Forward Bias

Part of The Transistor

Forward bias applies voltage across a PN junction in the direction that allows current to flow, reducing the depletion region and enabling the diode and transistor to conduct.

Why This Matters

The PN junction sits at the heart of every transistor and diode. In its natural state, the junction blocks current in both directions due to the depletion region — a zone stripped of charge carriers where a built-in electric field acts as a barrier. Forward bias is the act of applying external voltage to oppose this built-in field, collapsing the barrier enough for current to pour through.

Understanding forward bias is not merely theoretical. If you are building transistor amplifiers or switching circuits, you must bias the base-emitter junction into forward conduction. If you are testing semiconductors you have grown yourself, forward bias behavior tells you whether your junction is functional at all. A junction that refuses to conduct under forward bias has failed — either the doping is wrong or the junction was destroyed during fabrication.

Every active transistor circuit applies forward bias to one junction deliberately. Without grasping what is happening physically inside the semiconductor, circuit design becomes guesswork.

The Depletion Region and Built-In Voltage

When p-type and n-type semiconductor are joined, electrons from the n-side diffuse across the boundary and fill holes on the p-side. This leaves behind positive ions on the n-side and negative ions on the p-side, creating an electric field that opposes further diffusion. The region swept clean of free carriers is the depletion region.

The built-in electric field corresponds to a built-in voltage — approximately 0.3 V for germanium and 0.6–0.7 V for silicon. This is the voltage you must overcome before forward conduction begins.

Semiconductor	Built-In Voltage	Forward Threshold
Germanium	~0.3 V	0.2–0.3 V
Silicon	~0.6–0.7 V	0.6–0.7 V
Gallium arsenide	~1.4 V	~1.2 V

The depletion region width at zero bias is typically 0.1–1 micron for typical doping levels — too thin to see but electrically critical.

Applying Forward Bias

Forward bias means connecting the positive terminal of a voltage source to the p-side and the negative terminal to the n-side.

What happens physically:

1. The external positive voltage pushes holes on the p-side toward the junction
2. The external negative voltage pushes electrons on the n-side toward the junction
3. These free carriers crowd into the depletion region, reducing its width
4. The built-in electric field weakens as the depletion region shrinks
5. When external voltage exceeds the built-in voltage, the barrier essentially disappears
6. Electrons from n-side and holes from p-side recombine continuously at the junction, with the external circuit replenishing both

The result is a continuous current flow: electrons flow from the n-side through the junction into the p-side, where they recombine with holes, and new holes are injected into the p-side from the positive terminal.

The Diode Equation

Current through a forward-biased junction follows an exponential relationship:

I = I₀ × (e^(V/V_T) − 1)

Where:

• I = junction current
• I₀ = reverse saturation current (very small, ~1 nA for silicon)
• V = applied voltage
• V_T = thermal voltage ≈ 26 mV at room temperature (kT/q)

This exponential relationship is why diodes have a sharp turn-on threshold rather than a gradual increase. A small increase in voltage above the threshold causes a dramatic increase in current.

Practical implications:

• At 0.5 V (silicon): barely conducting, microamps of current
• At 0.6 V: milliamps begin
• At 0.7 V: full conduction, tens of milliamps or more
• Each 60 mV increase roughly multiplies current by 10x

Forward Bias in Transistors

In a bipolar transistor (NPN or PNP), the base-emitter junction must be forward-biased for any collector current to flow.

NPN transistor:

• Emitter (n-type), Base (p-type): this is a PN junction
• Apply positive voltage to base relative to emitter → forward bias
• Threshold: ~0.6–0.7 V for silicon, ~0.2–0.3 V for germanium
• Once forward-biased, electrons from emitter inject into the thin base region
• Most of these electrons, instead of recombining in the base, are swept into the collector by the reverse-biased collector-base junction

The forward-biased base-emitter junction acts like a valve: small changes in base voltage create large changes in collector current. This is the amplification mechanism.

Checking forward bias in practice:

1. Measure V_BE (voltage from base to emitter) with a voltmeter
2. Silicon transistor: V_BE should be 0.6–0.7 V when conducting
3. Germanium: V_BE should be 0.2–0.3 V
4. If V_BE is much lower, the transistor is cut off
5. If V_BE is much higher than threshold, check for faulty connections or damaged device

Practical Testing of Forward Bias

Diode Forward Test

1. Set multimeter to diode test mode (or low-resistance ohms range)
2. Connect red probe (positive) to anode (p-side), black probe (negative) to cathode (n-side)
3. A working silicon diode reads 0.5–0.7 V in diode mode
4. A working germanium diode reads 0.2–0.3 V
5. Reading near zero: diode is shorted (failed)
6. Reading “OL” or infinite: diode is open (failed) or you have probes reversed

Transistor Base-Emitter Test

1. Using a silicon NPN transistor
2. Apply 0.7 V from base to emitter (use a resistor divider from a 9V battery)
3. Measure collector current — should be substantial (mA range for small signal transistors)
4. Reduce V_BE below 0.6 V — collector current should drop sharply toward zero
5. This on/off behavior confirms the transistor is functional

Temperature Sensitivity

Forward voltage decreases with temperature by approximately −2 mV per degree Celsius for silicon. This matters in practice:

• A circuit designed at 25°C with V_BE = 0.65 V will have V_BE ≈ 0.50 V at 100°C
• This shifts the operating point and can cause thermal runaway in power circuits
• Use emitter resistors in amplifier designs to stabilize the operating point against temperature changes

Summary

Forward Bias — At a Glance

• Forward bias applies positive voltage to the p-side, negative to the n-side, opposing the built-in junction field
• Silicon junctions need ~0.6–0.7 V to begin conducting; germanium needs ~0.2–0.3 V
• Current rises exponentially with voltage — a 60 mV increase multiplies current roughly 10-fold
• In transistors, forward-biasing the base-emitter junction is what allows collector current to flow
• Measure V_BE to confirm a transistor is properly biased: 0.6–0.7 V for conducting silicon NPN
• Forward voltage drops ~2 mV/°C as temperature rises — account for this in amplifier designs