Collector-base Space Charge Region Research Articles

Collector–Base Junction Capacitance of Advanced Bipolar Transistors Operating at Avalanche Breakdown

Modeling of the collector–base junction capacitance of the advanced bipolar transistors operating at avalanche breakdown is developed. The comprehensive junction capacitance model accounts for high current and high field effects at the collector–base junction. The impact ionization generates tremendous amount of free carriers in the collector–base space-charge region which increases the collector–base junction capacitance at the avalanche breakdown regime. The present collector–base junction capacitance is useful for device and circuit design under avalanche breakdown prior to the actual fabrication of the circuit.

Collector signal delay time and collector transit time of HBTs including velocity overshoot

The collector signal delay time τ CT and collector transit time τ′ CT in the heterojunction bipolar transistor (HBT) are studied and discussed using the delay time concept. Based on the assumption that the free carrier travels with the saturation velocity ν s in the base-collector space-charge region, τ′ CT is conventionally modeled as 0.5 τ CT, which equals W C/2 ν s, where W C is the thickness of the collector depletion region. For an HBT in which velocity overshoot can occur in the collector layer, the constant velocity profile used conventionally is questionable. In addition to the constant velocity profile and the step-like velocity profile proposed more recently, the present study considers also a more realistic velocity profile consisting of a linear-dependent velocity in the velocity overshoot region and an average velocity in the rest of the collector space-charge region. This piecewise-linear velocity profile resembles closely that predicted by Monte-Carlo simulations reported in the literature. It is shown that τ′ CT can be notably smaller than 0.5 τ CT, particularly when velocity overshoot is significant. Cutoff frequencies calculated from the present approach using the piecewise-linear velocity profile are in better agreement with Monte-Carlo simulation than that calculated from the constant and step-like velocity profiles.

A phenomenological approach to estimating transit times in GaAs HBTs

A method is proposed for estimating transit times in heterojunction bipolar transistors (HBTs) which are fabricated from semiconductor materials in which the conduction band can be represented by a two-valley model. The transition times for exchange of electrons between the conduction band valleys are treated as phenomenological parameters and are shown to be linked by the electric field in the device. Incorporation of the transition rates into the continuity equations for upper and lower valley electrons yields a set of equations which can be solved under transient conditions to yield directly the transit times of carriers across the base and the base-collector space-charge region. Results from this approach are compared with Monte Carlo calculations and shown to exhibit good agreement. >

Parameter extraction for bipolar transistors

Novel methods for parameter extraction for bipolar transistors are presented. Emphasis is placed on reliable measurements of small-size transistors, examples are shown for Si integrated circuit transistors with emitters smaller than 10 mu m/sup 2/. Advantages over measurements on large-area test devices are discussed. Parameters measured are the current-dependent base and emitter series resistances, emitter contact resistance, base-emitter and base-collector space-charge region capacitances, junction temperature, and base and collector saturation currents. Applications to heterojunction bipolar transistors are demonstrated.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Charge-control analysis of the collector-base space-charge-region contribution to bipolar-transistor time constant τT

Charge-control analysis is used to show that the contribution of the collector-base space-charge region to bipolar transistor time constant τ <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">T</inf> is x <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">d</inf> /2v <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">d</inf> where x <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">d</inf> is the space-charge-layer width and v <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">l</inf> is the scattering-limited velocity of carriers.

An analytical model for the epitaxial bipolar transistor

In the n + pn − n + transistor, high-current effects in the base and collector regions are linked within the current ranges of practical interest. To describe such effects, we have derived an analytical model that is based primarily on five assumptions: (1) the structure is approximately one-dimensional; (2) recombination is negligible in the base and collector quasi-neutral regions, and in the three space-charge regions; (3) high-current effects are negligible in the emitter and n +-substrate regions; (4) the Fletcher boundary conditions (or the Misawa boundary conditions) can be used for the three space-charge regions; and (5) the ambipolar approach can be used for the base and collector quasi-neutral regions. The primary findings predicted by the n + pn − n + transistor model are: In current ranges of practical interest (usable current gain), the electron concentration profile has a significant “vertical step” located at the collector-base metallurgical junction for all values of collector current. In the limit of extremely-high-current operation, this step tends to vanish. In the current range where the current gain begins to decline rapidly with increasing collector current, the electron concentration at the base boundary of the collector-base space-charge region goes approximately as the square of the hole concentration at the collector boundary of the same region. Because of this relationship, a charge-control calculation is more difficult than a straightforward calculation of carrier concentration for a given degree of accuracy. The n + pn − n + transistor model (which consists of twelve algebraic equations) is particularly useful for the practically important case of an epitaxial bipolar transistor having a very thin, heavily-doped base region.