CORRESPONDENCE 905

0 b XM zw 3w Tlpsec)

T I M E

l n l

IA ..

4000 c I I \

100 200 300 11psec) T I M E

till

0 100 zw 3 w Tlpsec)

T I M E ( 4

0 100 200 300 T(psec)

T I M E ( f)

Fig. 2. Current waveform for each mode. (a)-(f) correspond to the same modes as in Fig. 1.

[9] C. Hammar and B. Vinter. ‘‘DiffuJion of hot electrons in n indium phosphide,”

perlattice and negative differential conductivity in Rlrrfvnn. 1.d.. “01. 9. nn. 9-10, Jan. 1973. simple design required to confine the discharge and produce the

necessary high density of electrons is described.

ductor. Warsaw, Poland: PWN- 437.

oscillations of conduction electrons, to be submltted. [12] T. Ohmi and M. Matsudaira, “Negpive differential mobility due to the Bloch

Room-Temperature Lasing of CdS Crystals in a Glow Discharge

F. H. NICOLL

Abstract-A very compact device has been made for excitation of lasing in CdS at room temperature. A low-pressure glow discharge is produced by high-voltage pulses to pump the CdS crystal. The

Manuscript received May 21, 1973. The author is with RCA Laboratories. Princeton, N. J.

Lasing in 11-VI compounds and other semiconductors in which suitable p-n junctions cannot be made has been obtained either by electron beam pumping or by optical pumping using a gas laser. Both of these pumping techniques require considerable equipment and are very bulky. Recently [l] a compact device has been described using a field-emission triggered-vacuum arc to provide electrons when a small capacitor charged to a few thousand volts is discharged across the high-vacuum space between a CdS crystal and the field emitter. With this arrangement CdS crystals have lased at room temperature.

It has now been found that pulsed lasins; at room temperature in CdS can also be achieved by using a low-pressure glow discharge designed especially to provide the high density of electrons necessary for pumping the CdS crystal. Focused gas discharges for electron beam welding have been described elsewhere [2] but the design is not readily adapted for low power where a very small diameter beam is required. Fig. 1 is a schematic showing the very simple structure necessary to achieve lasing with the small crystals usually used. The CdS sample is mounted with vacuum grease on the inside of the face-

906 IEEE TRANSACTIONS ON ELECTRON DEVICES, OCTOBER 1973

ANODE TUNGSTEN CATHODE IN CAPILLARY 4,

6 * I

t

VACUUM I U

PUMP

Fig. 1. Compact glow discharge tube for high-density excitation of CdS to produce lasing.

0.5 mm Fig. 2. Photemicrograph of lasing CdS crystal showing the end of the capil1ar.r

cated by the very intense vertical edge brightness. and a portion of the clrcular discharge outslde the crystal edges. Lasing is indi.

plate. Facing this is a small capillary tube with a tungsten w i n running from outside the vacuum to within about 5 mm of the oper tip. This wire is the cathode, the anode being provided to one side The CdS samples are thin platelets (0 10 pm) cleaved to give paralle’ sides spaced about 300 pm apart. The sample is spaced about 50 p m from the capillary, which has an inside diameter of about the crystal width.

Operation of the device requires appropriate gas filling and appli- cation of short high-voltage pulses. A suitable gas is admitted through a controlled leak with the pumps running. Nitrogen seems to operate best for pressures between 0.2 torr and 0.02 torr, while with helium the pressure required is between 2 torr and 0.2 torr. There is some evidence that the cathode dark space should be rather large so that high-voltage electrons can reach the CdS crystal. The high-voltage pulses were obtained by two different means. In one case, 100-ns pulses were obtained when a capacitor (20 pF) charged to a voltage of 5 to 15 kV was discharged through the anode-cathode space o f the tube [l] . A second method, equally satisfactory, was to apply the secondary of a modified Tesla coil to the cathode lead. The Tesla coil was altered in such a way that its capacitor at about 3 kV could be discharged through the primary to give a single train of high-voltage oscillations from the secondary lasting about 4 ps.

