Objective of the Experiment
The purpose of this experiment is to analyze the operating principles and output characteristic curves of representative transistors, BJT and MOSFET, using LTspice.Theoretical Background and Differences in Circuit Configuration
Both BJT and MOSFET are transistor devices that perform amplification and switching functions. However, there are clear differences in their internal carrier control mechanisms and device terminal nomenclatures.
A BJT consists of an emitter, a base, and a collector. Minority carriers injected from the emitter into the base region move to the collector through diffusion to form a current. In other words, it is a current-controlled device where a minute base current flowing into the input terminal determines the magnitude of the collector current at the output terminal.
A MOSFET consists of a source, a gate, and a drain. A voltage applied to the gate forms a vertical electric field in the substrate through the oxide layer, and this electric field induces an inversion layer channel on the surface, altering the conductivity between the source and the drain. In other words, it is a voltage-controlled device where the voltage at the input terminal determines the drain current at the output terminal.
- BJT Output Characteristics Analys is Measurement Method : The variation of collector current according to the collector-emitter voltage (V_CE) was measured using LTspice while increasing the base current in steps.
Characteristics of the BJT I-V Curve
Linearity in the Active Region : When V_CE is applied above a certain voltage (approximately 0.2~0.3V) and escapes the saturation region, I_C becomes insensitive to changes in V_CE and draws a flat curve.
Consistency of Amplification Ratio : The most notable point is that as I_B increases uniformly, the vertical spacing between each generated I_C curve remains almost constant.
This visually and perfectly demonstrates the linear current amplification characteristic of the BJT, represented by I_C = ßI_B (where ß is the current gain).
Physical Operating Principles of the BJT
Emitter-Base Junction (Forward Bias) : The emitter is highly doped (N+) with impurities.
When a forward bias is applied, the depletion region between the emitter and the base narrows, and the built-in potential barrier is lowered. This causes a massive injection of electrons, the majority carriers of the emitter, into the base region.
Base Region (Minority Carrier Diffusion) : The base region, a P-type semiconductor is very thin and lightly doped. Electrons transferred from the emitter become 'minority carriers' within the base and diffuse toward the collector due to the concentration gradient.
Because the base is thin, the rate at which electrons recombine with holes and disappear is very low.
Base-Collector Junction (Reverse Bias) : A strong reverse bias is applied to this junction, resulting in a wide depletion region and a strong electric field.
Electrons that safely pass through the base and reach the boundary of this depletion region are rapidly swept into the collector by the electric field (Drift).
Summary: A BJT is a transistor device in which a very small base current (replenishment of holes used for recombination) controls the potential barrier between the emitter and the base, thereby governing the massive amount of electrons transferring to the collector, with both electrons and holes participating in current conduction.
- MOSFET Output Characteristics Analysis Measurement Method : In the same manner, the variation of drain current according to the drain-source voltage was measured while increasing the gate-source voltage in steps.
Characteristics of the MOSFET I-V Curve
Non linearity in the Saturation Region : The saturation of current after the pinch-off phenomenon is similar to that of a BJT, but the pattern of curve spacing according to the input variable is completely different.
Confirmation of Square-Law Characteristic : Even though V_GS was increased at a constant voltage interval (1V), a nonlinear increasing trend was observed where the vertical spacing between I_D curves significantly widened toward the top.
This result accurately supports the fundamental semiconductor physics theory that the MOSFET's drain current is proportional to the square of the overdrive voltage (I_D ∝ (V_GS – V_th)²).
Physical Operating Principles of the MOSFET
Depletion State : When a positive (+) voltage is applied to the gate, holes, which are the majority carriers in the P-type substrate (bulk) below the oxide layer, are pushed away by the electric field, forming a negatively charged depletion region at the oxide-substrate interface.
Looking at the energy band diagram at this time, the energy bands near the substrate surface begin to bend downward.
Inversion Layer Formation : When the gate voltage is increased above the threshold voltage, the surface energy bands bend more severely, causing the intrinsic Fermi level (E_i) to drop below the Fermi level (E_F).
At this moment, the surface of the P-type substrate temporarily acts like an N-type, and an N-type channel (inversion layer) is formed through which electrons can move between the source and the drain.
Pinch-off : As the drain voltage increases, the reverse bias on the drain side increases, expanding the depletion region at the end of the channel.
Eventually, a pinch-off phenomenon occurs where the channel near the drain is blocked, and thereafter, the current no longer increases and becomes saturated even if V_DS is raised.
Summary: Unlike the BJT where both electrons and holes participate in current conduction, the MOSFET is characterized by the gate voltage creating a vertical electric field that bends the energy bands, and the charge in the induced channel moves by the horizontal electric field between the source and drain to create current, utilizing only one type of carrier and employing surface control via an electric field through the oxide layer.
- Conclusion
Through this simulation, the difference in physical control methods between BJT and MOSFET was cross-verified through the shapes of their I-V output characteristic curves. The BJT showed a linear current amplification with constant spacing (△I_C ∝ △ I_B) based on carrier diffusion among the emitter, base, and collector.
On the other hand, the MOSFET exhibited a nonlinear current increase (I_D ∝ (V_GS – V_th)²) based on the surface field effect in a gate-source-drain structure. Through this, it was confirmed that the criteria for device selection in circuit design must vary depending on whether linear gain is required or high-input-impedance voltage control is needed.
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