbol |
|---|---|---|
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Silicon Controlled Rectifier | A Silicon Controlled Rectifier (SCR) or Thyristor is a 3-terminal, four-layered PNPN semiconductor unidirectional device with its main terminals labelled Anode (A), Cathode (K) and Gate (G). Once triggered ON it remains conducting as long as current flows through it and can operate at higher voltages and currents |
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Triac | The TRIAC named from Triode for Alternating Current is a 3-terminal bidirectional device which can conduct current in both directions. Its main terminals are labelled MT2, MT1 and Gate (G) and can be triggering into conduction in either direction of the sinusoidal waveform |
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Diac | The DIAC named from Diode for Alternating Current is a 2-terminal bidirectional semiconductor device similar to a PNP transistor without a base terminal characteristic of two diodes back-to-back. Used together with a Triac to conduct current in both directions in AC phase-control, dimming, speed-control and power-control applications |
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Unijunction Transistor | The Unijunction Transistor (UJT) is a semiconductor 3-terminal, single pn-junction switching device with its main terminals labelled Base-1 (B1), Base-2 (B2) and Emitter (E). Programmable UJT’s use external resistors to set their switching parameters and are common in relaxation oscillators |