The attainment of lasing in the CdS was readily observed by eye, Fig. 2 reproduces a photograph taken through a microscope focused on the crystal. The end of the capillary can be seen with a portion of the circular discharge visible where it is not intercepted and ab- sorbed by the crystal. Fig. 2 shows the crystal lasing in a total inter- nal reflection mode, the strong laser light escaping at the right and left cleaved edges of the crystal. The far-field pattern is a fan-shaped beam with a horizontal spread of 180’ and a vertical width of about 10’. The light emitted while lasing is less than 10 A wide a t 5370 A, as observed on a hand spectrometer. Below lasing threshold the bright edges disappear, the far field becomes homogeneous in all directions,

and the emitted green light has a spectral width of about 300 A. It was readily demonstrated that the excitation of the CdS was mainly by electrons since a piece of quartz a few micrometers thick on top of the crystal stopped the excitation almost completely.

This new method of pumping semiconductors to lasing threshold offers the advantage of very small size and rather simple means for pulsed excitation. Efficiency is not high and should be similar to that quoted in [l], although those figures are believed to be pessimistic. As with all electron beam pumped lasers, there are problems with life, and in this version added damage occurs to the sample from the gas discharge. However, i t is becoming more and more apparent that there are many ways of achieving the pump-power density required to produce lasing in semiconductors.

REFERENCES [l] F. H. Nicoll, “Semiconductor lasers pumped by a pulsed electric discharge in

[2] G. G. Isaacs, “High power pulsed electron beam from a glow dwharge,” Eledron. vacuum, A p p l . Phys. Lett., vol. 22, pp. 363-364, Apr. 15, 1973.

Lett., vol. 3, p. 542, Dec. 1967.

Space-Charge-Limited Triode Using a Cr-Doped Semi-Insulating GaAs

T. MIMURA AND M. FUKUTA

Abstract-By providing ohmic contacts and a rectifying contact to a Cr-doped semi-insulating GaAs material, triode-like device characteristics were experimentally verified. The current drawn through the device is interpreted as space-charge-limited current (SCLC). Operation of the device is shown to be qualitatively in agree- ment with the vacuum triode analog.

For linear application a triode-like characteristic is favored over a pentode-like characteristic. Few solid-state devices with a triode- like characteristic have appeared up to this time. Zuleeg [ l ] fabri- cated a thin-film CdS dielectric triode with triode-like characteristics, utilizing the variation of current with voltage given by the solid-state analog of Child’s law [2] for space-charge-limited emission.

T o realize a good high-frequency performance of the device, how- ever, i t is required to use material with a carrier mobility as high as possible and planar techniques accomplishing fine patterns. Semi- insulating, i.e., compensated, GaAs is not the most promising ma- terial since the electron mobility is very low. The Cr-doped material was usedfor the experimental devices. T o realize high-frequency per- formance the most promising candidate is high-resistivity uncom- pensated n-type GaAs material which exhibits high electron mobility.

Fig. 1 depicts the cross-sectional representation of the device. The structure of the device is very similar to the standard FET structure, but i t does not have the conducting channel that a FET has. Semi- insulating GaAs with a resistivity of the order of lo8 Q*cm at room temperature and n-type conduction is cleaned in a NaOH system prior to the contact deposition. The source and drain contacts are prepared as ohmic contacts by alloying a 2000-A-thick gold-germa- nium film into the semi-insulating GaAs at 485OC for 180 s after depo- sition. The grid contact is prepared as a rectifying contact by deposit- ing a 3000 A aluminum film onto the semi-insulating GaAs. These contact geometries are defined using standard photoresist techniques. The devices are scribed and cleaved, thermally bonded on a TO-5 package, and gold wires are ultrasonically bonded to the source, drain, and grid.

Fjg.,2 (a) shows the nonlinear source-drain current-voltage char- acterlstlc (as stated in the next sentence with near square-law be- havior), and Fig. 2(b) shows the source-grid rectifying current- voltage characteristic. The source-drain I- V characteristic has no polarity dependence and has a near square-law dependence on ap- plied voltage, following (l), over the voltage range shown in Fig. 2(a), in spite of having a nonplane parallel electrode structure.

In the case of plane parallel electrodes the space-charge-limited- current (SCLC) in solids with shallow traps [3] is

I=-- . 9&pAV2 8d3 (1)

Manuscript received March 28, 1973: revised June 1. 1973. The authors are with the IC Design Department, Fujitsu Limited, Kobe, Japan.