Analysis of semiconductors with field-dependent mobility
Using the time domain analysis of Lagrangian approach, the small-signal behavior of a semiconductor with field-dependent mobility under various contact conditions is analyzed. A generalized closed form solution for an impulse current response is derived when the carrier velocity characteristic is expressed by a piecewise linear function. This result gives an accurate description of the transient performance of the device and can provide good insight into device operation. Numerical examples are given for a negative differential mobility GaAs crystal, considering the conditions under which negative resistances occur. A generalized small-signal impedance of the semiconductor is given, from which previous results reported for particular devices can be obtained as special cases.
Detailed theory of the negative conductance of bulk negative mobility amplifiers, in the limit of zero ion density
The complex admittance behavior is calculated for a bulk negative conductivity semiconductor, such as n-type GaAs, in the limit of zero doping and zero trapping, when all electrons are due to space-charge limited emissions from the cathode. Two different approximations are used: in the first, in closed analytical form, electron diffusion is neglected; in the second, by computer simulation of the internal space-charge dynamics, it is included. Both approximations agree at low frequencies where they predict a positive device conductance. Both predict a negative conductance, of slightly different magnitude, at frequencies around the reciprocal electron transit time. At higher frequencies the diffusionless theory predicts slowly damped conductance oscillations; diffusion effects strongly increase the damping.
Emitter controlled negative resistance in GaAs
A triode which uses the negative differential mobility of GaAs is proposed for various microwave applications and is analyzed as an oscillator. With proper bias conditions, this device has a microwave equivalent circuit that is simply a capacitance in parallel with a negative resistance. The magnitude of the negative resistance is controlled by injected current. The major characteristics as an oscillator are: 1 < −Q < 10 for nL ⋍ 1011 cm−2 and 107 < fL < 108, maximum device efficiency between 15 and 20 per cent, power density approximately 110 that of LSA mode device, absence of noise associated with domain formation, and third terminal control of negative resistance. The maximum efficiency as an oscillator is available over a finite range of current density, which decreases as nL increases.