Photodiode Symbol

Q: What is the difference between the photodiode symbol and the LED symbol?

Both start from the same diode triangle-and-bar. On a photodiode the two small arrows point toward the diode, showing light arriving at the junction; on an LED they point away, showing light being emitted. Functionally they are converses: an LED converts current to light and is operated forward biased, while a photodiode converts light to current and is normally operated reverse biased.

Q: Which way round does a photodiode go in a circuit?

For photoconductive (reverse-biased) operation, the cathode goes to the positive supply rail and the anode toward the lower potential — the opposite of how you would forward-bias a rectifier. Light then generates a photocurrent that flows through the external load. On the physical package the cathode is the marked or shorter lead, matching the bar of the symbol. In photovoltaic mode there is no bias; the anode is the positive output terminal, like a tiny solar cell.

Q: What is the difference between photoconductive and photovoltaic mode?

Photoconductive mode applies reverse bias: the widened depletion region gives fast response (nanoseconds), lower capacitance and excellent linearity, at the cost of dark current and its noise. Photovoltaic mode uses zero bias: the diode self-generates voltage/current like a solar cell, with zero dark current and lowest noise, but slower response. Instrumentation favours photovoltaic mode for precision at low frequency; communications and encoders use photoconductive mode for speed.

Q: What is the difference between a photodiode and a phototransistor?

A photodiode produces photocurrent directly with no gain — typically microamps — but responds in nanoseconds and is very linear. A phototransistor uses light as base drive and multiplies it by the transistor's current gain, delivering 100–1,000 times more current from the same illumination, but with microsecond response and poorer linearity. Choose a photodiode for speed and measurement accuracy, a phototransistor for simple, sensitive on/off light detection.

Q: What is dark current in a photodiode?

Dark current is the small leakage current (typically 1–30 nA for small silicon devices) that flows through a reverse-biased photodiode even with no light present. It rises exponentially with temperature — roughly doubling every 8–10 °C — and with reverse voltage. Because it is indistinguishable from photocurrent, it sets the minimum detectable light level; precision designs reduce it by using photovoltaic mode, cooling, or subtracting a matched dark reference diode.

The Photodiode symbol (IEC 60617 / ANSI Y32.2).

Definition: The Photodiode symbol represents a light-sensitive semiconductor diode with Anode and Cathode terminals, drawn per IEC 60617 as the standard diode triangle-and-bar with two small parallel arrows pointing toward the device to indicate incident light (the mirror image of the LED, whose arrows point away).

Also known as: photodiode, photo diode, light sensor diode, PIN photodiode, photodetector, optical receiver diode, avalanche photodiode (APD).

What the Photodiode symbol means

The photodiode symbol denotes a PN or PIN junction diode engineered so that photons reaching the junction generate electron-hole pairs, producing a current proportional to light intensity. It is the fundamental fast light sensor of electronics: where an LDR responds in milliseconds, a photodiode responds in nanoseconds, making it the receiver of choice for fibre-optic links, IR remote controls, optical encoders, pulse oximeters and light meters. The two incoming arrows are the entire semantic payload of the symbol — remove them and it is an ordinary diode; reverse them and it becomes an LED.

A photodiode operates in one of two modes. In photoconductive mode it is reverse biased: the light-generated photocurrent flows from cathode to anode through the external circuit, and the reverse bias widens the depletion region for faster response and better linearity. In photovoltaic mode it is unbiased and generates a small voltage like a tiny solar cell (a solar cell is, in fact, a large-area photodiode), which gives lower noise and zero dark current at the cost of speed.

How to identify the Photodiode symbol

Start from the ordinary diode symbol — a solid triangle (anode) pointing into a perpendicular bar (cathode) — then add two small parallel arrows angled at roughly 45°, pointing inward at the triangle. Arrows toward the symbol always mean a light receiver; arrows away always mean a light emitter (LED). Many drawings also enclose the diode in a circle representing the component envelope, particularly in older or European schematics.

IEC 60617 and ANSI Y32.2 / IEEE 315 use the same construction; the practical variations are whether the envelope circle is drawn, whether the arrows are straight or wavy (wavy arrows conventionally suggest radiation as waves, per older practice), and arrow placement. The cathode bar corresponds to the marked stripe or shorter lead on the physical device, exactly as with a rectifier diode.

Function in a circuit

In photoconductive mode the photodiode sits reverse biased in series with a load resistor or, far more commonly, feeds the virtual-ground input of a transimpedance amplifier (an op-amp with a feedback resistor), which converts its photocurrent into a clean output voltage: Vout = Iphoto × Rf. Because photocurrent is linear over many decades of light intensity, this arrangement is the basis of virtually every precision optical measurement front end.

Even in complete darkness a reverse-biased photodiode passes a small dark current (nanoamps at room temperature, doubling roughly every 8–10 °C), which sets the noise floor. PIN photodiodes insert an intrinsic layer to enlarge the depletion region for speed and sensitivity; avalanche photodiodes (APDs) run near breakdown to give internal gain for detecting extremely faint signals such as LiDAR returns and fibre-optic pulses.

