Understanding LED Forward Voltage and Color

The question asks in which of the provided options the forward voltage varies as the colour varies. Let's analyze the characteristics of each option.

Analyzing the Options

We need to consider how each option produces colour and its relationship with forward voltage.

• LED (Light Emitting Diode): An LED is a semiconductor device that emits light when an electric current passes through it in the forward direction. The colour of the light emitted by an LED is determined by the semiconductor material used to construct it. Different semiconductor materials have different band gap energies (\(E_g\)). When an electron and a hole recombine in the semiconductor, a photon is emitted with energy approximately equal to the band gap energy (\(E_{photon} \approx E_g\)). The colour of the light corresponds to the energy of the emitted photon. The forward voltage (\(V_f\)) required to cause current flow and thus light emission in an LED is related to the band gap energy by the approximate relationship: \(E_g \approx qV_f\), where \(q\) is the elementary charge. Therefore, LEDs made of different materials emitting different colours will have different band gap energies and consequently require different forward voltages to operate. For example, red LEDs typically have a lower forward voltage (around 1.8V - 2.2V) than blue or white LEDs (around 3V - 3.5V). This directly shows that in an LED, the forward voltage varies as the colour varies.
• LCD TV (Liquid Crystal Display Television): An LCD TV uses liquid crystals to modulate light from a backlight. The colour is created by colour filters (red, green, and blue) on the pixels. The liquid crystals control how much light passes through these filters. The operation of the liquid crystals themselves is controlled by voltage, but this voltage is used for controlling transparency, not directly related to the fundamental light emission color like in an LED. The backlight source (which might be LEDs in modern LCD TVs) has its own characteristics, but the LCD panel itself doesn't have a single "forward voltage" that varies with the colour being displayed on the screen.
• Colour TV (Cathode Ray Tube - CRT Television): Older colour TVs used cathode ray tubes. An electron beam strikes phosphor dots on the screen (red, green, and blue), causing them to glow and produce colour. The voltage involved in a CRT is primarily the high voltage accelerating the electron beam (thousands of volts) and voltages controlling the beam's direction and intensity. These voltages are not related to a "forward voltage" that varies with the colour produced by the phosphor. The colour is determined by which phosphor is hit by the electron beam.
• LED TV (LED-backlit LCD Television): An LED TV is essentially an LCD TV that uses LEDs as the backlight source instead of older technologies like CCFLs. As discussed, the LCD panel uses liquid crystals and colour filters. The LED backlight provides white light (or sometimes uses red, green, blue LEDs). While the LEDs in the backlight have a characteristic forward voltage, this forward voltage is fixed for the specific type of LED used in the backlight and does not vary depending on the colour being displayed on the screen pixels via the LCD panel.

Conclusion on Forward Voltage and Colour

Based on the analysis, the property where the forward voltage varies as the colour emitted varies is a fundamental characteristic of the Light Emitting Diode (LED) itself, due to the relationship between the semiconductor band gap energy, the energy of the emitted photon (and thus colour), and the required forward voltage for operation.

Therefore, the correct answer is LED.

Revision Table: Electronic Components and Colour Display

Additional Information: Semiconductor Band Gap and Light Emission

The energy band gap (\(E_g\)) of a semiconductor is the energy difference between the top of the valence band and the bottom of the conduction band. In a forward-biased p-n junction like an LED, electrons from the conduction band and holes from the valence band recombine. In direct band gap semiconductors, this recombination can release energy in the form of a photon. The energy of the emitted photon is approximately equal to the band gap energy: \(E_{photon} \approx E_g\).

The colour of light is determined by its wavelength (\(\lambda\)) or frequency (\(f\)). The energy of a photon is given by \(E_{photon} = hf = \frac{hc}{\lambda}\), where \(h\) is Planck's constant and \(c\) is the speed of light. Thus, the colour of light emitted by an LED is directly related to the band gap of the semiconductor material. Different materials like Gallium Arsenide (GaAs), Gallium Phosphide (GaP), Gallium Nitride (GaN), and their alloys have different band gaps and are used to produce LEDs of different colours (infrared, red, green, blue, UV).

The forward voltage (\(V_f\)) required to turn on the LED and drive current is roughly proportional to the band gap energy, specifically \(V_f \approx E_g / q\). This is why blue or white LEDs (higher \(E_g\), shorter wavelength, higher photon energy) have higher forward voltages compared to red or infrared LEDs (lower \(E_g\), longer wavelength, lower photon energy). This fundamental property is unique among the given options where the colour of the emitted light directly dictates the required forward operating voltage of the primary light source.