The symmetrical p^+-n-p^+ transistor of Fig. P 7-7 is connected as a diode in the four configura

step 1 we can conclude that that forward bias resistance of forward bias forward bias resistance of for

The symmetrical p^+-n-p^+ transistor of Fig. P 7-7 is...

The symmetrical $\mathrm{p}^{+}-\mathrm{n}-\mathrm{p}^{+}$ transistor of Fig. $\mathrm{P} 7-7$ is connected as a diode in the four configurations shown. Assume that $V \geqslant k T / q$. Sketch $\Delta p\left(x_{n}\right)$ in the base region for each case. Which connection seems most appropriate for use as a diode? Why?

Key Concepts

Bipolar Junction Transistor (BJT) Structure

A bipolar junction transistor consists of three regions with alternating types of doping. In a symmetric p?–n–p? transistor, both outer regions are heavily doped (p?) while the central n?region (serving as the base) is lightly doped. This structure is key in understanding how carriers are injected and collected when the device is repurposed as a diode, as the relative doping levels determine the injection efficiency, built?in potentials, and overall junction behavior.

Diode Connection in Semiconductor Devices

Connecting a transistor as a diode involves biasing two of its junctions in such a way that one functions as a standard p–n junction diode. The particular choice of which terminals are tied together and which junction is forward or reverse biased significantly affects the minority carrier injection, series resistance, and overall I–V behavior. Understanding these connection configurations and their impact on carrier distribution is essential to determine the most appropriate diode implementation.

Minority Carrier Injection and Diffusion

Under forward bias conditions (with V ? kT/q) in a p–n junction, minority carriers are injected from the heavily doped region into the quasi?neutral region of the lightly doped side. These excess carriers then diffuse away from the junction and recombine with majority carriers. Their spatial distribution within the base (or quasi-neutral region) is described by an exponential decay determined by the diffusion equation and the boundary conditions set by the junction, illustrating the true nature of carrier transport in semiconductor devices.

Excess Carrier Profile in the Quasi-neutral Region

The excess carrier concentration (?p in the n-region, for instance) follows a diffusion-recombination profile that typically exhibits an exponential decay or a linearized profile near the junction depending on the injection level and geometry. Drawing ?p(x) in the base region helps illustrate how carriers injected at one edge diffuse and recombine as they travel through the region, highlighting the effective diffusion length and the influence of recombination processes on the device’s performance.

Forward Bias and Low-Level Injection

When a forward bias (with V ? kT/q) is applied, the junction experiences low-level injection where the increase in minority carrier concentration is moderate enough to maintain a linear response in carrier recombination and diffusion theories. This condition simplifies the analysis, allowing one to use exponential functions to describe the minority carrier distribution and to directly compare the effects of different connection schemes on the injection efficiency and diode behavior.

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