Bipolar transistor

(PD) Image: John R. Brews
A planar npn bipolar junction transistor as might be constructed in a integrated circuit.

In electronics, the bipolar transistor, more completely the bipolar junction transistor, is a three terminal semiconductor device used for switching and amplification. In concept it consists of two back-to-back pn-diodes, forming either a pnp or an npn sandwich, where p refers to semiconductor doped to produce positively charge carriers (holes) and n refers to semiconductor doped to provide negatively charged carriers (electrons). However, the center region is thin enough to allow carriers injected from one of the end layers (the emitter E) to actually diffuse across the center region (the base B) and be collected by the other end region (the collector C).

Very small changes in the emitter-base junction voltage have an exponential influence over the number of carriers injected from the emitter, and so the base has enormous control over the current diffusing across the base to the collector. Moreover, the current consumed by the base in normal operation is very small, so the device serves well to amplify either a current or a voltage signal applied to the base.

History

The bipolar transistor was the historically first transistor invented. Prior to its invention in 1947 by Bardeen, Brattain and Shockley at Bell Laboratories, semiconductor devices were only two-terminal devices, like diodes and rectifiers. More of the history and development of this device can be found in an historical article by Shockley and a more recent history.

Operation

(PD) Image: John R. Brews
Band diagram for npn bipolar transistor biased in active mode.

The bipolar transistor can operate in a number of modes, distinguished by which junctions are injecting (forward bias of emitter-base or collector-base or both) and which are collecting (reverse bias of emitter-base or collector base, or both). Here focus is upon the active mode in which the emitter-base junction is injecting and the collector-base junction is collecting. This mode is commonly used in analog circuits.

Using a band diagram as shown to the right, the operation can be understood. The diagram shows an npn transistor. The conduction band labeled CB shows the lowest energy of an electron (in electron volts, or energy divided by electron charge) in the conduction band of the semiconductor as a function of position in the npn transistor. The valence band labeled VB shows the highest energy for electrons in the semiconductor valence band. These two energy levels are separated by the semiconductor energy gap, a region of forbidden energy for an electron. The CB and VB vary in position within the transistor for two reasons: if no bias is applied, the band edges vary because impurity atoms set the number of carriers, and the bands must adjust position to insure the correct carrier densities. For more detail, see the article semiconductor.

The majority carrier Fermi levels in the various regions are shown as determined by the impurity dopant levels: E_Fn for electrons in the field-free bulk of the emitter, E_Fp for holes in the field free portion of the base, and E_Fn for electrons in the field-free bulk of the collector. If the biases are reduced to zero, these Fermi levels all coincide.

When bias is applied, the relative energies of the different regions are modified, upsetting equilibrium and causing the band edges to adjust in response. The Fermi levels are separated by the application of bias voltages across the junctions. The forward bias V_BE splits the hole Fermi level in the base from the electron Fermi level in the emitter. Likewise, the reverse bias V_CB splits the electron Fermi level in the bulk collector from the hole Fermi level in the field-free region of the base.

The base-emitter junction is forward biased, that is, the base is made positive with respect to the emitter, attracting electrons. This forward bias V_BE reduces the barrier φ_n that opposes entry of electrons into the base. Because the barrier is smaller, electrons enter the base, raising the concentration of electrons in the base above the normal equilibrium level and setting up a concentration gradient of electron density across the base. That gradient drives a diffusion current of electrons across the base (transport according to Fick's law of diffusion), toward the collector. At the same time, the collector is reverse biased by a voltage V_CB with respect to the base, that is, made positive with respect to base, so it attracts electrons. This attraction reduces the electron density on the collector side of the base, adding to the gradient in electron density across the field-free portion of the base. The electrons diffusing across the base eventually reach the end of the field-free region, and enter the accelerating electric field created by the reverse bias on the collector. The transport of electrons then switches from diffusion due to the carrier gradient to drift under the action of the electric field.

The strong effect upon the collector current exerted by the base-emitter bias can be understood in terms of its large effect upon the electron density at the base-emitter interface. The number of electrons at the top of the barrier is a factor exp(−φ_n/V_th) smaller than the density in the emitter itself. Here V_th is the so-called thermal voltage given by:

$V_{th}={\frac {k_{B}T}{q}}\ ,$

where k_B is the Boltzmann constant and T is the temperature in kelvins. At 290 K, V_th ≈ 25 mV. Thus, a change in this barrier height by an applied bias to become V_BE smaller means the electron density at the top of the barrier becomes larger by a factor exp(V_BE/V_th), a large exponential increase.