Phototransistors

A phototransistor is a light-sensitive bipolar transistor that converts incoming optical signals into amplified electrical current. Unlike a photodiode which produces only a small photocurrent, a phototransistor uses the transistor's current gain (beta) to amplify the photogenerated base current, making it far more sensitive to low light levels. Phototransistors are widely used in proximity sensors, object detection, optocouplers, and industrial automation.

Core Concept of Phototransistor Operation

A phototransistor is most commonly an NPN bipolar junction transistor where the base terminal is left open (or the base connection is eliminated entirely) and light is allowed to fall on the base-collector junction. The reverse-biased base-collector junction acts like a photodiode: incident photons with energy exceeding the band gap generate electron-hole pairs in the depletion region of this junction. The photogenerated holes flow into the p-type base, raising the base potential. This increase in base potential forward biases the base-emitter junction, causing a much larger electron current to flow from emitter to collector.

The critical advantage over a photodiode is transistor gain. The photocurrent I_ph generated at the base-collector junction acts exactly like an externally injected base current. In normal BJT operation, the collector current is I_C = beta * I_B. In a phototransistor with open base, the photogenerated carriers replace the external base current, so I_C = beta * I_ph. The total collector current (including dark current contribution) is I_C = (beta + 1) * I_ph_total. For a transistor with beta = 100, this means the output current is 100 times larger than what a simple photodiode would produce from the same light input.

The device operates in common-emitter configuration for maximum current gain. The collector is connected to the supply voltage V_CC through a load resistor R_C. As light intensity increases, photocurrent I_ph increases, which increases I_C, which in turn increases the voltage drop across R_C and reduces the output voltage V_out = V_CC - I_C * R_C. This gives an inverted output characteristic: bright light produces low V_out, dark gives high V_out, making phototransistors naturally suited for digital switching applications.

Mathematical Expression for Phototransistor Gain

The collector current of a phototransistor in the active region is related to the incident optical power by combining the photodiode responsivity with the transistor current gain. If the responsivity of the base-collector photodiode is R (A/W), then the photocurrent is I_ph = R * P_opt. With transistor current gain beta (h_FE), the collector current becomes I_C = beta * I_ph = beta * R * P_opt.

The combined sensitivity of the phototransistor is characterized by its photosensitivity S = I_C / P_opt = beta * R (units A/W). A typical silicon phototransistor with beta = 200 and base-collector responsivity R = 0.5 A/W has a photosensitivity of 100 A/W. This is orders of magnitude higher than the responsivity of a discrete photodiode, making phototransistors ideal for detecting low light levels without external amplification.

The dark current in a phototransistor is also amplified by the transistor gain. If the base-collector junction dark current (I_CEO or I_CBO) is I_d, then the total collector dark current is approximately (beta + 1) * I_CBO. This means phototransistors have much higher dark current than photodiodes, which can be a disadvantage in precision detection. The dark current increases significantly with temperature, which limits the practical use of phototransistors in high-temperature environments.

Practical Understanding of Speed versus Gain Tradeoff

The primary limitation of phototransistors compared to photodiodes is their slower response speed. The response time of a phototransistor is determined by the transistor switching time, which includes the time to charge the base capacitance and the minority carrier transit time through the base. Typical phototransistors have response times of 5 to 50 microseconds, corresponding to bandwidths of 20 kHz to 200 kHz. Photodiodes, by contrast, can achieve bandwidths of hundreds of megahertz to tens of gigahertz.

This speed-gain tradeoff is fundamental. High beta means high photosensitivity but also means the base must be charged and discharged through a longer carrier lifetime, increasing response time. In applications where moderate speeds are sufficient (below 1 MHz) and high sensitivity is required, phototransistors are preferred. For high-speed optical communication above 100 Mbit/s, PIN photodiodes followed by transimpedance amplifiers are always used instead.

An optocoupler (also called an optoisolator) is a key practical application of the phototransistor. It packages an LED and a phototransistor in the same package, with the LED's output optically coupled to the phototransistor's base-collector junction. Current flowing through the LED creates light, which drives the phototransistor's collector current. The electrical isolation between input (LED side) and output (transistor side) can withstand several kilovolts, making optocouplers essential for isolating high-voltage circuits from sensitive control electronics.

Given:
Incident optical power: P_opt = 500 uW = 500 x 10^-6 W
Base-collector junction responsivity: R = 0.45 A/W
Transistor current gain: beta = 150
Collector supply: V_CC = 5 V
Collector load resistance: R_C = 1 kohm

Why this formula applies:
The phototransistor collector current is the photocurrent multiplied by transistor gain.
I_C = beta * R * P_opt
Output voltage: V_out = V_CC - I_C * R_C

Formula:
I_ph = R * P_opt
I_C = beta * I_ph
V_out = V_CC - I_C * R_C

Substitution:
I_ph = 0.45 * 500e-6 = 225 uA
I_C = 150 * 225e-6 = 33.75 mA
V_out = 5 - (33.75e-3 * 1000)

Calculation:
I_C = 33.75 mA
V_out = 5 - 33.75 = -28.75 V (impossible -> transistor saturates)
Saturation check: I_C_max = V_CC / R_C = 5/1000 = 5 mA
Actual I_C is limited to 5 mA (transistor in saturation)

Final Answer:
Photocurrent I_ph = 225 uA
Transistor saturates for this illumination level
Saturated collector current I_C = V_CC / R_C = 5 mA
V_out = V_CE(sat) approximately 0.2 V

Exam Tip: In GATE or university problems on phototransistors, always check if the transistor enters saturation by comparing beta * I_ph against V_CC / R_C. If beta * I_ph exceeds V_CC / R_C, the transistor is saturated and I_C = V_CC / R_C, not beta * I_ph. Ignoring saturation is the most common calculation error in phototransistor problems.

• Photons incident on the reverse-biased base-collector junction generate electron-hole pairs (photocurrent I_ph), which act as an injected base current.
• The transistor amplifies I_ph by its current gain beta, producing collector current I_C = beta * I_ph, far larger than any photodiode output.
• Overall photosensitivity S = beta * R (A/W). For beta = 200 and R = 0.5 A/W, S = 100 A/W, making phototransistors excellent for low-light detection without external amplifiers.
• The high gain also amplifies dark current, increasing noise floor. Response speed is limited to 20 kHz to 200 kHz due to BJT switching time, unlike fast photodiodes.
• Optocouplers use LED-phototransistor pairs for kilovolt-level galvanic isolation between high-voltage power circuits and sensitive control electronics.

Quick Revision

• Phototransistor = photodiode + BJT in same device. Photons generate I_ph at B-C junction, transistor amplifies it: I_C = beta * I_ph.
• Photosensitivity: S = beta * R (A/W). Much higher than a standalone photodiode due to transistor gain.
• Saturation check in circuit problems: if beta * I_ph > V_CC / R_C, transistor saturates, I_C = V_CC / R_C.
• Speed limitation: bandwidth is 20 kHz to 200 kHz due to BJT carrier lifetime, not suitable for high-speed optical communication.
• Dark current is also amplified by beta, raising noise floor compared to photodiodes at elevated temperatures.
• Optocoupler application: LED drives phototransistor optically, providing electrical isolation of several kilovolts between input and output circuits.
• GATE trap: Do not forget to check for saturation when V_CC and R_C are given. Also, do not confuse photosensitivity S (A/W) with quantum efficiency (dimensionless) or responsivity R (A/W of the junction alone).