Standards: IEC vs ANSI

IEC 60617	IEC 60617 defines the photodiode as the general diode symbol with two arrows directed toward it, optionally within an envelope circle; IEC 60747-5 covers semiconductor optoelectronic device specifications.
ANSI/IEEE 315	ANSI Y32.2 / IEEE 315 draws the identical diode-plus-inward-arrows form; JEDEC registration governs North American device outlines and part numbering (e.g. the classic BPW34-type registrations).
Key difference	No structural difference — both standards use inward arrows on a diode. Cosmetic variation is limited to the envelope circle (more common in IEC/European drawings), straight versus wavy arrows, and the Greek-lambda annotation some data sheets add. The critical universal rule is arrow direction: toward the diode = photodiode, away = LED.

Terminals / pins

Pin	Name
anode	Anode
cathode	Cathode

Typical values

Typical silicon photodiode figures: spectral response 400–1,100 nm peaking near 850–950 nm; responsivity around 0.5–0.65 A/W at peak; dark current 1–30 nA at 20 V reverse bias (small-signal types); junction capacitance 10–70 pF at zero bias falling to a few pF under reverse bias; rise times from ~100 ns (standard PN) down to <1 ns (PIN types like the SFH203). Reverse voltage ratings commonly 20–60 V. The ubiquitous BPW34 offers 7.5 mm² active area, ~50 µA photocurrent at 1 mW/cm², and ~100 ns switching.

Where the Photodiode symbol is used

IR remote-control and light-barrier receivers paired with an IR-emitting LED
Fibre-optic and free-space optical data receivers, where PIN and avalanche photodiodes recover megabit-to-gigabit streams
Pulse oximeters and heart-rate sensors measuring light transmitted or reflected through tissue
Optical encoders, slot sensors and tachometers detecting beam interruption from a rotating disc
Camera exposure meters, lux meters and ambient-light sensors with transimpedance front ends
Smoke detectors (photoelectric type) sensing light scattered by smoke particles in a chamber

Example

In a light-meter front end, the Photodiode symbol's Cathode pin connects to +5 V and its Anode pin to the inverting input of an op-amp wired as a transimpedance amplifier with a 1 MΩ feedback resistor; the reverse-biased diode's photocurrent of 0.5 µA per µW of incident light develops −0.5 V per µW at the output, giving a linear, nanosecond-fast light reading — connect it forward biased and it would conduct like an ordinary diode and sense nothing.

Key facts

The symbol is a standard diode with two arrows pointing toward it; the LED symbol is identical but with arrows pointing away — arrow direction is the only difference.
Photodiodes are normally operated reverse biased (photoconductive mode); photocurrent, not forward conduction, is the signal.
Photocurrent is highly linear with light intensity over several decades, which is why photodiodes are preferred for measurement over LDRs.
Photovoltaic mode (zero bias) trades speed for zero dark current — a solar cell is simply a large-area photodiode used in this mode.
Silicon photodiodes respond from roughly 400 to 1,100 nm with peak responsivity of ~0.5–0.65 A/W near 850–950 nm.
Dark current (nA-level) flows even in darkness and roughly doubles every 8–10 °C, setting the sensor's noise floor.
PIN photodiodes add an intrinsic layer for sub-nanosecond speed; avalanche photodiodes add internal gain for very faint signals.
The standard readout circuit is a transimpedance amplifier: the photodiode feeds an op-amp virtual ground and Vout = Iphoto × Rf.

Frequently asked questions

What is the difference between the photodiode symbol and the LED symbol?

Both start from the same diode triangle-and-bar. On a photodiode the two small arrows point toward the diode, showing light arriving at the junction; on an LED they point away, showing light being emitted. Functionally they are converses: an LED converts current to light and is operated forward biased, while a photodiode converts light to current and is normally operated reverse biased.

Which way round does a photodiode go in a circuit?

For photoconductive (reverse-biased) operation, the cathode goes to the positive supply rail and the anode toward the lower potential — the opposite of how you would forward-bias a rectifier. Light then generates a photocurrent that flows through the external load. On the physical package the cathode is the marked or shorter lead, matching the bar of the symbol. In photovoltaic mode there is no bias; the anode is the positive output terminal, like a tiny solar cell.

What is the difference between photoconductive and photovoltaic mode?

Photoconductive mode applies reverse bias: the widened depletion region gives fast response (nanoseconds), lower capacitance and excellent linearity, at the cost of dark current and its noise. Photovoltaic mode uses zero bias: the diode self-generates voltage/current like a solar cell, with zero dark current and lowest noise, but slower response. Instrumentation favours photovoltaic mode for precision at low frequency; communications and encoders use photoconductive mode for speed.

What is the difference between a photodiode and a phototransistor?

A photodiode produces photocurrent directly with no gain — typically microamps — but responds in nanoseconds and is very linear. A phototransistor uses light as base drive and multiplies it by the transistor's current gain, delivering 100–1,000 times more current from the same illumination, but with microsecond response and poorer linearity. Choose a photodiode for speed and measurement accuracy, a phototransistor for simple, sensitive on/off light detection.

What is dark current in a photodiode?

Dark current is the small leakage current (typically 1–30 nA for small silicon devices) that flows through a reverse-biased photodiode even with no light present. It rises exponentially with temperature — roughly doubling every 8–10 °C — and with reverse voltage. Because it is indistinguishable from photocurrent, it sets the minimum detectable light level; precision designs reduce it by using photovoltaic mode, cooling, or subtracting a matched dark reference diode.

Related symbols

Place the Photodiode symbol on a wiring diagram or schematic in the free online circuit diagram maker — no download required.