Photodiode vs. Phototransistor: Key Differences Explained

This page compares photodiodes and phototransistors, highlighting their differences, advantages, and disadvantages.

Introduction

Both photodiodes and phototransistors are photocells, devices that convert light signals into electrical signals. This light energy can be in the form of infrared or ultraviolet radiation. They are used in a variety of applications, including fire alarms, counting systems, and automatic control systems.

Photodiode

• A photodiode consists of a normal p-n junction housed in a small enclosure with a transparent window through which light can fall. The figure above depicts a photodiode symbol.
• A photodiode is operated in reverse bias, where the leakage current increases in proportion to the amount of light falling on the junction. This is a result of light energy breaking the bonds in the crystal lattice of the semiconductor, producing electrons and holes. This effect is similar to that of a photovoltaic cell.
• Photodiodes are used as fast counters and in light meters to measure light energy.
• The figure above depicts the circuit symbol and a photodiode manufactured by OSRAM.

Phototransistor

• A phototransistor is a transistor controlled by exposure to light, similar to a photodiode controlling an ordinary bipolar transistor.
• It can be either a bipolar transistor or a field-effect transistor (FET).
• The body is encased in resin, plastic, or a metal shell with a window.
• A phototransistor has two leads which connect internally with its collector and emitter (or source and drain in FET). The base of the transistor (or gate in FET) responds to light and controls the flow of current between the leads.

It can resemble a photodiode, providing current amplification due to transistor action. Some of these devices are molded in transparent plastic cases with a convex shape. This convex shape acts as a lens, focusing light on the transistor.

As a result, extra minority carriers are liberated at the reverse-biased CB junction (Collector to Base). This generated leakage current is then amplified. When used in this way, a connection to the base terminal is not needed. Hence, many phototransistors do not have a base lead. The figure above depicts the circuit symbol and an NPN phototransistor L14G2.

Difference Between Photodiode and Phototransistor

The following points highlight the difference between photodiode and phototransistor:

• A photodiode is more responsive to incident light than a phototransistor.
• A photodiode has a linear response over a much wider range of light than a phototransistor. Therefore, photodiodes are used for precise measurement of light over a wide range.
• Photodiodes pass less current compared to phototransistors and consume less current. Hence, photodiodes are used for battery-powered devices requiring less power consumption.
• A phototransistor is about one hundred times more sensitive than a photodiode.
• The dark current of a phototransistor is much higher than that of a photodiode.
• A phototransistor is used as a solid-state switch, unlike a photodiode. The saturation voltage of a phototransistor is the voltage drop between the collector and emitter, which seldom exceeds 0.5V.