06-03-2013, 10:11 AM
Transferred Electron Devices
[attachment=52712]
TEDs are made of compound semiconductors such as GaAs.
They exhibit periodic fluctuations of current due to negative resistance effects when a threshold voltage (about 3.4 V) is exceeded.
The negative resistance effect is due to electrons being swept from a lower valley (more mobile) region to an upper valley (less mobile) region in the conduction band.
Gunn Diode
The Gunn diode is a transferred electron device that
can be used in microwave oscillators or one-port
reflection amplifiers. Its basic structure is shown
below. N-, the active region, is sandwiched between
two heavily doped N+ regions. Electrons from the
Gunn Operating Modes
Stable Amplification (SA) Mode: diode behaves as an amplifier due to negative resistance effect.
Transit Time (TT) Mode: operating frequency, fo = vd / l where vd is the domain velocity, and l is the effective length. Output power < 2 W, and frequency is between 1 GHz to 18 GHz.
Limited Space-Charge (LSA) Mode: requires a high-Q resonant cavity; operating frequency up to 100 GHz and pulsed output power > 100 W.
Avalanche Transit-Time Devices
If reverse-bias potential exceeds a certain threshold, the diode breaks down.
Energetic carriers collide with bound electrons to create more hole-electron pairs.
This multiplies to cause a rapid increase in reverse current.
The onset of avalanche current and its drift across the diode is out of phase with the applied voltage thus producing a negative resistance phenomenon.
Microwave Transistors
Silicon BJTs and GaAsFETs are most widely used.
BJT useful for amplification up to about 6 MHz.
MesFET (metal semiconductor FET) and HEMT (high electron mobility transistor) are operable beyond 60 GHz.
FETs have higher input impedance, better efficiency and more frequency stable than BJTs.
Transit-time Mode
Domains move through the GaAs till they reach the positive terminal
When domain reaches positive terminal it disappears and a new domain forms
Pulse of current flows when domain disappears
Period of pulses = transit time in device
[attachment=52712]
TEDs are made of compound semiconductors such as GaAs.
They exhibit periodic fluctuations of current due to negative resistance effects when a threshold voltage (about 3.4 V) is exceeded.
The negative resistance effect is due to electrons being swept from a lower valley (more mobile) region to an upper valley (less mobile) region in the conduction band.
Gunn Diode
The Gunn diode is a transferred electron device that
can be used in microwave oscillators or one-port
reflection amplifiers. Its basic structure is shown
below. N-, the active region, is sandwiched between
two heavily doped N+ regions. Electrons from the
Gunn Operating Modes
Stable Amplification (SA) Mode: diode behaves as an amplifier due to negative resistance effect.
Transit Time (TT) Mode: operating frequency, fo = vd / l where vd is the domain velocity, and l is the effective length. Output power < 2 W, and frequency is between 1 GHz to 18 GHz.
Limited Space-Charge (LSA) Mode: requires a high-Q resonant cavity; operating frequency up to 100 GHz and pulsed output power > 100 W.
Avalanche Transit-Time Devices
If reverse-bias potential exceeds a certain threshold, the diode breaks down.
Energetic carriers collide with bound electrons to create more hole-electron pairs.
This multiplies to cause a rapid increase in reverse current.
The onset of avalanche current and its drift across the diode is out of phase with the applied voltage thus producing a negative resistance phenomenon.
Microwave Transistors
Silicon BJTs and GaAsFETs are most widely used.
BJT useful for amplification up to about 6 MHz.
MesFET (metal semiconductor FET) and HEMT (high electron mobility transistor) are operable beyond 60 GHz.
FETs have higher input impedance, better efficiency and more frequency stable than BJTs.
Transit-time Mode
Domains move through the GaAs till they reach the positive terminal
When domain reaches positive terminal it disappears and a new domain forms
Pulse of current flows when domain disappears
Period of pulses = transit time in device