Why we need these devices

• Normal devices are suitable for Transit time and other effects in High Microwave Frequency ranges. (>10^11Hz).

• So we need special devices, that can use different instabilities in semiconductor to produce negative conductance devices.

• Such devices are:-

High Frequency Devices

• Tunnel diode: Uses the tunneling phenomena.

• IMPATT Diode: Uses the Avalance and Transit time phenomena.

• GUNN Diode: Uses the Transfer of electrons from higher mobility to lower mobility.

TUNNEL DIODE

Degenerate semiconductor

• At very very high doping, (if) the conduction band electron concentration n exceeds the effective density of states Nc, the Fermi level no longer within the band gap rather it lies within the conduction band.

Peak Tunneling Current = IpCorresponding Voltage = VpPeak Valley Current = IvCorresponding Voltage = Vp

Figure of Merit = Ip/Iv

Voltage Spread = Vp/Vf

Peak Tunneling Current = IpCorresponding Voltage = VpPeak Valley Current = IvCorresponding Voltage = Vp

Figure of Merit = Ip/Iv

Voltage Spread = Vp/Vf

IMPATT Diode

IMPATT Diode

• IMPATT stands for Impact Avalanche And Transit Time

• Operates in reverse-breakdown (avalanche) region• Applied voltage causes momentary breakdown once

per cycle• This starts a pulse of current moving through the

device• Frequency depends on device thickness

Introduction• When the pn junction diode is reverse-biased, then current does not flow.• However when the reverse voltage exceeds a certain value, the junction

breaks down and current flows with only slight increase of voltage. This breakdown is caused by avalanche multiplication of electrons and holes in the space charge region of the junction.

• The pn junction in the avalanche breakdown condition exhibits negative resistance characteristics in the microwave frequency range.

• Since the negative resistance is based upon avalanche multiplication and transit-time effect of carriers, the device has been called the IMPATT (Impact Avalanche Transit-Time) Diode.

Device StructureThe original suggestion for a microwave device employing

transit-time effect was made by W. T. Read and involved an n+-p-i-p+ structure such as that shown in figure. This device operates by injecting carriers into the drift region and is called an IMPATT diode.

The device consists essentially of two regions: 1) the n+p region, at which avalanche multiplication

occurs, and 2) the i (essentially intrinsic) region, through which

generated holes must drift in moving to p+ contact.

How can we achieve Negative Conductance?

• Here an AC voltage will be applied with a DC reverse bias.

• To achieve negative conductance we have to get something like this-

So we have to delay the current somehow--

• How?There are two delays

1.A delay in avalanche process2.Delay in the drift region

We have to use the carefully so that current is delayed enough to be seen as negative conductance.

DC BIAS IS SET AT THE CRITICAL AVALANCHE POINT, SO MULTIPLICATION BEGINS AT t=0;

THE PULSE OF HOLE CONTINUES TO GROW AS LONG AS ELECTRIC FIELD IS ABOVE Ea, but time needed exponentially increases while field is above Ea.

• Now when field returns to Ea again, the generated pulse already entered the drift region; in the drift region it consumes some time reach next p+ contact.

ALTHOUGH THE VOLTAGE IS LOWEST NOW, BUT THIS FIELD IS ENOUGH TO MOVE THE HOLES FROM AVALANCHE REGION TO P+ CONTACT, BUT IT CONSUMES TIME, THIS TIME IS DEPENDED ON THE LENGTH “L”.

TIME CALCULATION

Time needed to pass this drift region =L/VdWe need this time to be equal to T/2

So, f=Vd/2L

IMPATT I-V Characteristics

0 T/2 T 3T/2 2T 5T/2

tI

t0 T/2 T 3T/2 2T 5T/2

IDC

VDC

V

t

vAC

iAC

Gunn Device

• Slab of N-type GaAs (gallium arsenide)• Sometimes called Gunn diode but has no

junctions• Has a negative-resistance region where drift

velocity decreases with increased voltage• This causes a concentration of free electrons

called a domain

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

Gunn Oscillator Frequency

• T=d/vT = period of oscillationd = thickness of devicev = drift velocity, about 1 105 m/s

• f = 1/T